“Foam sandwich” in gable walls
I have a ranch house in Zone 4A NY, with a gable roof that has been considerably upgraded over several years. Last fall, the house was re-insulated, sheathed and sided, with the following: 3.5 inches rockwool, 1/2 plywood, Henry Blueskin, 3/4″ tuff-r foil-faced polyiso (taped), Hydrogap, LP smartside.
The gable-end vents were the only attic-ventilation, and were removed in anticipation of an unvented attic (the air conditioning system is in the attic). We are now at that stage, and I have a couple of questions:
1) The contractor wants to spray 2″ of closed cell to the roof deck, and fill the remainder of the 2×10 cavity with rockwool. It doesn’t seem to me that this will meet code, which is R-15 of vapor-impermeable and R-49 total. The R-49 code was a change this year, the old code was R-38. It seems doing 3″ of closed-cell to get the R-15 of vapor-impermeable is non-negotiable. For the total-R assembly, I see two options – a) Use 3″ of closed-cell and 5.5″ of rockwool, for R-42.5 total. b) Use 3″ of closed-cell and 7.25″ of rockwool, furring down the rafters with rigid-foam (certainly more expensive), which brings us to
2) How important is it to address thermal bridging of the rafters? The contractor says they only ever over-spray the rafters in steel-framed commercial buildings. To do encapsulate in spray-foam would be very expensive as it would require closed-cell, open-cell and a fire-retardant spray.
3) Gable-end walls: The contractor wants to spray 2″ of closed-cell in the 2×4 stud bays. I am concerned about foil-faced foam on the outside and closed-cell on the inside. They don’t think this is a problem due to the unlikelihood of a leak in the wall cavity where there are no doors and windows (just exhaust fan and ERV penetrations). They are willing to do open-cell, but this might actually cost more as they would be using 3.5″ and the fire-retardant. They are not inclined to use mineral-wool in the gable-walls if spraying the roof.
Thanks!
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Replies
Hi Cody.
Why not increase rafter depth to allow for code minimum R-value and and mitigate thermal bridging? Some builders install strips of rigid foam along the bottom of the rafters which has two effects: it increases the depth of the cavities and helps with heat loss through thermal bridging. It would be similar to the approach shown for walls in this article: Breaking the Thermal Bridge.
When it comes to the gable walls, it is a judgement call. You contractor may be right that the gable walls, without windows and doors, and assuming the house has gable end roof overhangs, are not that vulnerable to water intrusion. And if the walls don't get wet, then they won't need to dry. But that is the more risky approach. The more conservative approach would be to make sure the walls can dry in at least one direction and in this case, that has to be inward. In other words, a vapor-permeable insulation would create a safer assembly.
Thanks for the suggestion. As a variation on your theme, and to avoid asking the contractor to do something "unconventional" like cutting foam strips, which they likely would charge dearly for, would it be a safe assembly to allow them to fill the cavity as planned (3.5" closed cell, 5.5" mineral wool) and then as a DIY project, install, say, 2" of mineral-wool board underneath the rafters? By my calculation, that would give a total R of 43.5. The only possible issue I can see is this may leave a .25" gap between the batts and the board - would this present any problems?
Thanks!
Hi Cody.
As long as you have enough closed-cell spray foam under the roof deck for condensation control, you should not have any problems as long as the rest of the assembly is vapor-open towards the interior.
The small air space will not cause a problem either. It may slightly diminish the R-value of the assembly--some GBA members can probably tell you how much exactly--but the layer of mineral wool boards over the rafter will help mitigate thermal bridging.
Another option would be to install the closed-cell and have the rest of the bays filled with dense pack cellulose, a more environmentally-friendly option that will completely fill the bays. Then install the mineral-wool board below the rafters.
Determining the best approach from an install, cost, environmental, and performance perspective is important, but again, what you are considering will not cause any problems.