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?
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In MA (US climate zone 5A) at least 40% of the total R has to be closed cell foam either above or below the roof deck for dew point control on 60% of more permeable insulation. So 2" (R12-R14) isn't really enough for dew point control on 8" (R24-R28) of open cell foam. The open cell foam would be at risk of accumulating moisture or even sustaining frost damage.
But 2" of closed cell foam directly on the underside of the roof deck is sufficiently low-perm to protect the roof deck. So 2" of closed cell + fluff or open cell foam that violates the 40/60 rule would need at the very least a Class-II vapor retarder or a "smart" vapor retarder to keep the permeable insulation sufficiently dry. Depending on the stackup even that won't always be moisture-safe.
But for any unvented solution there can't be an interior side vapor barrier. So a layer of Tuff-R on the interior side of the assembly demands that the roof deck MUST be fully vented, since unvented it would create a moisture trap.
There are many problems with your suggestion, and Dana has highlighted most of them. Read the following articles to find out about approaches that reduce the chance of moisture accumulation and rot.
How to Build an Insulated Cathedral Ceiling
Combining Exterior Rigid Foam With Fluffy Insulation
Seems like 3" closed with 7" open would be OK though, right? 4" closed with 6" open is even better.
I'm not sure what you mean by "for any unvented solution there can't be an interior side vapor barrier". Doesn't the Tuff-R behave as a vapor barrier, assuming the joints are sealed? Isn't it just closed-cell foam board? In that case, isn't it the same as closed cell under the roof, but with the barrier shifted down 7.25"?
Maybe you are talking about trapping the vapor that is permeating through the ceiling near the soffit which is outside of the Tuff-R envelope? If so, wouldn't a continuous soffit-to-ridge ventilation channel with Accuvent baffles solve this? But, I suppose there would have to be a gap somewhere in order to allow vapor permeating into the trap to be swept out through the ventilation channel.
Maybe I would need 4" of Tuff-R instead and just leave the bays open...
Yes, Tuff-R is a vapor barrier, which blocks all drying of the roof deck toward the interior. To protect the roof deck it then has to be able dry to the outdoors via a ventilation channel. Without the venting any moisture that seeps in under the shingles from wind-driven rain or weeks of snow load potentially stays for years, rotting the roof, since it can't dry very quickly through ~1 perm of shingles & #30 felt, and not at all through the Tuff-R.
If the roof deck is vented (Accuvent baffles work) with at least the code minimum inch of air space between any insulation and the roof deck, it'll dry just fine with Tuff-R on the interior side.
Actually, your comments about drying to the outdoors reminds me of a related question.
If I have 3" or 4" of closed cell directly under the roof sheathing, is it a problem to have ice and water shield on TOP of the roof sheathing and under the shingles? I ask because my roofer has given me the option of either 6' of ice and water shield OR over the entire roof all the way up to the ridge.
Here are the facts about unvented roofs with closed-cell spray foam and asphalt shingles:
1. Once you install closed-cell spray foam on the underside of the roof sheathing, there isn't going to be any inward drying.
2. And once you install asphalt shingles, there isn't going to be any outward drying.
As long as the spray foam is installed on a day when the roof sheathing is dry, this isn't really a problem.
Having a layer of Ice & Water Shield on top of the sheathing doesn't change the drying ability in any way. The asphalt shingles are already preventing outward drying.
That said, I'm not really a fan of roofs that are 100% covered with Ice & Water Shield. (The peel-and-stick complicates future repairs.) I would ask the roofer why the Ice & Water Shield is necessary.