Attic spray foam alternative
I have been considering having the underside of my roof spray foamed in the attic. I’ve done the BTU/hr heat loss calculations (designed for 68F indoor and 0F outdoor), and the payback in BTU/hr for each additional inch of closed cell foam really starts falling off after the 1st 2 inches which are needed for a vapor barrier to prevent condensation. The roof above my attic is only about 1044 sqft (simple gable roof), which is at best only 50% of the surface area of my house (i.e. ceilings and walls) that is exposed to outside temperatures.
If i could get away with only 2 to 4 inches of closed cell, that would be one thing, but I’m concerned about thermal bridging causing snow melt and ice dams, which is the reason I’m thinking about this in the 1st place. My rafters are 2×8 (i.e. 7.25″ deep), so one contractor proposal I had included 2″ of closed-cell to prevent condensation on the underside of the roof sheathing, followed by 8″ of cheaper open-cell to fill in the reset and cover the bottoms of the rafters. 8 extra of inches, even of open cell, is still a lot just to get the rafters covered. The contractor thinks that “wrapping” the rafters, while leaving the bays not totally filled, will be hard to execute consistently in the tight confines of the attic.
Other than to prevent ice dams, another motivation for doing this is that I can get rid of all the existing insulation on the attic floor and open up the whole space for semi-conditioned storage. The roof pitch is too low, so this will never be living area space.
As an alternative, I’ve been pondering the following:
Cover bottoms of rafters from the ridge down to the ceiling joists will 2″ thick Dow Tuff-R.
Cut out blocks to fill the spaces between the floor joists, and seal all the joints with Great Stuff foam and/or foil tape in order to seal the envelope.
Navigating around my bathroom fans may get tricky because they are close to the eaves/soffits and the roof only has a 5/12 pitch.
Cut holes in the Tuff-R near the ridge and fill in the rafter bays with ~6 inches of cellulose. Then plug the holes back up and seal with foam and/or tape.
I could even create a continuous cathedral-style ventilation channel from the soffit vents to the ridge using overlapped Accuvent baffles, and then fill in the space between that and the Tuff-R with cellulose. The only problem here is that the Accuvent baffles would probably need to be pushed in from the outside through the soffits and stapled to the outside edge of the top plate. I don’t know exactly how difficult this would be because I have never looked inside the soffit from the outside.
It seems like the cost of doing something like this would be WAY cheaper than the 2″ closed-cell with 8″ open-cell combo since Tuff-R is only $0.63 per board foot (i.e. $40 per 4’x8’x 2″ board).
I understand that this would need to be perfectly air tight, which may be much more difficult than with spray foam, but at least you’d be working with a mostly continuous flat surface.
Also, regarding the spray foam option, is 2″ really enough to prevent condensation in my climate (northern Massachusetts)? Does it even matter if there is condensation sandwiched between the closed and open-cell foam as long as it doesn’t touch the roof sheathing? Will the open cell just dry out on its own?