Insulating a partially vented attic with closed-cell spray foam sprayed on top of ceiling instead of roof deck
Hi, looking to insulate and air seal our attic in PNW zone 4M, in 2 story 113 year old house. Attic ridge is vented (added when previous owners had house re-shingled) , but there are no vent openings in the soffet. It doesn’t get very cold here, but house has no insulation for the most part and is very leaky. I am removing the siding, insulating, sheathing with plywood, and air sealing at the sheathing plane. Don’t want to insulate roof with external insulation as shingles are in good shape. Instead of doing a flash and batt style insulation of the roof deck, is there any reason not to use 2″-3″ of heat lok HFO cc spray foam on ceiling plane and then add loose cellulose to R65 on top? Would air seal the top plate to the roof at same time with the cc Foam. Nothing is in the attic except the old knob and tube wires, and I will remove those and run new wiring after the insulation is installed.
Thanks,
Dave
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Replies
If you don't have ducts in an attic, keeping the attic unconditioned will be less expensive and have less heat loss (even at the same R value). Spend some of the saved money on blower door testing.
I agree with Jon.
You might not need so much insulation, but that's up to you. To both keep the attic safe from moisture, and help stop air leaking thr0ugh your newly sealed ceiling, you should add soffit vents. At least the same amount as you have at the ridge, preferably 30% more.
>"...is there any reason not to use 2″-3″ of heat lok HFO cc spray foam on ceiling plane and then add loose cellulose to R65 on top?"
Yes.
Reason #1: There is no point to spending $2.50-$4 per square foot just for air sealing. There are many cheaper ways to get there.
Even if using spray foam is for some reason deemed the way to go, 3"-3.5" of half-pound OPEN CELL foam will achieve the same degree of air tightness for a buck a square foot, using only 1/4 the amount of polymer, at about the same R value as 2" of HFO blown closed cell foam.
Reason #2: You don't need (or particularly want) the low vapor permeance of closed cell foam at the ceiling plane in your climate. (And even if you did, painting the ceilings with vapor barrier latex would get you there at tiny fraction of the cost.)
If there are no signs of mold or moisture accumulation in the rafters roof deck, there isn't a burning need to add soffit vents, but best practices would be to add more soffit vent area than ridge vent to keep the ridge vent from depressurizing the attic relative to the conditioned space below. With the attic floor/ceiling plane LESS than air tight the ridge vent could be drawing conditioned space air into the attic, but if it's going to be ultra-air sealed with foam that risk is low going forward. Adding the ridge vent without soffit venting was a mistake (a reversible mistake).
No matter what there needs to be baffles added to keep any insulation other than closed cell foam (which I'm encouraging you NOT to use) from contacting the roof deck.
David,
I agree with Dana: The best approach is usually air sealing + cellulose. That will cost less than installing spray foam.
For more information on air sealing, see this article: "Air Sealing an Attic."
Ok, thanks for the advice, I'll add soffit vents. In my case I think the cost of spray foam may not be as significant an impact, as I need a newly built small cathedral style roof with a shed dormer to also be air sealed and insulated with spray foam. I haven't taken quotes yet, but assume there will be an up charge for such a small job ( (~750 bdft)) , or I may not be able to find someone even willing to take the job within the time frame I would like it done. I'll look into getting quotes for air sealing ( I am too busy to do it myself), but if it turns out that spray foam for the attic is cheaper and easier I still may have them foam the attic floor.
Once the spray foam contractor is already onsite, the cost situation changes a bit. Since you have a cathedral ceiling, where spray foam is really the best option, you have a need for that contractor to be onsite -- adding a bit more to the job won't be as big of a deal.
Dana is still correct about using open cell foam in your "seal the floor" application instead of closed cell foam. The same contractor should be able to do both types of foam. BE SURE they used closed cell in the cathedral ceiling part. You should be able to have them use open cell on the attic floor, and they should be able to apply the open cell foam for significantly lower cost per square foot.
If your attic is open and accessible, air sealing in more traditional ways (canned foam and caulk) won't be difficult, and will be much cheaper than spray foam. Most of the work would have to be done either way (removing any existing insulation and getting the floor clean).
Bill