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Roofing Insulation Question

GBA Editor | Posted in Energy Efficiency and Durability on

I saw some similar questions, but it is not quite the same. Please help to finalize the design:

The setting: Midwest, low-pitch (2/12) roof with almost no insulation, cathedral ceiling (some roof decking is 2×6 tongue and groove, some is plywood) . We plan to frame a higher pitched roof (6/12) over existing, the newly created attic will be vented and insulated with 2 layers of recycled ISO (total of about 6″). Existing shingles will be stripped to roof decking.

Proposed design (from bottom up):
Old roof decking (T&G or plywood)
1st layers ISO (glued, seams taped)
2nd layer ISO (glued to first,staggered, seams taped). There will be some bracing to hold ISO in place (connected to rafters above, not belove to avoid thermal bridging)
Attic (vented)
New roof decking/roof.

How does this plan look ?

I have 2 questions:
1. Do I need to put any air barrier between Old roof decking and 1st layer ISO ?
2. Is it better to use fiber-faced ISO ? Would I benefir from aluminum faced one (at least on top ) ?
3. What material I should use for taping ISO seams (in case I use fiber-faced ISO or aluminum-faced one) ?

Thank you so much in advance.

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  1. Doug McEvers | | #1


    I had a similar project a few years ago, the house had a 2/12 pitch. The look was dated and the pitch is not suitable for shingles in a cold climate. The existing roof (vaulted ceiling) was 2x8 rafters with fiberglass batts which I left intact. I stripped the shingles, added a nailing plate above the wall line and added 4/12 pitch scissor truss rafters to create a new roof line and add insulation space. The new trusses had an energy heel, I installed a windwash barrier and blew insulation over the original roof. The new roof was vented at both the eaves and the ridge. I did not add any additional air barrier, the drywall ceiling served the purpose.

  2. Alex L. | | #2

    I considered this option (insulation-wise) but was concerned about blown-in insulation rolling down from the ridge of old roof and/or plugging the soffit vents. Also, at wall plate there would not be much insulation (I have 2ft overhangs) and ISO would provide a better R-value. Other than insulation my design is very similar to yours.

  3. Doug McEvers | | #3

    You missed a couple of key points, the windwash barrier will hold the insulation out of the soffit. The energy heel allows for plenty of insulation at the plate line. With a new 6/12 pitch rafter, the rafter tail should intersect the old roof at the facia, this will determine the height of the energy heel. I would never use foam insulation when blown is an option, the cost is much lower and greener if you use cellulose. Blown insulation of any type will not "roll down" a 2/12 pithch.

  4. Alex L. | | #4

    I am getting ISO (recycled) at less about 60% off new product price, which will come to about 2.5K for 6" ISO (2 layers of 3") - and this was comparable to the quotes I got for about 3ft of blown in insulation that I would have to get. Access to the new attic (or lack of thereof) and sequence of steps are other reasons I decided to go with ISO (but you are now seeding some serious last minute doubts in my mind).

  5. homedesign | | #5

    Could you describe "recycled ISO" ?
    How do the recycle ISO ?... I am assuming you mean Polyiso?

  6. Alex L. | | #6

    It is either "used" polyiso boards (installed once, but they are structurally sound and are almost "like new") or so called "factory seconds" (boards that did not pass QA control during manufacturing process).

  7. UukdueJwuK | | #7

    Any other feedback ? Thank you guys in advance.

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