# Insulating the inside of concrete basement walls?



## Knots (Mar 13, 2013)

Hi - I've lurked here plenty (and learned a lot), but now that I'm building a house around a wood stove I need to ask some questions.

The question: The builder seemed reluctant to put the foam board insulation on the outside of the foundation and instead wants to put 3" Thermax foam board on the inside.  Does anyone have experience with this?  I worry about mold/condensation between the foam and the concrete.

House/stove details:  1400 sq ft ranch with walk-out front.  House will have 2x8 walls with blow-in icynene insulation.  Lotsa insulation in the attic.  Chimney roughly in the center of the house with the stairwell nearby.  Jotul F55 Carrabassett for a stove.

I'm not even close to starting the house yet and I bought the stove already!


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## begreen (Mar 13, 2013)

Hi and welcome. Sounds like you are correct. If the foundation is raw, I'd like it to be exterior insulated. But it would be good to understand his concerns to see if they are valid and addressable. It could just be he is more familiar with one method and wants to provide you a product he knows how to do well.

What is being done to eliminate thermal bridging on the studs? Will this be a staggered 2x4 wall equaling 8" thick or will there be an insulation wrap on the building? If yes, I think the stove may be overkill. You might want to rethink it to something that can run cleaner at lower output like a small Woodstock cat stove. Or is 1400 sq ft just the main floor and with basement is it 2800 sq ft?

Get the heatload calcs done on the house soon. You might be surprised how little heat it takes to heat a small, super-insulated home.


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## Knots (Mar 13, 2013)

Thanks.  Yeah - I just have visions of weird stuff happening at that interface, especially in the summer.


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## begreen (Mar 13, 2013)

A good source of information on these kind of questions is www.buildingscience.com

http://www.buildingscience.com/search?SearchableText=basement+insulation


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## Knots (Mar 13, 2013)

Doh!  My first thread and I put it in the wrong forum  Thanks for moving it.

I will dig through that site this weekend.


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## begreen (Mar 13, 2013)

LOL, common mistake. Tell us more about the house. Can you answer some of my second part questions about the construction and total heated area size?


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## Knots (Mar 13, 2013)

The first floor will be 1400 sq ft.  The stove will be roughly in the middle of the unfinished basement.  It will also be heating the 26x28 two-bay under garage.  And, when needed I will be sending some of the heat to the 28x28 garage hanging off the end.

The thermal bridging will be mitigated by 3/4" of foam between the foil-backed drywall and the 2x8 studs.  Lemme see if I can find a pic.


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## Knots (Mar 13, 2013)




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## save$ (Mar 13, 2013)

My brother had foam insulation put under the cement floor and on all the exterior of the cement walls.  It was coated and sealed.  There is an apartment in the basement and never cold or damp.   Very easy to heat.


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## Knots (Mar 13, 2013)

save$ said:


> My brother had foam insulation put under the cement floor and on all the exterior of the cement walls.  It was coated and sealed.  There is an apartment in the basement and never cold or damp.   Very easy to heat.


Nice.  When you say "coated and sealed" do you mean that the foam was glued to the concrete or something more than that?


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## begreen (Mar 13, 2013)

For sure keep us posted on how it works out. Don't insulate the basement ceiling and consider a couple key returns to the basement near the outside walls. Cooler air will sink down these returns to replace the warm air heading up the stair case. If you can make that stairwell open and with an oversized door at the top, go for it.


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## Coog (Mar 13, 2013)

begreen said:


> A good source of information on these kind of questions is www.buildingscience.com
> 
> http://www.buildingscience.com/search?SearchableText=basement+insulation



I second building science.  When I am not reading posts here at hearth.com I actually hold a day job and have consulted with them professionally.  Very knowledgable but very expensive.  They have a lot of free resources  as well.  May want to tap into that.  

I gotta ask,why 2 x 8 walls? I built my house 6 years ago and my exterior walls are 2 x 6.  I ran calcs comparing 2 x 4 walls with 2 x 6 walls and figured there would be a 20 year payback at higher energy costs.  I have never compared 2 x 8 walls but I gotta wonder if there is a whole lot to gain.  Just a thought.


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## velvetfoot (Mar 13, 2013)

I put 4" of foam board on the inside walls of my basement and I think it's drier in summer.  Thinking about some foam on the floor, with something or other on top.


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## save$ (Mar 13, 2013)

Knots said:


> Nice.  When you say "coated and sealed" do you mean that the foam was glued to the concrete or something more than that?


Yes it was coated with some sort of black stuff below grade and something gray stuff above grade.  Foam left unprotected from daylight will degrade.  I think the foam is a couple in. thick and tongue and groove.   My parents lived in that apartment for 30 years without problems. The house was built into the side of a hill so the top level is street level then you drive down around back and apartment is walk out ground level.  The attached garage was also set up te same way so there were cars parked upstairs and down.   Both living areas are mostly heated with two pellet stoves.


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## woodgeek (Mar 14, 2013)

Lots of folks have done foam on the inside. The trick is not having wood members in contact with the cold/wet concrete. The Thermax is fire rated such that it doesn't need to be drywalled over, which is presumably why he chose it. I wouldn't want bare foil/foam walls in my basement....they will look crappy fast.  E.g. one of your kid's little friends will visit and punch it full of fist holes for fun.

I think I would want a vapor permeable board (no foil liners) like EPS or XPS so it could 'dry inward'. But then you need to drywall over for the fire code. If you ever want to finish the basement walls (e.g. yourself after construction to save $$) give a thought to how you will hang wall boards. I guess you could tapcon wood furring to the face of the foam board and hang the wall on that.

I blather. Lots of options on the inside and pretty conventional, neither concrete nor foam nor tapcons molder.

Just don't forget to do under the slab. at least a couple inches.


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## peakbagger (Mar 14, 2013)

I dont recomend foam on the outside as carpenter ants love to nest in it. If its outside any exposed foam is covered with a stucco coating but that doesnt stop the ants. Once they get in the foam, they will tunnel just under the surface and inevitably they will find a route into the house. I have 1" of foam on the outside that runs from ground line to the footings on all foru walls and they havent gotten into it but on the two sides I ran up the base of the siding, I wish I had never done it. I ended up cutting a 2" wide gap on top below the siding and after a few years of hitting it with pesticides, I got the buggers out of the foam. One the other two walls which are exposed concrete above ground line, I glued foam boards from the sill down 4 feet on the inside. There is still a thermal bridge via the concrete but it did make a big difference. Spray foam wasnt availlable in my area when I did it but I defintiely would have the interior sprayed next time.


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## velvetfoot (Mar 14, 2013)

woodgeek said:


> I guess you could tapcon wood furring to the face of the foam board and hang the wall on that.


That's what I did, only I routed a groove so it would be flush and then put another 2" layer over that.  The drywall, attached by screws to the furring strips, is holding well.  Whether it was cheaper than building a wall offset from the concrete and spraying, I don't know.  You can run into my drywall and bounce off though.


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## shoot-straight (Mar 14, 2013)

i would do a really good job with water proofing the outside with a membtrane, make sure i have good drainage as well. then stud the off the wall at least and inch and then sprayfoam behind.


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## woodgeek (Mar 14, 2013)

velvetfoot said:


> That's what I did, only I routed a groove so it would be flush and then put another 2" layer over that. The drywall, attached by screws to the furring strips, is holding well. Whether it was cheaper than building a wall offset from the concrete and spraying, I don't know. You can run into my drywall and bounce off though.


 
Me too. They make 1.5" XPS that's precut: http://www.homedepot.com/p/t/100320...&productId=100320293&R=100320293#.UUHbmxnzI7A

I opted a little different (on a small area). I put up 1" continuous and taped XPS held in place by 1"x1" furring (over) with tapcons, and put a second layer of 1" XPS in the spaces between the furring. And then drywall into the furring. On a larger area, I would use the precut stuff, but I wanted 2" for some reason.

FYI, XPS uses a HCFC blowing agent so it has a large global warming footprint....they are working on replacement blowers, but for now XPS is not green.


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## 5 Furnace (Mar 14, 2013)

Hi,

I am an architect and a Certified Passive House Consultant.  You are absolutely right to install insulation on the exterior of the foundation walls, under the ENTIRE slab, and ideally across the top of the footing before the block wall is laid up.  You will find most builders are not into this, because this is way out of the normal for them.  But you will find lots of helpful reference for such details at the Building Science Corporation's website: http://www.buildingscience.com/index_html

Getting the insulation on the outside is a no-brainer on new construction- the reason everyone insulates on the inside is that either they are doing so to existing basements, or they don't know any better.  Once you backfill it is much more difficult to go back and do it over again, so get it right the first time.  For those reasons get the best insulation, best vapor barrier, and best waterproofing you can find, and have them installed meticulously.  Also check that sub-slab ventilation is not needed in your area- this would mostly concern radon or VOCs in the ground- such as from soil contamination.

The reason why you want to insulate any usable space is that the dew point happens around 54 degrees with air heated and humidified to room temperature and active use.  That is also right about the average soil temperature- so it should be no surprise to anyone that basements are wet.

Insulation UNDER the block wall?  Yes- Foamglas by Dow Corning is a load-bearing insulation board that you can use either under the block/on top of the footing, or under the spread footing.  Good luck convincing your building inspector and your concrete guy- but point out to them that it has a greater bearing capacity than the soil you are bearing on.

Cheers!


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## Swedishchef (Mar 14, 2013)

I agree with the above comments. Insulating the outside is the best option. However when I built 4 years ago I was not aware of this... (I learned lots since then).

Instead what I ended up doing in spraying 1.5 inches of closed cell, high density spray foam on the concrete on the inside and then placing R14 roxul between the studs that butt up against the insulation. I even sprayed in the rim joists to create a very air-tight seal.

The spray foam creates enough of a thermal barrier to prevent condensation on the inside (same concept of a cooler in the summer...how often do you see condensation on the outside when there's ice on the inside??). Works great for me.

Andrew


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## velvetfoot (Mar 14, 2013)

woodgeek said:


> FYI, XPS uses a HCFC blowing agent so it has a large global warming footprint....they are working on replacement blowers, but for now XPS is not green.



I'm not sure that's true anymore.  I recall some printing on the later panels I installed about a new formulation and I copied this from the Owens Corning website:
• Zero ozone depletion potential
with 70% less global warming
potential than our previous
formula.


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## Knots (Mar 14, 2013)

Coog said:


> I gotta ask,why 2 x 8 walls? I built my house 6 years ago and my exterior walls are 2 x 6. I ran calcs comparing 2 x 4 walls with 2 x 6 walls and figured there would be a 20 year payback at higher energy costs. I have never compared 2 x 8 walls but I gotta wonder if there is a whole lot to gain. Just a thought.


 
My best guess is that I will be in this house for at least 25 years. I think it will pay in that amount of time.


Wow!  Lots of good info here!  It looks like I will be doing a lot of reading this weekend!


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## woodgeek (Mar 14, 2013)

Knots said:


> My best guess is that I will be in this house for at least 25 years. I think it will pay in that amount of time.


 
Why not use a proven high eff design and not guess? My concern is that 2x8 framing still gives disappointing assembly R-values due to thermal bridging if you don't put continuous foam on the outside. If you do put continuous foam on the outside, you could drop to 2x6 and put the money you saved toward making the foam on the outside thicker. IOW, 2x6 with FG or cellulose + foam sheets on the outside will be cheaper than 2x8 with cavity foam and perform much better.

A lot of people have done this optimization....the reason 2x8 with cavity foam is not done is because there are better, cheaper designs.


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## Knots (Mar 14, 2013)

In the picture above it shows the continuous foam on the inside that allows the icynene to be between the drywall and the studs.


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## woodgeek (Mar 14, 2013)

Sorry...missed that.  Is that stackup really cheaper than 2-4" recycled polyiso on the outside with 2x6 studs and cellulose?  If you weren't in a monosyllabic state I would give you a hard time about the foil backed drywall (a vapor barrier).  In Maine, I will stay mum.


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## Swedishchef (Mar 14, 2013)

woodgeek said:


> Sorry...missed that. Is that stackup really cheaper than 2-4" recycled polyiso on the outside with 2x6 studs and cellulose? If you weren't in a monosyllabic state I would give you a hard time about the foil backed drywall (a vapor barrier). In Maine, I will stay mum.


 lol...what is wrong with monosyllabic states? I gotta admit, I have never seen drywall with foil...


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## Coog (Mar 14, 2013)

woodgeek said:


> Why not use a proven high eff design and not guess? My concern is that 2x8 framing still gives disappointing assembly R-values due to thermal bridging if you don't put continuous foam on the outside. If you do put continuous foam on the outside, you could drop to 2x6 and put the money you saved toward making the foam on the outside thicker. IOW, 2x6 with FG or cellulose + foam sheets on the outside will be cheaper than 2x8 with cavity foam and perform much better.
> 
> A lot of people have done this optimization....the reason 2x8 with cavity foam is not done is because there are better, cheaper designs.



I second foam on the outside wall continuous.  Much thermal loss is through the stud.  Not as bad a steel studs but still.  Foam on the outside provides a nice thermal break.  

Of course the other major thermal loss is through the windows.  You may spend as much attention to the insulation of your windows as you do the walls.  Add brick as exterior facade and you will add some thermal mass to the structure as well.  Now we are talking.


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## jdp1152 (Mar 15, 2013)

Too late to consider ICFs?


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## Knots (Mar 15, 2013)

I was unaware of the effect of monosyllabism on insulation...


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## velvetfoot (Mar 15, 2013)

With the exterior you have to protect the foam from damage while making it look good as well as be concerned about carpenter ants.  I can definitley see the foam under the concrete floor slab because then you wouldn't have put something on the concrete like I might; it'll be strong enough so can you drive cars on it and stuff, right? 

I've noticed less stinkiness in the basement since I put the 4" of foam on the walls.

Are you sure about the stove heat setup?  I'm sitting here by the insert and it got toasty quick after putting in some splits, nice looking flames, etc...value added, not totally just heat.  It might take a while with the basement setup and you have to have grates, keep the basement door open etc.  Are you going to be down in the basement a lot?  Conserving wood, and dollars and time, can't be a bad thing.What kind of backup heat will you be using?  Maybe you can tie something into that, but the indoor gasifier route seems expensive, especially for a not too big house.


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## SmokeyTheBear (Mar 15, 2013)

woodgeek said:


> Sorry...missed that. Is that stackup really cheaper than 2-4" recycled polyiso on the outside with 2x6 studs and cellulose? If you weren't in a monosyllabic state I would give you a hard time about the foil backed drywall (a vapor barrier). In Maine, I will stay mum.


 
Sort of like PA say??


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## Knots (Mar 15, 2013)

velvetfoot said:


> With the exterior you have to protect the foam from damage while making it look good as well as be concerned about carpenter ants. I can definitley see the foam under the concrete floor slab because then you wouldn't have put something on the concrete like I might; it'll be strong enough so can you drive cars on it and stuff, right?
> 
> I've noticed less stinkiness in the basement since I put the 4" of foam on the walls.
> 
> Are you sure about the stove heat setup? I'm sitting here by the insert and it got toasty quick after putting in some splits, nice looking flames, etc...value added, not totally just heat. It might take a while with the basement setup and you have to have grates, keep the basement door open etc. Are you going to be down in the basement a lot? Conserving wood, and dollars and time, can't be a bad thing.What kind of backup heat will you be using? Maybe you can tie something into that, but the indoor gasifier route seems expensive, especially for a not too big house.


 
It seems like an equal number of pluses and minuses to putting the foam on the inside or outside.  I'm definitely going to pursue putting foam under the slabs.

When you say "gasifier" do you mean a wood stove with secondaries?  I'm not familiar with the term.

I will be in the basement a lot and it will be heating the two bays under some of time.

Back-up heat will be LP.


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## velvetfoot (Mar 15, 2013)

Knots said:


> It seems like an equal number of pluses and minuses to putting the foam on the inside or outside.  I'm definitely going to pursue putting foam under the slabs.
> 
> When you say "gasifier" do you mean a wood stove with secondaries?  I'm not familiar with the term.
> 
> ...



I meant like a boiler or furnace that uses gasification technology (see the boiler forum).   I was just saying that you'll probably have to have it cranking in the basement to heat the upstairs, and you could tie something in to your central LP fired backup.  That game could get expensive though.


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## Knots (Mar 15, 2013)

velvetfoot said:


> I meant like a boiler or furnace that uses gasification technology (see the boiler forum). I was just saying that you'll probably have to have it cranking in the basement to heat the upstairs, and you could tie something in to your central LP fired backup. That game could get expensive though.


 
 I'm going with a Jotul F55 in the basement.  Water will be on-demand LP.  Back-up heat will be direct vent wall units - one in the basement - one upstairs.


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## velvetfoot (Mar 15, 2013)

Sounds great!


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## peakbagger (Mar 15, 2013)

For some illustration of why foam on the outside is not a great idea in rural Maine


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## Knots (Mar 15, 2013)

peakbagger said:


> For some illustration of why foam on the outside is not a great idea in rural Maine


 
Whoa!  Can you tell me any more about that?


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## Swedishchef (Mar 15, 2013)

Looks like ants to me..


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## Knots (Mar 16, 2013)

Swedishchef said:


> Looks like ants to me..


 
Yes - but was it completely below grade?  What brand of foam was it?  How long was it installed?

This really makes me reconsider having it on the outside.


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## sheetmetaldan (Mar 16, 2013)

Knots said:


> Yes - but was it completely below grade? What brand of foam was it? How long was it installed?
> 
> This really makes me reconsider having it on the outside.


 
I just found this thread lots of very good informative posts. I would not do the foam on the outside for fear of Carpenter ants. I added a 25`X23` ft addittion to my home a few years ago and did not use any foam on exterior of my foundation just tar painted on concrete for water protection. I have seen some high end homes use a black plastic material like a bubble wrap with a black felt cover, you would place the bubble side against the foundation (foundation is painted with tar or similar material) and felt side facing soil water passes through and runs down to french drains keeps basement dry inside. I wish I knew the name of this stuff maybe others will know what I am talking about and chime in. The french drains definatley keep my basement dry I highly recomend having them!

 One day I will finish my basement. I am thinking of using metal studs with a 1 inch gap off the foundation wall and putting astro foil between stud wall and foundation or sprayfoam between studs and foundation, not sure which way to go yet. The floor I am thinking of using 2" high density interlocking foam and a floating type finish laminate floor on top of that.


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## peakbagger (Mar 16, 2013)

For those who wonder, that shot was taken from some foam above grade in a dry location, actually inside the house. One end was butted up tight to a nail hole running through a rim joist to the outdoors. The hole lined up with some of the more serious tunnelling on the exterior of the house. The exterior foam on my walls has a paint on stucco so the tunnels are not as obvious from the outside. How I originally discovered they were in the above grade foam with the coating on it was just a few small holes with some frass usually on edges where the stuuco was thin or nonexistent (sawdust and ant waste) falling out of it. I had been having yearly infestations up on my second floor in some foam and couldnt figure out how they were getting in. One day I took a knife and cut out the area around one of the holes and found the tunneling.

I am not the only one, a friend in Millinocket had Iso board foam on the exterior of his house under the siding. He had serious annual ant issues until he brought in a pro who tracked the ants. They ripped off the vinyl siding and found one whole wall of foam tunneled out and loaded with ants. He doesnt think it was moist.

This is contrary to the advise that with wood, if you see carpenter ants they are just following wet/rotten  wood. I have heard references to bug  proof foam but have yet to find any boards or contractors who spray it. I expect Borate might work but dont know how it would affect the foam as it usually has a polyethylen glycol carrier. Great Stuff canned foam advertises a bug proof foam but I have never seen it at a store.


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## sheetmetaldan (Mar 16, 2013)

peakbagger said:


> For those who wonder, that shot was taken from some foam above grade in a dry location, actually inside the house. One end was butted up tight to a nail hole running through a rim joist to the outdoors. The hole lined up with some of the more serious tunnelling on the exterior of the house. The exterior foam on my walls has a paint on stucco so the tunnels are not as obvious from the outside. How I originally discovered they were in the above grade foam with the coating on it was just a few small holes with some frass usually on edges where the stuuco was thin or nonexistent
> (sawdust and ant waste) falling out of it. I had been having yearly infestations up on my second floor in some foam and couldnt figure out how they were getting in. One day I took a knife and cut out the area around one of the holes and found the tunneling.
> 
> I am not the only one, a friend in Millinocket had Iso board foam on the exterior of his house under the siding. He had serious annual ant issues until he brought in a pro who tracked the ants. They ripped off the vinyl siding and found one whole wall of foam tunneled out and loaded with ants. He doesnt think it was moist.
> ...


 

I think termites can do this type of damage as well! I think I recall my father telling me stories of this happening to places in Florida


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## Knots (Mar 16, 2013)

peakbagger said:


> For those who wonder, that shot was taken from some foam above grade in a dry location, actually inside the house. One end was butted up tight to a nail hole running through a rim joist to the outdoors. The hole lined up with some of the more serious tunnelling on the exterior of the house. The exterior foam on my walls has a paint on stucco so the tunnels are not as obvious from the outside. How I originally discovered they were in the above grade foam with the coating on it was just a few small holes with some frass usually on edges where the stuuco was thin or nonexistent (sawdust and ant waste) falling out of it. I had been having yearly infestations up on my second floor in some foam and couldnt figure out how they were getting in. One day I took a knife and cut out the area around one of the holes and found the tunneling.
> 
> I am not the only one, a friend in Millinocket had Iso board foam on the exterior of his house under the siding. He had serious annual ant issues until he brought in a pro who tracked the ants. They ripped off the vinyl siding and found one whole wall of foam tunneled out and loaded with ants. He doesnt think it was moist.
> 
> This is contrary to the advise that with wood, if you see carpenter ants they are just following wet/rotten wood. I have heard references to bug proof foam but have yet to find any boards or contractors who spray it. I expect Borate might work but dont know how it would affect the foam as it usually has a polyethylen glycol carrier. Great Stuff canned foam advertises a bug proof foam but I have never seen it at a store.


 
Yikes!  I know a guy who has continuous foam from his footings all the way up the side of the house.  I'm going to do him a favor and NOT show him that picture.  Ignorance, bliss, and all that...


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## jdp1152 (Mar 16, 2013)

Thanks for posting this.  I've been going going back and forth on insulation upgrades.  We went ahead and spray foamed our three attics, garage ceiling, and rim joist.  The results have been outstanding, so I've been looking into what I want to do with walls.  Rebuilding the window boxes and using exterior foam boards was the direction I was leaning, but given the amount of carpenter ants we have in this area, I think I'll reconsider and caulk the heck out of all my board seams when I reside the house this spring.

I imagine their infestation on those boards was largely just hunkering down for winter and not seeking out a food source.



peakbagger said:


> For those who wonder, that shot was taken from some foam above grade in a dry location, actually inside the house. One end was butted up tight to a nail hole running through a rim joist to the outdoors. The hole lined up with some of the more serious tunnelling on the exterior of the house. The exterior foam on my walls has a paint on stucco so the tunnels are not as obvious from the outside. How I originally discovered they were in the above grade foam with the coating on it was just a few small holes with some frass usually on edges where the stuuco was thin or nonexistent (sawdust and ant waste) falling out of it. I had been having yearly infestations up on my second floor in some foam and couldnt figure out how they were getting in. One day I took a knife and cut out the area around one of the holes and found the tunneling.
> 
> I am not the only one, a friend in Millinocket had Iso board foam on the exterior of his house under the siding. He had serious annual ant issues until he brought in a pro who tracked the ants. They ripped off the vinyl siding and found one whole wall of foam tunneled out and loaded with ants. He doesnt think it was moist.
> 
> This is contrary to the advise that with wood, if you see carpenter ants they are just following wet/rotten wood. I have heard references to bug proof foam but have yet to find any boards or contractors who spray it. I expect Borate might work but dont know how it would affect the foam as it usually has a polyethylen glycol carrier. Great Stuff canned foam advertises a bug proof foam but I have never seen it at a store.


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