Advice: Insulating underside of a roof, conditioning attic
I’m in the process of moving our HVAC system from our basement (which is valuable space) to our attic (which is unused). I understand the extreme efficiency penalty that such a move would cause if I did not insulate & airseal the attic, turning it into conditioned space. Hence, I’m looking for a simple, cost effective strategy to bringing our roof a reasonable R-value.
I realize the most effective way to do this would be to insulate on top of the roof deck. That’s just too much work. Our shingles are in good shape and I’m not interested in completely rebuilding our soffits.
I’ve looked at two options to insulate from below the roof:
1) Closed cell spray foam. I got a few bids and 3″ of spray foam applied between the joists will run $7-$10K. Yikes. Even DIY canisters are $6500.
2) Install 4’x8′ sheets of 3″ recycled polyiso with a shiplap edge (to help airseal the seams). I can get the material for under $2K. That’s much more reasonable.
Our roof is a simple design with nothing challenging other than some plumbing vents to navigate. So I’m leaning toward option 2.
My idea is this:
1) I don’t need the clearance so rather than a “cut and cobble” job within the rafters, I’m thinking of applying the foam in full sheets to the bottom of the rafters. The roof would remain vented (the entire 2×6 rafter would be the channel). I’d install a ridge vent along the top of the roof to ensure each rafter bay is ventilated. Each foam has a shiplap edge so I think I can simply airseal most seams with tape (especially since this is a vented assembly).
2) At the top (ridge) and bottom (where rafters meet the soffit/top plate) I would use a canister of closed cell spray foam to seal these areas (This would be two-part DIY closed cell, not Great Stuff). The area where the attic joists, rafters, and sill plate meet would be my only “cut & cobble” job. My plan is to wedge pieces of rigid foam between the joists at the point where it meets the top plate. Then I’d spray foam over that entire assembly.
My questions are:
1) I think it would make sense to install a radiant barrier before the insulation goes up. For $300 I can buy 2000sqft of breathable foil radiant barrier. I think this would help performance. Does it matter if this is permeable or impermeable? I’m thinking permeable/breathable is better so that any moisture doesn’t get trapped.
2) I have gobs of 8″ thick R30 unfaced fiberglass insulation rolls (it currently covers the entire attic floor). I was thinking of securing these somehow beneath the 3″ of polyiso but I haven’t a clue how I’d keep them tight against the roof (gravity will not be my friend). Any thoughts on how to attach this? This stuff is essentially free since I’ve already purchased it. Plus re-using it would keep it out of a landfill (and be cheaper than hauling away).
Lastly, I live in Chicago (Zone 5).
Thank you,
Brian
Replies
from the sounds of your description, I would consider enclosing your units.
Build a platform (floor) and four walls large enough to contain and maintain your units. Then insulate this new utility room. by doing this you can then layer your polyiso and air seal penetrations with cans of great stuff, or a smaller slow rise foam kit.
My suggestion comes because I generally believe you get what you pay for, so 10k vs 2.5k insulation jobs more than likely fairly reflect the effectivness of the material used.
Would the foam insulation require fireproofing?
My understanding is that foam insulation must be fireproofed with a layer of drywall or similar. I too am in Chicago and have a similar issue with knee wall attics.
Apparently sheet foam is very flammable and must be covered if it is inside the conditioned envelope.
I too am in Chicago.
Good luck.
Spray foam (and sheet foam) requires a thermal barrier
Norman is correct--spray foam needs a thermal barrier.
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/72240/Does-Your-Spray-Foam-Insulation-Need-a-Thermal-or-Ignition-Barrier
http://www.buildingscience.com/documents/guides-and-manuals/irc-faqs/irc-faq-basement-insulation
You can use DOW Thermax, which is an insulation sheet covered with aluminum:
http://building.dow.com/en-us/products/thermax-sheathing/?sc_itemid=d93988d5-73d9-4988-b1a1-b48282f3c7b7
I special ordered my Thermax through Home Depot. I think the minimum order was 32 sheets. I have it installed in my crawlspaces and it will soon be installed in my attic (over cut and cobbled sheet foam / spray foam between rafters).
You may stumble over what 'occupied' space means too. If you just have equipment in the attic, it might not be considered 'occupied'. If you have storage in the attic, it then may be considered 'occupied'. Your building inspector will give you an answer if asked--just not the one you may want.
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