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This summer I will be converting the attic of a one and a half story builder bungalow in Seattle WA. Existing roof framing is 2×4 rafters at 24″ o.c., holding the existing loads quite well, so I intend to keep with it. Shingles have at least five years left in them, so don’t plan to replace yet, but will need to do complete tear off when the time comes.
After reviewing options w/ FG, cellulose, and rigid boards, I am considering an installation of either spray foam or similar such as Icynene. I would have the insulation contractor come in and fill the rafter bays from ridge to eave, leaving no venting gap. There would be no venting at the eave or ridge.
My question to the board is: do you all think I’m nuts? I know there are strongly differing positions out there on venting and insulating and I’d welcome all comments. There has been a considerable amount of information coming out that cast doubts on the need to ventilate in all situations. My thinking is the foam will air seal the roof assembly thereby reducing or eliminating the potential for condensation. I know basic rules suggest a vapor retarder should be placed on the warm side of the insulation too, but I’m thinking of eliminating that as well to allow any small amount of moisture to diffuse back to the interior. In fact, I’m half toying with the idea of leaving the foam filled rafter bays open to view (sort of an “industrial” look) or maybe using some other type of finish effect other than drywall.
The upside of this approach is simplicity and maximizing the R-value to be attained within the 3-3/4″ of the rafter depth, thereby not losing any head room clearance in the existing space, which is important as the framing is quite low. Those who could warn me of the downsides or who have experience with results from similar attic conversions, I’d appreciate your thoughts.
Mike Chi
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I don't think the Icynene is very good at stopping moisture - I seem to recall it is very permeable. The foam Gene L uses is very good at stopping both moisture and air and should work fine. When you dod do a tear off, I assume you will reuse sheathing or sheath over.
*Due to fire codes for 'exposed foam' your inspector may make you cover it with drywall. But if you look in these archives you'll find recent discussion about the flamability and the moisture permeation of spray on foam insulations for you to make up your own mind.
*I hadn't thought of the fire/smoke hazard potential. That's a good point and I will check into it. It seems you all are not that concerned about the "venting" issue. Are there any special requirements for sealing the interface between the foam and rafters for the long term? Also, another issue I have is the existing roof has skip sheathing. If foam is applied it will fill the "skip" voids and I will have a heck of a time resheeting with plywood in a few years as I intend to do so. I'm wondering if I could put some Tyvek or perhaps thin asphalt board against the underside of the skip sheathing before spraying the foam, to keep it out of the gaps...
*Most folks around here would leave the skip sheathing and sheet right over it with OSB or ply, I always do.
*I confirmed the Icynene is not a good vapor retarder compared to some other foams and things like foil and plastic.This is not to say it isn't good - just don't count on it to stop vapor. On the other hand Icynene is not a serious burning concern - air and cement basically.I reccommend sheathing inside with polyiso and blowing cells above for a tolerant and air tight ceiling - and definitely no venting. Space the polyiso about 3/8" and spray foam in joints to seal. Cover with drywall screwed through foam (after blowing cells). Air tight is most important.
*Mike, Another idea would be to foam as you have outlined and then add an inch of rigid insulation over the new sheathing when you reroof. This would keep your roof deck warmer and should further reduce the potential for condensation. I think Bill is right about the Icynene...you may be able to leave it exposed. Other foams should definitely be covered. Best to check with your inspector.
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This summer I will be converting the attic of a one and a half story builder bungalow in Seattle WA. Existing roof framing is 2x4 rafters at 24" o.c., holding the existing loads quite well, so I intend to keep with it. Shingles have at least five years left in them, so don't plan to replace yet, but will need to do complete tear off when the time comes.
After reviewing options w/ FG, cellulose, and rigid boards, I am considering an installation of either spray foam or similar such as Icynene. I would have the insulation contractor come in and fill the rafter bays from ridge to eave, leaving no venting gap. There would be no venting at the eave or ridge.
My question to the board is: do you all think I'm nuts? I know there are strongly differing positions out there on venting and insulating and I'd welcome all comments. There has been a considerable amount of information coming out that cast doubts on the need to ventilate in all situations. My thinking is the foam will air seal the roof assembly thereby reducing or eliminating the potential for condensation. I know basic rules suggest a vapor retarder should be placed on the warm side of the insulation too, but I'm thinking of eliminating that as well to allow any small amount of moisture to diffuse back to the interior. In fact, I'm half toying with the idea of leaving the foam filled rafter bays open to view (sort of an "industrial" look) or maybe using some other type of finish effect other than drywall.
The upside of this approach is simplicity and maximizing the R-value to be attained within the 3-3/4" of the rafter depth, thereby not losing any head room clearance in the existing space, which is important as the framing is quite low. Those who could warn me of the downsides or who have experience with results from similar attic conversions, I'd appreciate your thoughts.
Mike Chi