Insulation calculations

[Ashful]

Interesting...

I can't tape to the joists, because they're rough sawn. I was just going to pneumatic staple them to the furring strips and let the spray foam be the vapor barrier. I do have concerns about interior humidity reaching the underside of that cold metal roof, which is part of what steered me away from cellulose and toward closed cell foam.

Light bill? December is a big month for us... too many outside lights.

 

[woodgeek]

I think the spray foam plan will make a robust assy.

 

[semipro]

Just tossing out another idea (which I hate to do because I irritate myself 2nd guessing plans )
This would be easy to install, would act as vapor retarder/barrier, allow plenty of roof ventilation, and address thermal bridging issues. The foam been the rafters could be omitted if a lesser R value was okay.
In general I'm a big fan of cellulose but not where water is more likely to enter (like roofs).

Edit: foam panels with OSB already attached can also be purchased.

 

[Ashful]

I like that, semipro, and have actually been looking at doing a foam layer under rafters, but headroom is already a little tight. I don't think I want to spare the room for it. Also, was looking at maybe recessing ceiling (would have to use drywall instead of OSB) between rafters. This plays well with spray foam.

 

[Ashful]

Just coming back to this, after being away doing other things awhile. I think I went about this entire thing wrong. I came here asking questions to make my chosen (spray foam) method of insulating work. I should have asked how one might best insulate this structure, as spray foam may not be the best course.

So, let me lay it out there... this is an 1850'ish timberframe barn, photos above, with new-work second floor and interior framed walls, new doors and windows, etc. Roof is raised seam metal (agricultural screw-down type) on 1x3 purlins, on 2x6 rough joists. New 2x4 framing within old structure, with framing for gable end walls (structural) set in several inches from sheathing, creating wall cavity depth of 6" - 10". New 2x4 framing on front and rear broad walls is against sheathing (not structural), and notched around all old timber knee braces, nailers, etc.

New windows and doors to be installed, followed by house wrap (TBD), then furring and wood siding. Had considered just using fiberglass batts for insulation, but concerns with condensation due to cold metal roof and air infiltration (sheathing will be old and leaky 2x6, not plywood) pushed me toward spray foam. I think this is a good choice, but am admittedly not the most educated on these matters. One thing I do dislike about spray foam is its permanence, in that this is an old structure, and some future owner 100 years from now may not appreciate having 2014 spray foam stuck to their 1850's timberframe.

How would you insulate this structure?

 

[woodgeek]

Or the 2014 sprayfoam may have crumbled to dust in 30-40 years.

Missing info...do you intend to heat this structure 24/7, so it makes sense to give it house levels of insulation, or do you want a shop you can heat quickly from time to time, leading to a low thermal mass, lower R value design?

 

[mellow]

Sounds like a plan....for cheap foam board, you can buy 'used' from: http://www.insulationdepot.com/ and save money and be 'green'.

That outfit doesn't do small lots, usually trucks a large shipment, but it sounds like you want a lot of foam board, and don't care if its a little scuffed.

Just a heads up I tried getting a quote from them for 2000 sq ft of board, the guy got my info and said they had a site in Western MD they were pulling board out of and could bring it to me, never heard back from him, he sounded like they only want HUGE industrial jobs. Just an FYI.

 

[Ashful]

Thanks, Mellow. After talking this over with two friends who both work for builders, they're recommending I just stay simple and go with fiberglass bats. One recommended styrene baffles in the rafter spans behind the bats, and the other liked the idea of cutting foam board to go in behind the bats.

On a more interesting note, I did some calculations on loss due to garage doors of various R-value. I'll post that info tonight. I was surprised how little the difference was between R9 and R16. In fact, it's almost irrelevant in terms of heating cost, until you go real low, like < R4. This is of course based only on ~140 sq.ft. of door, not the entire barn.

 

[woodgeek]

Yar. Your insulation cost goes like R-value, and your savings go like the reciprocal...1/R-value (which is U-value)

16 is a lot more than 9 that is more than 1 (R-vaules), but 1/16 is not very different than 1/9 when compared to 1 (U-value) .

Same as going to a 30 mpg car from a 20 mpg car saves a lot less gas than going from a 10 mpg car to a 20.

 

[woodgeek]

Ok, nothing wrong with fiberglass. The FG haters might talk about convection induced R-value drop at low temps. In our climate with FG batts (versus Wisconsin with loosefill FG) NOT a problem. Without a baffle there might be some wind-washing loss of R-value...don't know if its worth the effort/cost of doing prefab or DIY baffles.

I would insist, however, on a careful air-barrier. Stapled FG Kraft paper does NOT count. IF however, you taped all the Kraft paper seams (including the staples and any tears during install) and then OSB'ed under I suppose it would be ok.

Do they make tongue and groove edged OSB to keep the seams lined up?

How deep are your joists again?

One tip....sometimes different thickness batts are the same price, and compressing the 'thicker' batt (to a higher final FG density) does yield slightly higher R-value per inch, and is less likely to collapse (in my imagination) than a ultra-low density product. I've done that myself.

 

[Ashful]

Ok, nothing wrong with fiberglass. The FG haters might talk about convection induced R-value drop at low temps. In our climate with FG batts (versus Wisconsin with loosefill FG) NOT a problem. Without a baffle there might be some wind-washing loss of R-value...don't know if its worth the effort/cost of doing prefab or DIY baffles.

I would insist, however, on a careful air-barrier. Stapled FG Kraft paper does NOT count. IF however, you taped all the Kraft paper seams (including the staples and any tears during install) and then OSB'ed under I suppose it would be ok.

Do they make tongue and groove edged OSB to keep the seams lined up?

How deep are your joists again?

One tip....sometimes different thickness batts are the same price, and compressing the 'thicker' batt (to a higher final FG density) does yield slightly higher R-value per inch, and is less likely to collapse (in my imagination) than a ultra-low density product. I've done that myself.

I do remember you mentioning compressed batts before. Definitely worth consideration, in a vaulted ceiling install.

Joists are full 2 x 6 rough sawn.

I have only seen T&G OSB in 3/4" thickness for subfloors, never in 7/16" thickness for a ceiling or wallboard install, but I'm sure someone must make it. I just don't know how easy it would be to purchase.

Re: air barrier. This is where I come back around to the spray foam. The only time I will have appreciable indoor humidity is when I'm painting, as this shop has no kitchens or baths, but air infiltration can still cause havoc in terms of condensation. Closed cell spray foam is absolutely superior, in this regard... but 100 years of building with batts has shown they can work well enough when installed simply with stapled paper barrier. What am I missing?

 

[woodgeek]

Forget the idea of a vapor barrier...you want an air barrier....little cracks and seams can move a lot of heated air, and bypass the insulation. In 70-80 years of stick building with batts, houses had 100% higher heat loss due to poor airsealing!

The seams in the OSB and the gaps under the stapled Kraft paper will (in typical construction) leak enough air to largely defeat the insulation.

You need to at least decide on what the air barrier will be, and then try to make it continuous. You are not going to tyvek the place, or put in a taped layer of poly, or tape or caulk all the OSB seams. Being careful with the Kraft paper...overlapping the paper edges and taping the seams, should provide a decent air barrier without too much added effort or cost.

Regular OSB is fine of course....I was just free-associating....

 

[Ashful]

Woodgeek, I will be tyvek'ing the outside walls, before installing new siding. Existing (leaky) siding will become sheathing. However, you are correct on the roof, it's steel on purlins on joists. If doing batts in roof, it would be cutting 1" foam to fit between joists, then batts with stapled Kraft. I could spray foam (canned) seams of 1" foam board to joists, but I'd worry about this becoming a secondary vapor barrier, and trapping moisture. I think it best to have a single vapor barrier at sheathing.

I could do a secondary barrier at sheating, eg. tyvek or poly sheeting. I really know nothing about modern building science, beyond the basic principles of condensation, 100% of my work has been repairing old construction.

 

[Ashful]

This related thread may be of interest to those reading here. I graphically show what woodgeek was describing in post #35.

https://www.hearth.com/talk/threads/overhead-door-insulation-calculations.138654/

 

[woodgeek]

So, on the side walls, you can tape the tyvek seams with....tyvek tape, and that is your air barrier. Even taped tyvek needs to be lapped carefully for drainage, esp around windows, but I expect you are an expert on that. No problem with the air barrier being on the outside (in this case) because tyvek is vapor permeable.

Assuming you want to vent the steel roof, you will need a good airbarrier on the (sloped) ceiling plane because any leaks in that plane will have ready access to the vented cavity, wind driven pressure, etc.

I'm assuming you want to go as simple/fast as possible....sounds like thin polyiso stapled to the roof purlins. If all you want to do is define a venting space (by keeping the FG from pushing up to the roofing) I'd go with 1/2" xps or polyiso, which would also avoid windwashing the FG. I'd cut the foam all the same width, enough under the joist spacing so that each piece does not need to be custom fit (PITA). If you define your air barrier at the Kraft paper, then the (0.25"?) gaps on the sides of the polyiso don't matter. And allow the FG to dry to the vented space. But, to be a broken record, I would tape the Kraft seams if you are going to rely on it being your primary air barrier.

At 6" overall (old lumber dimensions) you'd prob get R-20 center of cavity, maybe R-15 with thermal bridging by the joists. Certainly not code, but workable if airsealed. If you wanted to go to higher R.....you could put some polyiso under the joists, and 2" would get you to R27 or so. You'ld need longer screws for the OSB, is all. Or you could spring for foam-OSB bonded panels and save a step (if you can source them). In the foam case you could not sweat the Kraft paper at all (or get unfaced) and tape the foam seams as an airbarrier.

IF R-15 is good enough for you....the FG seems like a simple and 'conventional' solution. If you want significantly more than that, you are back to where the thread started....building up sheet foam, the joists, or going spray foam.

You will have to detail the transition from vented roof assy to your wall assy carefully to connect the air barriers.

 

[Hansson]

This is a pretty good page for calculate U-value.
http://www.energiberakning.se/
Go to U-Value. Press the English flag

 

[Ashful]

Okay... roof insulation finalized, thanks to a lot of back-room help from woodgeek. Here's the plan:

1. 1" XPS sheets cut 22" wide and fastened to underside of purlins. Also fastened to top side of collar ties (to create small flat ceiling in second floor space), and boxed in around soffits.
2. R-30 closed-cell spray foam on XPS in each rafter bay. The XPS is really just there to keep the spray foam off the underside of roof, and facilitate future roof replacement.
3. 7/16" OSB ceiling, pref T&G, if it can be sourced.
4. UltraVent product (soft brillo pad type material) installed under ridge vent, to keep out critters, insects, snow.
5. Corrugations caulked shut (could not source a prefab trim-board for this roof) at soffit, and louvered vents installed at underside of soffit.

Ideally, it would have been nice to just spray foam to the roof, and eliminate need to ventilate, along with any concern of condensation. However, semipro's comment about roof replacement got to me, as I surely would replace with a different roof system, when the time comes.

I still need to decide on a secondary barrier behind OSB, but decided to not worry about thermal bridging. Calculations showed thermal bridging bringing the assembly down from R-35 to R-25, which will only amount to $5 per year of additional heating cost, given the temperatures I intend to maintain.

 

[woodgeek]

I don't know what you mean by secondary barrier....if its sprayfoam, the assembly is airsealed and vapor impermeable. The joists can dry to both sides.

You can also go to 1/2" foam board, saves money and even easier to cut, tack, etc.

 

[semipro]

However, semipro's comment about roof replacement got to me, as I surely would replace with a different roof system, when the time comes

That credit belongs to Bret Chase, post 7.
I appreciate the thought though.

 

[woodgeek]

Re the thermal bridging, when you replace the roof in 25 years, you can (1) see if the spray foam shrank (2) add another layer of sheet foam as underlayment to the new roof to bring it to R-50, and (3) get an energy rebate for the upinsulation from the Bieber administration (don't ask).

 

[Ashful]

That credit belongs to Bret Chase, post 7.
I appreciate the thought though.

You are correct! Sorry, Bret!

1/2" foam board... what I originally suggested. I thought I had let you talk me into going thicker, tho!

... and (3) get an energy rebate for the upinsulation from the Bieber administration (don't ask).

"Ronald Reagan? The actor? Then who's vice president? Jerry Lewis? I suppose Jane Wyman is the First Lady!" - Dr. Emmett Brown

 

[woodgeek]

"Ronald Reagan? The actor? Then who's vice president? Jerry Lewis? I suppose Jane Wyman is the First Lady!" - Dr. Emmett Brown

In an enlightened future, ScarJo would be veep. Might make the SOTU speech even harder to follow.

 

[Ashful]

Next up... how to handle ceiling / insulation around a lighted cupola. Cupola has windows on all sides, and is sealed tight on air, insects, etc., but not insulated. It straddles a vented ridge, as shown in photos.

Ridge and soffits are vented, with foam board and spray foam on purlins, as previously discussed. There will be collar ties, and a ceiling, approximately as shown in the attached sketch. Roofing is opened up to cupola (i.e. prior owner cut hole in roof to allow cupola lighting into barn). Initial thought is to make the lighted cupola part of the un-insulated attic space, sealed off from the heated space. Pro's are ease of construction, and better insulation. Con's are loss of light, and a potential place for insects (stink bugs, hornets, etc.) to congregate out of sight.

Thoughts?

 

[ironpony]

seal the cupola off from the attic with a trap door on bottom of cupola, so you still have access. then leave as part of attic space.

 

[woodgeek]

I like daylighting. Maintain the opening and cover it with a transparent sheetgood like plexi, suntuff or lexan twinwall. Or even coroplast from a sign store. Will diffuse the light, and provide enough insulation. Cover the inside walls of the cupola shaft with foil or glossy white paint beforehand for better throughput.