Closed-cell spray foam
We are preparing to build a home in the mountains of North Carolina, CZ4A. We are planning to insulate the cathedral ceiling using CCSF. I recently viewed an Instagram post showing CCSF separated from framing members creating a gap. A number of comments suggested the pictures depicted a very poor installation, but others indicated that they often found such failures in the course of performing renovations and repairs. Some suggested that due to thermal expansion/contraction and/or settling, these gaps were inevitable.
1. Are such gaps inevitable? If so, wouldn’t that mean spray foam is not an effective means of air sealing?
2. Someone wrote a comment stating that if the framing and sheathing were sprayed with water prior to applying the CCSF, there would be no separation issues. Any comments about this approach?
We are considering the use of Aerobarrier to create as tight an envelope as reasonably achievable. The question arises as to whether this product would be any less susceptible to shrinkage and settling failures than the CCSF. Any thoughts?
And finally, if we do use Aerobarrier, any thoughts about whether it should be applied before or after drywall? If it matters, the walls will be 2×6, 24” on center, fiber glass batts, and Zip R6 sheathing.
Thank you in advance for any information you can provide!
Glenn
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Replies
>" We are planning to insulate the cathedral ceiling using CCSF."
Don't.
Closed cell foam between framing is a waste, due to the thermal bridging of far more conductive R1/inch wood penetrating the R6-R7/inch foam.
Instead of 7"-8" of 2lb foam to hit R49, install 6-8" of half pound open cell foam on the underside of the roof deck, and 3-4" of rigid polyiso above the roof deck. As long as more than 30% of the total R is above the roof deck the roof deck can be allowed to dry toward the interior using semi-permeabl materials & finishes, making the assembly more moisture resilient. A continuous R18-R24 over the rafters improves the overall performance.
8" of half pound foam uses the same amount of polymer of 2" of closed cell foam, and it's blown with water instead of climate damaging HFCs (or more benign but more expensive HFO1234ze). If the rooftop polyiso were 4" of 2lb roofing polyiso it would be the same amount of (similar) polymer as 6" of closed cell foam, but the polyiso is blown with fairly benign pentane.
HFC blown closed cell foam needs to be installed in 2" lifts for quality (yes, sinks /voids and separation) and it takes 8" to hit R49. HFO blown foam can usually be installed in a single pass of 7" to hit R49.
Half pound open cell can be safely installed in lifts of 6", which would be about R22-R23. To make code R49 on an R-value basis would take R26, which would be 4.5" of 2lb roofing polyiso, or (still acceptable for residential roofs) ~4" of 1lb foil faced polyiso. But due to the thermal break of continuous foam over the rafters you can get there on a U-factor basis with 3" of foil faced or 3.5" of 2lb fiber faced roofing polyiso held down with a 1/2" or 5/8" nailer deck through-screwed to the structural roof deck.
With 8" / R30 of half-pound foam (installed in 2 lifts) you could arguably get away with about an inch less rigid foam above the roof deck, but with less dew point margin. The 30% number is a minimum, not an optimum.
Dana,
Thank you for your advice. This sounds like the way to go, just need to get my builder on board. FYI, his initial advice was to spray open cell under roof deck with no over the deck insulation. He says that is common around here and two other builders have confirmed that to me. I feel like using only open cell below with no insulation above may be asking for problems down the road. I’m not interested in creating future problems!
Glenn
For info on getting any type of spray foam installed correctly, take a look at this resource: https://www.buildinggreen.com/blog/foam-place-insulation-7-tips-getting-injection-and-spray-foam-right.
Using AeroBarrier: the AB folks make it pretty clear that you can't use AB as the PRIMARY air barrier system; their system is to TUNE your air tightness. I am not sure what the recommend but after drywall is what I always see in an AB installation.
Peter