Hi, I’m performing an energy retrofit
thrifttrust
| Posted in Green Building Techniques on
Hi, I’m performing an energy retrofit. My house has tiny roof overhangs and will have none after external wall insulation is applied. An unvented attic with internal spray foam and/or external rigid insulation is too expensive. I’ve found a product called….
My house is in Detroit (zone 5a).
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Thrift,
It looks like you tried to cram too many sentences into the title of your post, and the software cut off your last sentence (and your question).
I have moved most of your long title to the body of the post, but we still don't know what your question is. Post a comment on this page with the missing information so we can help you.
I apologize, I realized my error too late.
Instead of soffit vents I'm considering In-Vent. It's like a half shingle over ridge vent that is installed under the first shingle row above the ice shield. I'd then spray foam the difficult to seal/vent/insulate eave area and use cellulose on the attic floor. Is this a viable plan?
Have you considered changing over to an unvented attic? If you are planning to reroof, this might be a better option.
So you want to create a closed cell foam filled/sealed area above the eaves and a cellulose on the floor, vented attic in higher areas. This hybrid sounds OK to me.
Also take a look at "SmartVent" under shingle attic vents.
Thanks for your advice guys.
I have considered an unvented attic Steve, but the cost of spray foam to R40+ seems prohibitively expensive.
I'm happy for the vote of confidence Jon. SmartVent looks like a better thought out and less conspicuous system than In-Vent.
Thrift,
Did you get the SmartVent? How did it work out? I'm considering SmartVent to convert my attic to passive ventilation. Unlike you, I have big soffits, but there isn't much clearance between the tops of the exterior walls and the roof. Otherwise, my house is a perfect candidate for passive ventilation. Contractors have recommended SmartVent (or competing products) but I haven't found much information on this site regarding them.
Thanks,
Thad
"I have considered an unvented attic Steve, but the cost of spray foam to R40+ seems prohibitively expensive."
Rather than R40+ in spray foam under the roof deck, try putting R20+ above the roof deck using RECLAIMED rigid polyisocyanurate foam, and up to R30 of cheap fiber insulation under the roof deck. It zone 5 as long as at least 40%+ of the total R is above the roof deck.
https://detroit.craigslist.org/search/sss?query=rigid+insulation
Derate roofing polyiso to R5/inch in that climate when half or more of the insulation is on the interior side of the assembly.
If you have 2x6 rafters R23 rock wool can fit under the roof deck, and with 4" of used 2lb roofing iso above you'd be at R43+ at center cavity, but it would outperform R49 between joists or rafters due to the R20 thermal break over the framing.
It's more expensive than R49 cellulose on the attic floor, but you probably don't have ROOM for R49 that extends all the way out over the top plates of the exterior walls. It's still dramatically cheaper than 7-8" of 2lb spray polyurethane under the roof deck, thermally bridged by rafters.
On a deep energy retrofit I was involved with a handful of years ago they went with 3" of used roofing polyiso + 1" foil faced goods on the walls + spray foam cavity fill (though I was advocating for cellulose) that was continuous with a double layer of 3" used roofing iso on the top with spray foam in the rafter bays (again, I was pushing cellulose, and lost) putting all of the structural wood completely inside of at least 4" of polyiso. The cost of the virgin stock 1" foil faced on the walls was more than the 3" of reclaimed goods, and the total 4" on the exterior came in quite a bit cheaper than the ~4" of sprayed goods in the wall cavities.
Exterior foam is certainly a "perfect" solution but really expensive. The cost of batts and their installation alone would be several times that of cellulose on the attic floor. The roof would need another deck and then the fascia and/or the soffits would have to be redone. I thought of the spray foam as a cost saving compromise. Only the first few inches at the heel would be foamed. However, I'm uncomfortable with the idea anyway. I'm not sure even foam would eliminate ice damming at the heel, which might back up to the smart vents. If I had soffits I'd put in air shuts, and insulate as best I could.
Anyway, my roof has more serious issues. It's a hip roof, and through a series of poorly conceived additions the roof design became tortured, with an oddly draining low slope section. Then they built an attached garage. So now the house drains toward the garage gable wall. I plan to remove the entire roof structure and replace it with a simple gable design using raised heel trusses that accommodate soffits and external wall foam. The garage will then safely meet a vertical wall.