Low-Slope Roof Insulation Retrofit
Zone 5 (Ohio)
A section of my house has a low-slope roof (just under 2:12), with a couple layers of built up roofing of some sort (asphalt-y / granular lower layer, somewhat more rubbery but still tarry upper layer). We are planning on adding 6″ of reclaimed polyiso on the exterior to supplement dense-packed cellulose in the 2×6 rafter space to the interior. I understand that the key to this assembly is a robust air barrier at the structural sheathing (under the exterior insulation); on the remainder of the house (walls & steeper roof) we’ve been stripping everything back to the sheathing and applying Proclima Adhero to the sheathing as the primary air barrier.
However, on the low-slope area, after investigating a couple small areas, the existing roofing is proving very difficult to remove — the upper layer can be peeled off, but the lower layer is firmly adhered (as well as nailed) to the sheathing, wants to break into pieces, and can only be slowly pried off inch by inch.
Is the existing roofing, which is in reasonable shape / isn’t leaking anywhere, a sufficient air barrier on its own, such that I could just attach the Adhero coming up the walls to the roofing, or would I need to cover the entire roof (over all the existing roofing) with Adhero as well? Are there any other potential downsides to just leaving the existing roofing in place and doing my insulation and new roofing stackup on top of it?
Thanks!a
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Hard to tell without photos and a brief description. Btw, a built up roof is hot tar/asphalt and loose laid gravel on top, it's common in large commercial, very rare in residential. It sounds like you might have a touchdown modbit roof.
Anyways, That's a judgement call on your behalf on the condition of your existing roof. But if you think it's good, I personally think it's fine to leave in place.
Yes it is vapour closed and waterproof. But you'll have to think about how you're going to fasten the foam layers to your roof membrane. If you're screwing down then it won't be so waterproof anymore, which is only useful as a backup layer to your new roof layer. This still might be your best bet vs glueing down the foam cause it might not stick to the modbit roof. Again hard for me to tell from where I am.
Then consider your parapets, and the details around that. Details around Water management, (ie are you blocking scuppers? Or will you have to redo your gutters, and what are you going to affix it to)
And then what is your new roof layer going to be? You'll probably need a protection board on top... If it's mod bit torch down, you'll probably need asphalt protection board so your roofers dont light all that insulation on fire. Or something like tpo, I'd still suggest putting down plywood so walking on it doesn't ruin the insulation or footsteps don't stretch the membrane. And then details like how that ties into your parapets, etc.
Possibly Consider some weep holes somewhere for the foam. if your primary roof membrane does leak at some point, your polyiso will soak up water and itll diminish it's insulation ability. And you'll never know it it's all soaked up cause you have the bottom roof layer stopping the water.
Jamie
Thanks Jamie! I've attached few photos of the situation to this comment.
Thanks for the clarification on terminology, after googling around a bit I think you're right, its either mod-bit or rolled roofing (or one on top of the other).
The plan has always been to screw down the foam, so there aren't any issues with adhesive compatibility there, and there were always going to be fasteners penetrating whatever the air barrier membrane layer was. For simplicity, we're going with the same stackup on both the low-slope and steep-slope roofs -- two 3" layers of polyiso, with 2x4's embedded in the upper layer, fastened through to the structure below; 1/2" plywood sheathing nailed to the 2x4s. The low-slope part will get an EPDM membrane. It's not a true "flat roof" with parapets and those types of details, it's more of a big "shed dormer" with a slightly less than 2:12 slope -- I do have to redo the gutters and everything; we cut off the old overhangs and are building new overhangs.
Good point on not noticing potential leaks -- you convinced me to leave a couple little holes near the edge that could be periodically inspected for signs of water leakage.