# eps or epx foam under slab



## curtis (Apr 2, 2013)

As i continue to research and plan for my garage i have come to the point where i am trying to figure out how to insulate the slab. I have read that the epx foam board at 27 bucks a sheet for 2 inch is the way to go. But have also read that the 12 dollar sheets of eps foam will work just as good. The epx is a r-10 and the eps is a r-7.5 in 2 inch think sheets. Has anyone done this before and can tell me witch way to go?  Im thinking i will need around 48 sheets so the $15 price difference will add up fast.


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## heaterman (Apr 2, 2013)

Curtis we did a 9,000 sq ft church this past fall/winter. The only place we used 2" foam was around the edge of the slab down vertically about 20". The entire floor was done with 1". It heats like a dream.
By far the vast majority of heat loss from a slab is on the edge rather than the interior of the slab itself. Just think about this for a minute... The edge is going to see ground temperatures next to it that may be well below freezing while the ground underneath the interior of the slab only drops to maybe 50-60*. When you have temperature differentials that low in the slab to ground interface you don't need all that much insulation to stop the heat flow.

A local business asked me to give them a price on doing a 10,000 sq ft equipment repair garage for their logging operation. I priced it out like above and lost the job because someone came in and told them all they needed was bubble foil under the slab and nothing on the edge because there was so little area exposed. Long story short, after burning up a couple of the huge Centrals inside of 6 years they are now building their own boiler that will take logs in 8' lengths and there is a 4 foot wide strip of green grass all the way around the building even in the dead of winter.

Take care in insulating the edge and use some 2" there. The interior of the slab is not as critical.


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## curtis (Apr 2, 2013)

Are you using the eps foam or the epx?


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## BoilerMan (Apr 2, 2013)

XPS, expanded polystyrene, or blue board as most of us in the trade call it.  The white "beadboard" does not have the same compression strength and water will penetrate it over time.  Use the blueboard or whatever color it is in your area.  As Heaterman pointed out once it's done it's done all you can change it the heat source, unless you like big jackhammers. 

In my northern climate it's 2" everywhere in any residential slabs,  which are relitivly small square footage, therefore not wide enough to justify the transition to thinner foam in the middle.  Large slabs (over 40' wide) I'd consider thinner once your 8 or 10 feet in.  And leave NO exposed concrete to the elements, the heatloss is HUGE!  I use double 2" on the vertical perimiter of monolithic slab pours (my house included).

TS


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## Fred61 (Apr 2, 2013)

I personally always use a vapor barrier beneath the insulation.


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## BoilerMan (Apr 2, 2013)

Fred61 said:


> I personally always use a vapor barrier beneath the insulation.


 
Same here at least 8mil poly or better.

TS


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## curtis (Apr 2, 2013)

Taylor, how do you insulate a monolithic slab? I thought of doing that type of slab instead of a footing with block but wasn't sure how to go about insulating it.


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## Fred61 (Apr 2, 2013)

My best friend is having a large house built on a lake in New Hampshire and after some words back and forth with his contractor he had to raise his voice in order to get him to lay vapor barrier. Contractor kept telling him it wasn't needed and they never install it.


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## BoilerMan (Apr 2, 2013)

I insulate the whole thing, I drew a crude picture of how I've done it and not had any issues doing it this way. The thickened edge is 12-16" thick and goes into the slab 12" plenty of steel in all of this with the rebar bent into hooks tied to the thick edge lateral rebar as well.  And as Fred said, I use poly under the whole thing, I like to tape the poly and overlap it at least a foot, does it do anything?  Don't know but it's cheap insurance and a damp slab is no fun!

TS


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## BoilerMan (Apr 2, 2013)

Oh and for what it's worth, the slab the chair is on which I'm trying from is done this way and has been here for 4 winters and I havn't had any sheetrock crack yet!  The sub-grade is the critical factor as well as quality concrete.  Dig out any organic matter and fill with coarse gravel.  I use regular 4,000 pound concrete with fiber in it as well as 1/2" rebar no wider that 24" O.C.   Over-kill in this stage is money well spent in my opinion, a building is only as good as the "rock which it is built on".

TS


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## curtis (Apr 3, 2013)

Is there a reason for that one horizontal piece to extend out past the wall that far?


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## ewdudley (Apr 3, 2013)

curtis said:


> Is there a reason for that one horizontal piece to extend out past the wall that far?


In order for frost to get under the slab it has to travel down to the level of the horizontal piece, then the frost has to travel horizontally the length of the horizontal piece.

When I proposed this type of slab to our building inspector in NY he dug into the code and found specs.  In NY it's down 1' and out 2' for a heated space, and out further for an unheated space.  In NY I had to leave out a foot of insulation under the perimeter beam to allow some heat flow out to meet the frost headed in.

And FWIW, in my experience for high and dry Corning pink is OK, but for beneath grade it's gotta be Dow blue.


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## jebatty (Apr 3, 2013)

I think it is a big mistake not to put 2" under the entire slab, especially in a cold climate like N.MI. Assuming ground temp is around 45-50F before being cooled further by the cold of winter, would you not insulate your walls and ceiling in a climate with outside air at this same temp? The floor has the same square footage as the ceiling, same area of endless heat loss to 45-50 "climate." You likely will insulate your ceiling much more than the R factor of 2" foam. So leave the floor uninsulated? Makes no sense.

With my 32 x 48 pole barn shop, I used 2" foam vertical down 4' around the outside perimeter, 2" foam between the concrete and the inside perimeter of the shop, and 2" foam under the entire slab. I set the floor temp to constant 61F as read by a sensor in the middle of the concrete about 3' into the slab from the perimeter; with a mixing valve supply the floor with 100F water; and the shop keeps an air temp between about 53F (when it's well below 0F outside), and about 60F (outside temp +20F and higher).

Do it the first time. You never will have a 2nd chance to add insulation under concrete.


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## maple1 (Apr 3, 2013)

On the vapour barrier - are you saying put it on the ground then the insulation on top of it? That's what I was reading - but doesn't vapour barrier always go on the warm side of insulation? Which would mean insulation down then vapour barrier.


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## jebatty (Apr 3, 2013)

Vapour barrier, then foam, then concrete. The vapour barrier keeps water from penetrating the foam and therefore reduces heat transfer to the ground via water. It also would be good to tape your foam seams.


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## Fred61 (Apr 3, 2013)

Don't forget that the soil prep comes first. Vapor barrier and insulation base must be high and dry which means good drainage and very well draining material under the slab and around the perimeter. That's another place that you can't re-work later. Doing this incorrectly could rob you for a lifetime and you may not even be aware of it.

I think I have kept the local crushed stone supplier in business over the years. It's worth the extra effort and the extra bucks. Crushed stone is about 90% compacted as dumped so compacting is a small job.

I once had this wild idea of using epdm pond liner instead of polyethelene because it's less porous but for some reason during that phase of construction it's rush--rush--rush and there's no time to screw around.


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## peakbagger (Apr 3, 2013)

Dick Hill and Tom in Maine (a hearth member) did a TV show several years back on frost heaves and slab insulation. I dont think the video is available anymore but Dicks paper is 

http://www.hotandcold.tv/professor hill.html

Tom stocks and sells lots of surplus foam to contractors in Maine for this type of insulation. It may not look pretty but the price is right (unfortunately I dont think it would be worth shipping to the OPs location.


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## Bret Chase (Apr 3, 2013)

Taylor Sutherland said:


> I insulate the whole thing, I drew a crude picture of how I've done it and not had any issues doing it this way. The thickened edge is 12-16" thick and goes into the slab 12" plenty of steel in all of this with the rebar bent into hooks tied to the thick edge lateral rebar as well. And as Fred said, I use poly under the whole thing, I like to tape the poly and overlap it at least a foot, does it do anything? Don't know but it's cheap insurance and a damp slab is no fun!
> 
> TS
> 
> View attachment 98606


 
how did you get that past codes?


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## BoilerMan (Apr 3, 2013)

What do you mean?  That was code then.  Actually above code standards, I'm not one to just follow rules, I like to make up better ones   LOL

Now that we have the MUBC well I guess we'd have to have a 6' frost wall.  I can't stand wasting time and material on things that have NO bearing on anything.

TS


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## Bret Chase (Apr 3, 2013)

I don't know what code your municpality was using, but a 4' frost wall has been code at this end of the state for decades.


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## curtis (Apr 3, 2013)

Im not sure on houses but I know for a garage you can pour a monolithic slab here if it isnt attatched to a building but if it is you have to go down 42'' with a footing.


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## heaterman (Apr 3, 2013)

Using the church I spoke of as an example..........when you do the math on it you'll find there is less than 5000 btu difference in heat transfer between 1" and 2" foam used under the slab. That's on a 9,000 sq ft building.
R-value of the product is of significant importance but realize that when talking about underslab insulation you are dealing with a strictly conductive heat loss and R-value as a measurement does not express that factor very well.


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## 711mhw (Apr 3, 2013)

curtis said:


> Im thinking i will need around 48 sheets so the $15 price difference will add up fast.


 
Bite the bullit!
I've used these folks several times. http://www.insulationdepot.com/ They may have a warehouse near you. They will negotioate, but like to sell trailer loads. I have allways picked up at their warehouse with a large trailer but not a full 48' van. I did a turned down slab like Taylors (excelent) drawing and used a product that they (insul. depot) had that was I think removed from a commercial flat, rubber roof, & that it is used for it's insulation and instead of the old stone "ballast" that they used to use. It was 2'x4' t&g panels that had 2" of Dow "blue" with a little 3/8 to 1/2 in concrete surface on it that look's like a fine exposed aggregate finish. I used it under the slab, and vertically along the perimeter. It really worked well, it did 2 things for me. 1. it is ridgid enough to be the formwork. 2. The concrete finish looks nice above grade and protects the foam.


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## Bret Chase (Apr 3, 2013)

curtis said:


> Im not sure on houses but I know for a garage you can pour a monolithic slab here if it isnt attatched to a building but if it is you have to go down 42'' with a footing.


 
In Maine, or at least southern Maine.... municipalities using a variety of IBC and IRC codes since I entered the industry in '89 requires a 3' 10" frost wall with a footing for any building that has human occupation as a use. The only monolithic slabs I have poured have been for external slabs... or a fuel containment structure...


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## easternbob (Apr 4, 2013)

I used EPS under my slabs and it is fine. You have to make sure to get the higher density EPS for under the slab though. Under the slab is 1.5 lb foam, the foam I used on the inside of the basement walls is 1.0 lb.
This is the company I got my foam from.  http://www.thermalfoams.com/default.asp  They were very helpful.


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## eauzonedan (Apr 4, 2013)

Curtis:
This is how I handled it on my Northern Wis Shop.  I used a 1' high perimiter curb wall formed as part of the slab to allow me to go with 9' clear interior height and still use 8' studs/sheet goods.  That approach also allowed me to better match surrounding grades and still keep the siding from being too close to the ground.  If you read the fine print on the load capacity of the foams - they tell you to de-rate the stated capacity to 1/3 the nominal carrying capacity for *continuous* loading.  So I upped the foam to the 60 psi rated stuff under the perimiter load bearing areas of the building that carried the weight from the upper floor and roof (the area of the slab perimiter typically thickened for that very reason).  It's readily available from anybody that would supplly comercial  building materials.  The 25 psi foams should have plenty of capacity if the only loading is the floor slab (and not the building loads).  I wrapped the full perimiter area of concrete with 4" of rigid also.......  Have yet to fire the in-floor up, so can't tell you how well the heat loss worked, but after two years there is no sniff of any cracking.  My thought was better safe than sorry ........after the building is up it's pretty tough to try to re-do .  The foam doesn't like UV for long exposure, so I covered it with alluminum and installed a z strip to keep the rain off it.  Ended up looking pretty decent.  I only ran a single piece of 2" between the O/H doors and the aprons.  Thought the 4" gap to be a bit strong if driving over it. Good Luck!
Dan


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## Bret Chase (Apr 4, 2013)

easternbob said:


> I used EPS under my slabs and it is fine. You have to make sure to get the higher density EPS for under the slab though. Under the slab is 1.5 lb foam, the foam I used on the inside of the basement walls is 1.0 lb.
> This is the company I got my foam from. http://www.thermalfoams.com/default.asp They were very helpful.


 
No, the regular blue dow foam is just fine....  it gives you the same 3000#/sq.ft as the assumed ground load..... there is generally no need to go to Dow Hiload 100... (14,400PSF) at $108 a sheet...


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## BoilerMan (Apr 6, 2013)

Personally I have to say, I'm glad the IBC hasn't made it this far north.  I was just at a town meeting and we voted out the MUBC as we are a towm of less than 4,000 residents.  It's interesting that a "human occupancy" has to have a frost wall, but a "detached building" can be monolithic.  The base (gravel) has all to do with the frost's heaving force and foam can stop frost penetration.  Drainage is also a deciding factor, I'm on blue (hard) ledge and excivated a foot of topsoil and filled and leveled with bank run gravel.  To me a monolithic slab is stronger as it's all one pour and the load bearing is transferred into the "floor surface" to further spread out the building load, rebar should be bent into the thickened edge and all tied to the lateral rebar in the shollow footer making it much stronger than a sepetate footer and floating slab IMHO.  But I can't tell the code that.............. This is America isn't it. 
Rant over, I'm still fired up from the town meeting about this subject, and none of the selectmen having building skills.

TS


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## Fred61 (Apr 6, 2013)

Taylor Sutherland said:


> Rant over, I'm still fired up from the town meeting about this subject, and none of the selectmen having building skills​


 Most of our laws/rules are made by people who don't know crap about the subject.


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## ewdudley (Apr 6, 2013)

Taylor Sutherland said:


> To me a monolithic slab is stronger as it's all one pour and the load bearing is transferred into the "floor surface" to further spread out the building load, rebar should be bent into the thickened edge and all tied to the lateral rebar in the shollow footer making it much stronger than a sepetate footer and floating slab IMHO.  But I can't tell the code that.............. This is America isn't it.


I was making the same argument to my code enforcement officer here in NY and he said code is footers 42" below grade.  But he liked the idea and said he might be able to do something if I came back with more engineering to back it up.  

Meanwhile he dug into the NY code and found a section that covered insulated grade beams specifically as an approved alternative to footers.

So 'everybody knows' footers are required, including the zone enforcement experts, but it is possible that what everybody knows is contradicted by the actual code.  And it is possible that a building inspector might go the extra mile if you're lucky enough that they turn out to be an ally.

Here's some photos of the forming.  Unfortunately all the gravel work and the horizontal insulation were covered over by the time I got around to taking photos, but the basic idea is to have tiled drainage below a gravel layer with the insulation next, extending 2' down and away from the base of a 1' x 1' reinforced insulated concrete beam.  (Would have much preferred Dow blue board but couldn't get what I needed in the time frame available.)

http://summerwages.blogspot.com/


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## BoilerMan (Apr 6, 2013)

Nice work EWD!  There are few to no sturctures in that era here, most go back to 1880s with a huge boom in the roaring 20s and again in the 60s.  I used the Pink "Owens Corning" board as that is what my building supplier carried at the time.  I was not aware of any differences other than color and manufacturer O.C. vs. Dow. 

TS


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## ewdudley (Apr 6, 2013)

Taylor Sutherland said:


> I used the Pink "Owens Corning" board as that is what my building supplier carried at the time.  I was not aware of any differences other than color and manufacturer O.C. vs. Dow.


Before I built the shed over where the 'rain harvesting system' tanks are, I used a bunch of scrap insulation board to cover the area where the pipes came in to the house.  I used gravel, then plastic, then insulation, then more plastic, topped off with a couple inches of grave.  With rain water and snow melt from above and condensation from below, it completely failed to keep the insulation dry.  

There was a mix of Dow blue and Corning pink, and when I dug the mess up to do the shed the Dow blue was crisp and bone dry while the pink was flexible and water-logged, so that's where I get my opinion.  But I am betting that the pink will do the job if it's kept well drained.


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## BoilerMan (Apr 6, 2013)

Gulp...........

TS


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## ewdudley (Apr 6, 2013)

Taylor Sutherland said:


> Gulp...........
> 
> TS


I wouldn't be too worried, I had built that patio off to the right a few years before and used pink to form the 48" concrete footings.  When we did the shed I exposed some of that.  Backfilled with gravel and under the wide eaves the pink was in real good shape.  I'm just saying given a choice blue is better in adverse conditions.


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## Bob Rohr (Apr 6, 2013)

Here is a link to some excellent reading and codes regarding underslab insulation.

http://www.healthyheating.com/Radiant_heating_designs/insulating-underslabs.htm#.UWBuqnD6pJ0


Consider also having a bug repellent sprayed before insulating.  Here in Missouri I detected 3 different bugs boring into my Blueboard and also the foam ICfs I used for the foundation.  It seems warm conditions under a slab are a big draw for bugs to take up residence.

Also consider a tight tube spacing.  I'd even go to 6" on center so you could run that slab with supply temperatures as low as possible, perhaps 90F.  To really leverage your tank output charge to 180- 190, and run it down to 90F with good insulation and tight tube spacing.  That provides for a 100 degree ∆T from your tank temperature!


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## Floydian (Apr 6, 2013)

Bob Rohr said:


> Here is a link to some excellent reading and codes regarding underslab insulation. http://www.healthyheating.com/Radiant_heating_designs/insulating-underslabs.htm#.UWBuqnD6pJ0


 
My favorite paragraph from the above link:

"When you build to code you essentially get a “D” grade – that is, you didn’t fail but you missed the opportunity to earn an A or A+. If you don’t understand the words, ‘missed opportunity” then you have never considered what it takes to increase the insulation under a concrete slab after it has been poured."

Noah


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## nate379 (Apr 6, 2013)

Looks way better than current codes around here for a SOG



Bret Chase said:


> how did you get that past codes?


 
We used 2" XPS foam under my house and garage when the slab was poured.  I forget the brand now, but it was about 1/2 the price of the blue board.  Something around $18 a sheet vs $36


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## Bret Chase (Apr 6, 2013)

Taylor Sutherland said:


> Personally I have to say, I'm glad the IBC hasn't made it this far north. I was just at a town meeting and we voted out the MUBC as we are a towm of less than 4,000 residents. It's interesting that a "human occupancy" has to have a frost wall, but a "detached building" can be monolithic. The base (gravel) has all to do with the frost's heaving force and foam can stop frost penetration. Drainage is also a deciding factor, I'm on blue (hard) ledge and excivated a foot of topsoil and filled and leveled with bank run gravel. To me a monolithic slab is stronger as it's all one pour and the load bearing is transferred into the "floor surface" to further spread out the building load, rebar should be bent into the thickened edge and all tied to the lateral rebar in the shollow footer making it much stronger than a sepetate footer and floating slab IMHO. But I can't tell the code that.............. This is America isn't it.
> Rant over, I'm still fired up from the town meeting about this subject, and none of the selectmen having building skills.
> 
> TS


 
The worst of the IBC was the 2000 edition, which is what Sanford had adopted for a while... it added at LEAST 20% to the cost of the building for basically NO benefit whatsoever... During that era, I put up a building by the airport... the code driven design ended up requiring the purlins to be 18" OC.... and this building stands next to 20# buildings from the 70's... and their 6' purlin spacing... and those 40 year old buildings, with 2" insulation are still standing.

monolithic slabs have their purpose, but they are certainly NOT stronger from a load bearing and seismic perspective.


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## Bret Chase (Apr 6, 2013)

Bob Rohr said:


> Here is a link to some excellent reading and codes regarding underslab insulation.
> 
> http://www.healthyheating.com/Radiant_heating_designs/insulating-underslabs.htm#.UWBuqnD6pJ0
> 
> ...


 

This is why I like Tuff-R (Polyisocyanurate) for below ground....  if the bugs eat it, they die... lol


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## Bret Chase (Apr 6, 2013)

nate379 said:


> Looks way better than current codes around here for a SOG
> 
> 
> 
> We used 2" XPS foam under my house and garage when the slab was poured. I forget the brand now, but it was about 1/2 the price of the blue board. Something around $18 a sheet vs $36


 
My comment about codes had nothing to do with the insulation... it was with the monolithic slab and a complete lack of a frost wall, which is generally mandatory in this state...


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## BoilerMan (Apr 6, 2013)

Bret Chase said:


> This is why I like Tuff-R (Polyisocyanurate) for below ground.... if the bugs eat it, they die... lol


 Ployiso, takes on water and is NOT for below grade at all.  The fact that Dow puts the aluminum over it is to keep the moisture from destroying the R-value, it is water permiable. 

TS


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## Bret Chase (Apr 6, 2013)

Taylor Sutherland said:


> Ployiso, takes on water and is NOT for below grade at all. The fact that Dow puts the aluminum over it is to keep the moisture from destroying the R-value, it is water permiable.
> 
> TS


 
it still makes bugs die...


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## nate379 (Apr 6, 2013)

Must be something new.  I lived in Maine for almost 20 years....



Bret Chase said:


> My comment about codes had nothing to do with the insulation... it was with the monolithic slab and a complete lack of a frost wall, which is generally mandatory in this state...


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## Bret Chase (Apr 6, 2013)

nate379 said:


> Must be something new. I lived in Maine for almost 20 years....


 
well... I've seen exactly one monolithic slab... with a "human occupation" use, since 1989....


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## nate379 (Apr 6, 2013)

I can think of 5 or 6 just in the town I grew up... a town of about 2000 people. Don't really have building codes "per say" though.

That photo with the insulation is exactly how my house and garage is, as well as most of the 12 houses in my neighborhood.


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## BoilerMan (Apr 6, 2013)

I've lived in Maine all my life, and I can speak of more than a dozen monolithic slabs that are for "humans"........  My house, uncle's place, old apartment we lived in before we built, some friends from church, come to think of it, it's only commercial buildings that I've with floating slabs and footings.  Slab houses are not at all common around here, the houses I speak of are all under 20 years old.  There are many, if not all garages are monolithic pours though. 

My reasoning for a slab:
 I'm an electrician and I've been _way too many_  wet, damp, flooded basements, which are useless IMHO
 Wet radiant heat with high mass is unparrelled for comfort and inherrant storage for wood heat
 No blasting required if ledge is close to surface
 Radon which is rampent in our area
 Simplified construction for novice (me) builders who do everything themselves
 Keeps my hoarding temdencies in check   LOL

Bottom line is insulate under any slab even if you don't plan to heat the building, because someday someone may turn it into living space and I lived in a converted two car garage (apartment mentioned above) with carpet right over that cold uninsulated concrete and it ain't no fun............

TS


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## nate379 (Apr 6, 2013)

My Dad did not insulate or vapor barrier the slab in the first garage he built.  In the summer it sweats so bad that you have to squeegee the water out.  Even with a fan going and doors/windows left open it does it.  Most of his tools are equipment are pretty rusty from that.

Garage #2 he put 6 mil plastic down and no problems.  The 2 garages are about 50ft apart.


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## Bret Chase (Apr 7, 2013)

Taylor Sutherland said:


> I've lived in Maine all my life, and I can speak of more than a dozen monolithic slabs that are for "humans"........ My house, uncle's place, old apartment we lived in before we built, some friends from church, come to think of it, it's only commercial buildings that I've with floating slabs and footings. Slab houses are not at all common around here, the houses I speak of are all under 20 years old. There are many, if not all garages are monolithic pours though.
> 
> My reasoning for a slab:
> I'm an electrician and I've been _way too many_ wet, damp, flooded basements, which are useless IMHO
> ...


 
I'm not arguing against insulating under a slab... at all.... it's been S.O.P. for us for many, many years.... but we still do have to build to the print/specs...  if the specs call for insulation only in the outer 4'.... that's what they get...  I hear owner complaints all the time.... All I can tell them is that their building is operating as designed, I built what they paid me to build.  Getting blamed for what the architect designed... I just *love* that one.... but anywho...

I've got one of those damp cellars.... not a basement.... the cellar in my nearly 150 year old home has a rubble/granite foundation and a dirt floor.  it's damp all the time, but is only really wet in the spring.  the house leaks so much air that Radon really isn't a concern for me... it might in the future as I tighten this thing up.


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## Highbeam (Apr 16, 2013)

Back to the OP of the thread... You can certainly use EPS, the white stuff, under the slab. The manufacturer rates their product for this application. It works great and is much cheaper than XPS (pink/blue). I bought the 25 psi EPS from the factory and put it under my 30x60 slab with radiant heat tubes.


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## curtis (Apr 16, 2013)

Highbeam said:


> Back to the OP of the thread... You can certainly use EPS, the white stuff, under the slab. The manufacturer rates their product for this application. It works great and is much cheaper than XPS (pink/blue). I bought the 25 psi EPS from the factory and put it under my 30x60 slab with radiant heat tubes.


 
Thanks highbeam, are you by chance a member on garagejournal? I think i remeber that name from there about topics just like this.


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## Highbeam (Apr 16, 2013)

Yes, that's a great site for this type of thing.

I used EPS and know the plant manager at the EPS factory so I was able to pick his brain. I am also a civil engineer and have used EPS as lightweight fill under roadways above gas mains and at railroad crossings. There is nothing at all wrong with putting EPS underground including those places where moisture may be present. 25 psi EPS is just as strong as the bearing capacity of most subgrades so strength is not an issue. Concrete, 6", only weighs 0.5 psi.

The trouble is when you wreck a truck and pour fuel into the foam. Then it shrinks. Probably happens with XPS too but I've never tried it.


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## mikefrommaine (Apr 28, 2013)

http://www.gologic.us/videos/passive-house-foundation/


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## tom in maine (Apr 28, 2013)

I derive some income from selling foam. I have been associated with a number of companies over the years.
A couple thoughts/comments from my experience:

Most foams are rated for at least 20psi. That is about 3000psf. There are very few applications that we would encounter that would not be well served with 20psi products. This would assume a reinforced slab, which is just common sense for any concrete project.
I believe even 1 pound density EPS would meet this criteria. There are many road projects that use EPS for filler to build up a substrate.
I am extremely nervous about bugs in foam.Once they get there it is scary. They are going to nest in it because it is easy to chew through. I would NEVER use foam that is contiguous from the ground to the wooden part of the house, especially where termites are common. I do not trust a flashing shield to keep them out. This goes for ICF's too.
There seems to be inconsistent results from treated foam in ground keeping bugs at bay.
I think the only real way to keep them out is baited traps and "nuclear" medicine if they show up.

Polyiso can be used under slabs. Realize that if you are doing a slab on grade, it should be very well drained.
Polyiso can pick up water, but so can the styrenes. If it is on a properly drained substrate, it will work.
I would use it under a slab, not on the edges where it sees daylight.
Polyiso can pick up up to 10 times its weight in water. At this point it will lose half of its R value.
And it will retain half of its R value. Once it dries out, it regains that loss. It can dry out, over time.
EPS and EPS can also pick up water. EPS more than XPS, but they both do with similar loss in insulation value.
They are slower to pick up and discharge water.

Regardless of what insulation I was using for under a slab, I would always use a reasonably good grade vapor barrier.
It only makes sense.
Joe Lstiburek felt that Polyiso made sense under slabs. He and I disagreed on where the barrier would go.
I would put it under the foam, he thought it should go over it.

My 2 cents worth from 30 years of dealing with foam as a buyer and seller.


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## DaveBP (Apr 28, 2013)

If the vapor barrier protects the concrete from moisture, why not put it below the foam and protect the foam as well? I agree with you, Tom.

But an equally important factor, I think, is that if it's under the foam the film is protected from the mud crew.


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## eauzonedan (Apr 28, 2013)

Tom in Main:

This is a copy of the design info for the Foamular products.  What's your read on the long term dead load capacity?  I interpreted this to de-rate the nominal ratings to about a third of the nominal load for long term dead loads like maybe a building sittin on it.....(see note 5)  The table got a bit messed up when imported, but the last three items for each of the product items is Live, Dead and Ultimate.  Here is a link to the original http://foamular.com/assets/0/144/172/174/c273fe0e-e0c8-4bb2-9ed3-6e3ba390a12c.pdf

I used only a couple sheets of the 60 psi foam, just under the grade beams of my building and everything under the center portion of the slab was 25 psi.......under a couple hundred bucks.......would like to hear your thoughts if I wasted the money?   After 4 years no cracking, but that's not to say the foam was the reason......  I'm just a silly surveyor and not much a a structural kind of guy....

Dan


The Foamular Foundation Properties table provides engineering data for individual project analysis if needed. *Foamular Foundation Properties *
Foamular Product

Foundation Modulus (pci)1, 2, 3

Compressive Stress (psi)


Thickness (in.)

Live5
Recommended 
Dead5
Recommended 
Ultimate4     


150

590 
550 
500 
450 
400 
300 
3 
5 
15 
250

750 
710 
675 
595 
565 
510 
5 
8.3 
25 
400

1100 
1000 
900 
780 
680 
650 
8 
13.3 
40 
600

1520 
1400 
1275 
1150 
1040 
790 
12 
20 
60 
1000

NA 
-- 
2600 
NA 
NA 
NA 
20 
33.3 
100




1.
Foundation modulus is a measure of deflection at given loads, expressed as inches deflection per inch of thickness or "pci". 
2.
For insulation installed in multiple layers, assuming the layers are identical, the foundation modulus for the system equals the foundation modulus for one of the layers divided by the total number of layers. 
3.
For insulation systems that utilize a variety of thicknesses, the system foundation modulus is determined by adding the reciprocal of the foundation modulus of the individual layers. The total is the reciprocal value for the foundation modulus of the entire system. 
4.
Ultimate compressive stress is measured at 10% deformation or yield, whichever occurs first. For thinner product (1"), yield typically occurs first. For thicker products (1.5" and thicker), yield typically occurs first with 3% to 4% deformation. 
5.
Recommended stress (load) levels will limit long term compressive creep to not exceed 2% in 20 years.


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## eauzonedan (Apr 28, 2013)

sorry about that ........once I sent it - the table got completely corrupted for some reason.........best to follow the link to the Foamular site.........
Dan


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## Bob Rohr (Apr 28, 2013)

tom in maine said:


> I derive some income from selling foam. I have been associated with a number of companies over the years.
> A couple thoughts/comments from my experience:
> 
> Most foams are rated for at least 20psi. That is about 3000psf. There are very few applications that we would encounter that would not be well served with 20psi products. This would assume a reinforced slab, which is just common sense for any concrete project.
> ...


 
I argee with you Tom, about the bugs.  I'm surprised it is also a concern in a cold northern climate.

Here is some foam ICF I put around my shop about 6 years ago.  It was wrapped with an aluminum bug shields.  I also sprayed around them when they were installed.  I think the spray needs to be replensihed every few years.  A fairly safer Borax product is what I used but it washes away.

I have also seen the BlueBopard product turn into a swiss cheese after 10 years from bug bores.

Years ago I recall that had a very toxic bug repellant in many of the underground foam prodcts.  Since that was banned the bugs have found a nice warm nesting spot under, and around radiant slabs.

I met a contractor from Australlia years back.  He told me all the foam foundations or foam insulation sheets needs to be wrapped with a fine stainless mesh product to keep the bugs out, down there.  It is also available with a tar coating to act as a water shield and bug shield.


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## tom in maine (Apr 29, 2013)

Dan,
I am not a structural engineer either. I do refer to them when I need to. Fortunately, we have a really good one who answers my questions at the University of Maine. He is a consultant for work NASA is doing on a moonbase(!) They built one in Orono, Maine!

There is the potential for foam to creep over time but I suspect the most important factors are a solid substrate (this is most important for any slab) and reinforcing the slab.
There is always the factor of manufacturers tailoring their specs for the best impression of whatever they sell. This is my first impression after only one cup of coffee this morning. I will look it over again. I think the average of 20 psi is real and stable.
Residential loads are minimal. If you have a reinforced slab over 4" thick(I believe mesh is adequate), I think the potential for impact on the foam is minimal.
This is really a good question for an engineer. I have a good friend who is a code guy and will run this by him as well.


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## BoilerMan (Apr 29, 2013)

This is interesting indeed, I agree with you Tom about the vapor barrior placement as well, under the foam seems best. I have been cleaning up the yard as the snow has FINALLY MELTED  up here. I found this, it's been on the ground for about two years as far as I can figure. Pink "foamular" Owens Corning 2" thick. These holes are a bit bigger than 1/4" scary as this is what's under my house.............. I have and will continue to spray pesticide 4' out around the entire house every month in the summer.

TS


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## tom in maine (Apr 30, 2013)

So, I will throw out a pet theory on foam underneath slabs:
The Passiv Haus guys and others are using a LOT of foam under slabs. We all have seen or heard the horror stories of critters in foam.
If one built a well drained floating slab with no insulation underneath it, and then insulated on top of the slab, with either sleepers to carry the load to the floor or a tongue and groove deck over top, the insulation is inside the house and not in contact with the soil.
It would be more expensive, but simpler in some ways. It makes the slab install quicker. It would be critical for it to be well drained. It functions a bit more like a crawlspace but is filled with foam.It should not ever move because it is insulated on top and is on a well drained base.

What I think about in the evenings....


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## BoilerMan (Apr 30, 2013)

tom in maine said:


> So, I will throw out a pet theory on foam underneath slabs:
> The Passiv Haus guys and others are using a LOT of foam under slabs. We all have seen or heard the horror stories of critters in foam.
> If one built a well drained floating slab with no insulation underneath it, and then insulated on top of the slab, with either sleepers to carry the load to the floor or a tongue and groove deck over top, the insulation is inside the house and not in contact with the soil.
> It would be more expensive, but simpler in some ways. It makes the slab install quicker. It would be critical for it to be well drained. It functions a bit more like a crawlspace but is filled with foam.It should not ever move because it is insulated on top and is on a well drained base.
> ...


 
I like the idea, but you'd loose all the benifits of the huge thermal mass of the slab, which is one of the reasons I chose to build on a slab.  I've seen it done as you say with PT sleepers and foam in a retrofit living space on an otherwise uninsulated slab.  Like people converting a garage into living space. 

TS


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## Seasoned Oak (Apr 30, 2013)

WHat about using salt under the foam. I know some people use it to drive away termites.most if not all bugs DO NOT thrive in salty soil and actively avoid it. Salt does have a bad effect on concrete ,but if its under the foam it will not come in contact with the concrete.


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## Chris Hoskin (Apr 30, 2013)

and some thoughts from Siggy here: http://www.pmmag.com/articles/91791-mountaintop-experience?v=preview


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## Frozen Canuck (Apr 30, 2013)

tom in maine said:


> So, I will throw out a pet theory on foam underneath slabs:
> The Passiv Haus guys and others are using a LOT of foam under slabs. We all have seen or heard the horror stories of critters in foam.
> If one built a well drained floating slab with no insulation underneath it, and then insulated on top of the slab, with either sleepers to carry the load to the floor or a tongue and groove deck over top, the insulation is inside the house and not in contact with the soil.
> It would be more expensive, but simpler in some ways. It makes the slab install quicker. It would be critical for it to be well drained. It functions a bit more like a crawlspace but is filled with foam.It should not ever move because it is insulated on top and is on a well drained base.
> ...


 
I have seen a variation of this on some of the newer net zero homes being built in Edmonton. No foam under slab. 2" high density foam on top of slab then pregrooved underlay with aluminum radiant plates in most cases but not all, on top of slab. Then tubing in gooves. Then floor covering usually 12mm laminate but other flooring options are doable like tile for instance. So far results are good, next to zero heatloss downwards, tubing is accessable in the event of a need, zoning is easy, accurate & perhaps most important given the life cycle of the building "changeable" for future owners with a different idea. All in all a better way to look at radiant floors IMO. Only real issue is that the buildings are getting so efficient that tubing in some cases does not yield that "warm floor" as the btu requirements of the building are too low. In most cases <10 btu/sq ft. When you think about it this is simply the old tried & true practice of insulating the warm side of a structure rather than the colder underside of the slab in this case. Makes sense, & takes the "bugs" out of the equation. A


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## Frozen Canuck (Apr 30, 2013)

Sorry must have miss clicked. Last sentence was to be that all one has to do as a builder is plan for the increased height of the basement floor, not really an issue with many wanting 9, 10, or 12' ceilings in a modern home.


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## mikefrommaine (Apr 30, 2013)

Frozen Canuck said:


> I have seen a variation of this on some of the newer net zero homes being built in Edmonton. No foam under slab. 2" high density foam on top of slab then pregrooved underlay with aluminum radiant plates in most cases but not all, on top of slab. Then tubing in gooves. Then floor covering usually 12mm laminate but other flooring options are doable like tile for instance. So far results are good, next to zero heatloss downwards, tubing is accessable in the event of a need, zoning is easy, accurate & perhaps most important given the life cycle of the building "changeable" for future owners with a different idea. All in all a better way to look at radiant floors IMO. Only real issue is that the buildings are getting so efficient that tubing in some cases does not yield that "warm floor" as the btu requirements of the building are too low. In most cases <10 btu/sq ft. When you think about it this is simply the old tried & true practice of insulating the warm side of a structure rather than the colder underside of the slab in this case. Makes sense, & takes the "bugs" out of the equation. A


 

Does the foam go down before the walls go up? --- is it under the exterior walls on a slab?  Or is this just for basements.


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## Frozen Canuck (Apr 30, 2013)

Framed as per normal Mike. Yes just for basement floors, thats +95% of market here (full basement), never seen foam under a load bearing wall, no reason though to not put it under partition walls in new construction, non load bearing of course & that's what I see in the newer net zero homes, total isolation of the living space from the concrete floor (conductor). In areas where a basement is not practical one could do this on a floor over a crawl space for example, or for a slab on grade. I for one have never got the flywheel thing. If I had a flywheel that was losing 10-20% of it's energy to the ground I would fire it for non performance issues.


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## tom in maine (Apr 30, 2013)

We have done several floors with foam on top of the slab.
We (and some others) have grooved polystyrene foams and put tubing in the grooves. I had made a heated "groover" with some cheap wide soldering irons. It worked well and was quite smelly, if you have never heat cut EPS, it is very smelly! I was nervous about higher temps affecting the foam, and melting into the foam. I never did a post mortem since it is still in service. Nothing happened that seemed to be really bad.

I did another odd install that was installed at the University of Maine in a heating project about 20 years ago.
This one used polyiso foam like Tuff R, which was foam without fiberglass in it. Polyiso like Thermax used to have fiberglass embedded in it and would not work.
This gets weird, but it did work.
I used foam that was 2" thick, took 1/2" polybutylene tubing (PEX might work), taped it to the foil facer with 3" wide foil tape and beat the tubing into the foam with a rubber mallet until it was flush with the foam surface.
We then covered it with a product like cement board (this was before cement board was readily available.
The board was glued to the foam and then a second layer was installed over the first, with the joints offset.
It worked well and did heat reasonably well.
It is still in service. Dick Hill had us do it. It was tedious.
This was in a house that we did a lot of heating system tests called the "Parallel Systems House".
The guy who built it claimed we were trying to kill him.
He was a real character who was the most untechnical person I ever met. He was an English professor who taught Stephen King.
Although he was a complete Luddite, he wanted us to do this project and funded it. (He was so opposite Dick and I, he could not figure out how to plug in a telephone!)


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## Frozen Canuck (Apr 30, 2013)

Sounds like fun Mike.

In future you can use a hotknife for grooving foam in a situation like that, we use them when we do a SIP home for wire grooves/elec boxes etc, which I am happy to say the younger generation is far more interested in than mine. It's hard to express just how impressed I am when a young 20 something couple comes forward & says we want to build a very energy efficient home to start our family in - how do we best do that? They will likely be close to a million dollars ahead of my generation just on the money saved in utilities by the time they reach my age. Spend it upfront then bank it for 50+ years. Not like my generation at all.

The idea of more (thicker) high density foam has been kicked around by a few of us old dogs & then just like you said flooring right over the foam. We know we can achieve no measurable downward loss, the only kicker has been at high impact zones such as the bottom of the stairs, that I am sure we can work around. 

Sounds like an interesting project with folks you enjoyed Mike.


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## arbutus (Apr 30, 2013)

Frozen Canuck, thank you for posting that.
I am planning to retrofit my new to me home with slab insulation in a similar fashion.


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## Frozen Canuck (Apr 30, 2013)

No problem, hope it helps. The only consideration on a retrofit is the uneven tread height on the last step of the stairs due to increased floor thickness in the basement. If you have new stairs as part of that retrofit then problem solved.


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## BoilerMan (Apr 30, 2013)

Frozen Canuck said:


> I for one have never got the flywheel thing. If I had a flywheel that was losing 10-20% of it's energy to the ground I would fire it for non performance issues.


 
Isn't storage kinda like the "flywheel" like a slab?  I know you can't really regulate the emmisivity of a slab like you can dray or not draw from storage, but the concept is simmilar.  I build one fire a day, the air temperature changes very little.  Seems like I have some type of storage going on.  I wonder what the real loss factor of a slab is with R10 foam under it.  I'd think the 10-20% may be high.  The critter issue is a good case for a floating slab inside and isolated from frost walls poured around the perimiter as load bearing walls.  I wonder just how deep sub-terrainan bugs go, that is at what depth would foam be considered inherrantly protected if there is such a thing.

TS


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## arbutus (Apr 30, 2013)

Frozen Canuck said:


> No problem, hope it helps. The only consideration on a retrofit is the uneven tread height on the last step of the stairs due to increased floor thickness in the basement. If you have new stairs as part of that retrofit then problem solved.​


 Haha!

New stairs are in the works, though for a different issue!


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## Frozen Canuck (Apr 30, 2013)

BM. 10-20% would be for slabs in direct contact with soil. Probably a conservative number & yes plenty of other variables as well, clay/sand/gravel, water table height & on & on.

Up here the local water well driller says down to 10 feet he finds ants in granular soils so it's probably safe to consider exterior foam as bug food/attractant in many areas, no termites here thankfully (yet).

Most homes burn fossil rather than wood so buffer tank of say 40 gallons to shield the boiler from the direct loads, but yes I get your analogy to storage in a cordwood system, way easier to measure that loss accurately though & control/limit it.


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## tom in maine (May 1, 2013)

Interesting thread. I keep remembering things. Guess I am getting old...
The groover I made did about three feet wide with tubing slots every 6". The grooves were for 1/2" tubing. It went very quickly.
The other thing was another floor which was all foam. It was basically a structural panel that we did with pressure treated lumber. There was PT on the bottom, a core of foam and then plywood on the topside. It was glued and screwed together. It was basically a panel on its side. To try to mitigate the ant issue, I wrapped the slab in .060" EPDM underneath and up the sides of the slab. It was then covered on the sides with PT plywood. I used tapered foam since the site was not level and I was too lazy to grade it.
It was a 12'x12' building and it had never moved.
No heat in the building, but it never freezes.

The advantage of the foam floor is although it is low mass, it responds very quickly.
That being said, a concrete slab has a significant heat storage ability.


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## Frozen Canuck (May 1, 2013)

For sure Tom, I always enjoy threads of this sort.

My grandkids should be able to live in what we now consider McMansions +4000 sq/ft & heat the thing with a little 20K btu boiler in the future. We are already close, <10 btu sq/ft is acheivable by many here now & those of us who are really working on it are looking 5 btu sq/ft square in the face now. 1 advance in window tech & we have it. As that is the largest loss.

For them (grandkids) choosing a very high quality heating appliance will be important as with these low consumption/run times the unit is quite likely to last 3 times as long simply because it sits & waits for a call for heat for so much of it's life.

Heck when we can get it down to 5 btu sq/ft here I think the heating industry (may) just switch over to a roof mount solar HW heating system & the fossil boiler will just be there for backup. Most homes have enough roof space to heat a home that only needs those numbers.

Time will tell but we are close already.


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## BoilerMan (May 1, 2013)

FC, I enjoy these type of threads very much too!  One of the good things the internet has brought us, collective minds and collective discussions from people from all over the world.  I'm at 14btu/ft at -45F here, and I bu no means did a whole lot unconventional.  Tried and true tech (DOW Tuff-R) and strapping over fiberglass (would use Roxul now).  It kills me to hear of people burning 1,000+ gal of fuel and having to make payments year-round to the oil man.  Ah well, great thread!

TS


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## Frozen Canuck (May 1, 2013)

For sure BM. I have always believed that adding insulation, even poorly done had a better ROI than any heating system. Felt like a voice in the wilderness through the 60's to 90's, not so much anymore. Now lots of folks are getting REAL about saving energy & money. That's good keeps old dogs like me on top of our toes so to speak. Current with new tech.


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## Floydian (May 3, 2013)

This was just posted today-seems relevant to some of the discussion in this thread.

http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-thermal-mass

I am sure some lively debate will follow in the coming days in the "comments" at the bottom of the page. I am curious how the Passivhaus folks will respond as super(duper) insulating+solar gain+thermal mass(how much?) is a important part of achieving the ridiculously low Passivhaus standard of 4.75 kbtu/sq ft/year.

As far as low heat loads,

Here http://www.greenbuildingadvisor.com/blogs/dept/green-building-news/striving-passivhaus-affordability is an example of a Passivhaus that beats the standard by a margin. At 3.1 kbtu/sq ft/yr (heating) for a 1600 sq ft house, that is a whopping 5 MM btus/yr in a 7300 HDD climate. Simple heating systems, very low energy demand for the life of the building and comfort is a given.
Honestly I don't care about some certification, I care about performance and this is about as good as it gets. Now for someone building their own house and committed to burning wood, I think building to Passivhaus standard is overkill. If someone is hiring out the building of a new house and is not interested in the commitment to burning wood, Passivhaus or near passivhaus starts to make a lot of sense.

I really like the "Pretty Good House" approach: http://www.greenbuildingadvisor.com/blogs/dept/guest-blogs/pretty-good-house

Noah


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## heaterman (May 4, 2013)

At the energy conference in Austria there were a number of manufacturers with boilers (pellet only) that fired well below 10,000btu. Can you imagine buying only a ton or two for an entire years worth of heat and hot water?

Getting back to the original posters question.........I've used both with good results. The density of the product is the main concern as far as holding up goes. Lately we've been buying an EPS product that has a poly type scrim on one side and an aluminum scrim on the other. Very resistant to moisture because the only area exposed is the edge of the panel. I can even order it with bug killer impressed right in the foam.


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