Spray foam questions/concerns
I intend to insulate my garage, which has living space upstairs. I’m told by a local contractor that I should go with either 0.5 lb or 2.0 lb foam in the attic. His claim is that 3″or more of the 1.o lb foam will provide a vapor barrier. I have my doubts whether this is true. However, I have seen so many different opinions on whether a vapor barrier is a good or bad thing. I live in NW PA, and the roof has titanium felt and ice shield under the architectural shingles and plywood sheathing. I don’t know if that qualifies as a vapor barrier. The heating system will be in the attic (under the insulation). I can add a supply and return in the attic. Should I apply 0.5, 1.0 or 2.0 foam, considering I have lots of room for the foam. Do I need some form of vapor barrier, and if so, where. Should I vent from the soffit to the top of the roof (vent ridge), or have no ventilation (envelope the attic & roof)? I appreciate any advice you can give me. Thanks.
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The Ice & Water Shield is a vapor barrier, but it's on the cold-in-winter side of the roof deck, and doesn't protect the roof deck from accumulating moisture from the indoor air during the cold months. The roof needs something on the interior side to slow down that moisture accumulation, but not with such low vapor diffusion that it can never dry.
1 perm meets the Canadian code definition of "vapour barrier", and is the boundary between Class I & II vapor retardency. That is sufficiently vapor retardent to be protective of the roof deck in your climate, but 3" of 1lb foam isn't much R-value.
Read and re-read this document, then take a careful look at Tables 3 & 4:
https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf
The 2lb foam modeled in those simulations was 1.2 perms @ 1" . A 1" shot was sufficient to protect the roof deck in 5A (Chicago) as long as the assembly was air-tight between the fiber insulation layer and interior, so if the product truly is 1 per or lower at the depth you're installing it will be enough.
Installing just half pound foam can work if one also installs a "smart" vapor retarder such as Intello Plus or Certainteed MemBrain or "vapor barrier latex" primer on gypsum board on the interior side.
If the rafter framing isn't blocked and a 1"-2" soffit-to-ridge channel can be established in every rafter bay, a vented roof deck is more resilient, and half pound foam with any interior side vapor retarder (even cheap polyethylene vapor barrier) can work. But there has to be both soffit & ridge venting, with more free flow area at the soffits than at the ridge.
So, if I vent from soffit to ridge in every rafter space, and then cover that and the joists with either 0.5 or 1.0lb foam, and use an interior vapor retarder (such as vapor barrier latex primer) on the drywall ceiling below the attic, I would be OK? The ceiling of the floor below would be the first place I would have interior drywall, not in the attic. Should I still have a supply & return in the attic (where the furnace will be), which would be above the vapor primered drywall or membrane? Or, should I use the vapor paint directly on the foam (above the furnace)? Not sure what you mean by more free flow at the soffits. I'm new to the foam discussions, but I do like the idea of no air flow. I just want to make sure I'm not dismantling the structure 10 years from now, due to mold, rot, or anything else. I'm already too old for that!! I just want to better understand what I need to do - what foam, what barrier (and where), whether to put a heat duct & supply return in the attic, how much ventilation (if any) and where, etc. These same questions apply to the walls below, and the floor between the garage and 2nd floor. The garage floor will be heated (in-floor radiant), generally to about 60 degrees F. Not sure whether to insulate the floor or not. Thanks for your patience and understanding for this rookie.
1. if you have the room, soffit to ridge vents are a good thing. Dana is suggesting more square area of venting at the soffit than at the ridge (I am not sure that having more area at the ridge affects actual air flow, but I don't see how it can hurt; I am actually testing soffit-to-to-ridge air flow).
2. Make sure you have a continuous air control layer; don't let the soffit-to-ridge venting "communicate" with any attic air. This is particularly important at the soffits, where--depending on the pitch of your roof--it can be the hardest to keep them separate.
3. Move the air around in your now cathedralized attic (cathedralized is when the air and thermal control layers are at the roof line, but the attic has a floor with a finished ceiling below). Supply and return is best practice, but you just need to introduce air movement and exchange in this space; it does not need to be fully conditioned like an occupied space.
4. vapor retarder paint on face of open-cell foam; you won't need that paint with closed cell spray foam.
Peter