Contractor recommends Open Cell for Conditioned Attic in 4A. Is this okay?
Ryan_Huddleston
| Posted in Energy Efficiency and Durability on
I am building a new house that will have a conditioned attic. I was planning on doing a flash and bat combo of 2″ closed cell and R30 fiber glass. The spray foam contractor is recommending R40 open cell with a vapor barrier paint applied.
Are there any advantages to doing a flash and bat instead of open cell? Should I be worried about roof rot problems in 4A?
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Neither assembly meets the code requirement for R-49 in CZ 4a. 3" of CCSPF plus R-30 batts gets you close. Open cell foam has been problematic in colder climates. The vapor retarder paint will help, but isn't a guarantee. The problems I've seen with open cell in roof assemblies are more related to gaps in the insulation than to diffusion through the insulation. Open cell foam, especially in thick layers, tends to form large air bubbles and/or to shrink away from the framing members during curing. These open channels can allow significant convection that deposits moisture from interior air directly onto the backside of the sheathing.
Your contractor's suggestion can work, but it is risky. If you decide to go that way, let the contractor know that there will be zero tolerance for gaps and holes. A well detailed smart membrane air/'vapor barrier instead of the paint would also be an option, though probably a bit more expensive.
Martin comments in this thread that Building Science Corporation doesn't think applying vapor retarder paint to open cell foam is effective: https://www.greenbuildingadvisor.com/question/spraying-vapor-barrier-paint-on-open-cell-foam
The most conservative approach would be to stick to having 30% of your total R-value be in the form of closed-cell spray foam, as per:
https://www.buildingscience.com/documents/building-science-insights/bsi-100-hybrid-assemblies
However, in climate zone 4A you might be able to get away with just open-cell foam (like your spray-foam contractor is suggesting), with some caveats regarding conditioning the attic to remove moisture. There is a comment about this just below Figure 3 in the hybrid assembles article, with links to two more BSC articles to read:
https://www.buildingscience.com/documents/insights/bsi-077-cool-hand-luke-meets-attics
https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-016-ping-pong-water-and-chemical-engineer
The short version is that open-cell foam + no dedicated attic conditioning is a bad idea. Open-cell foam + conditioning your attic might work, read the articles, see what you think, see what other commenters think. The closed-cell foam might still be worth the peace of mind if it's not outrageously expensive :)
price wise both options are close.
2" of closed cell is $2.00/sf and an R30 batt is about $1.1/sf total price = $3.10/sf for R43
11" of open cell R40 is $2.85/sf
My second floor air handler will be located in the attic so conditioning the attic would be easy.
What's the big attraction with spray foam?
In new construction in zone 4A you can hit code-min on a U-factor basis with 3" of exterior roofing polyiso (~R17), and R23 rock wool between 2x6 rafters. With the 3" roofing polyiso you have more than sufficient dew point control for R30 rock wool in 2x8 rafters (or R30 fiberglass in 2x1os) , which would beat code-min with real margin on a U-factor basis, and come arguably close to meeting code on an R-value basis.
It's easy to finish the roof edges with a 1x4 facia board when the exterior foam is only 3". In zone 4A it only takes R15 to have enough dew point control for up to R34, per the prescriptives in the IRC. R17 exterior foam and R30 in the rafter bays has margin, but even R38HD fiberglass in 2x1o rafter bays (R55 total) still makes it from a dew point control perspective.
R40 open cell foam on the interior side can meet code on a U-factor basis in a 2x4 trussed roof due to the minimal thermal bridging, but it will definitely need something other than sprayed on vapor barrier paint. Vapor barrier paint only meets spec when it can soak in a bit where it binds to the substrate, such as paper facers on gypsum board or applied directly to plywood, etc. It's at best 5 perms when applied to open cell foam, and needs to be under 2 perms to avoid ping-pong attic moisture, under 1 perm to meet code for unvented roofs.
Dana,
Very helpful post. Honestly I wasn’t considering exterior foam because it seems like a foreign concept to me.
Would you recommend a certain product. I looks like I can get 1.5” for $24. Two layers would get me to R15 at $1.50/sf, adding $1.10 for the R30 fiber glass batts brings the total to $2.60/sf which is the cheapest option. Tape, screws, furring strip sheathing and a roofing fabric would probably push it over $ up to 3.50 or so but replacing roof sheathing in the future would be a breeze.
Would this work?
https://www.homedepot.com/p/202085958
Another question,
I had planned to used zip for roof sheathing. Would there be any benefit to that since the it would be under polyiso and a second layer of sheathing?
Check craigslist for reclaimed foam dealers. I got pricing for 3” polyiso at $20 per sheet reclaimed, 3” Dow thermax seconds at $30 sheet.
This pricing is in MA.
That’s awesome. That would really reduce cost.
If your roof is reasonably simple, the lowest cost is a vented assembly. Going with 11 7/8 TJI, you can staple 1" foam to the bottom of the top flange for a vent baffle, then go with 2x4 followed by a 2x6 batt. With mineral wool batts, that gives you an R39 whole assembly, with fiberglass that drops a bit to R36. You can get the fiberglass assembly up to R38 by going with polyiso for the foam baffle.
If you roof is low enough slope (I think it is less than 4 in 12) you can also loose fill the bays, you would have to bump up the TJI to 14" to get enough depth.