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Re-roofing and re-insulating with spray polyurethane foam

Berning | Posted in General Questions on

I’ll try to make this as succinct as possible: I’m having my roof re-done and re-insulated. It is covered by my insurance company (with a few exceptions) as the whole thing is infested with squirrels and their droppings (I got lucky I had coverage for this).

The roof is a 12-12 pitch, covered with standing seam steel with 2×12 rafters on 24-inch centers. Roof system sits on timber frame bents visible from inside. On these sit 2×6 tongue and groove, visible from inside. On top of this is a poly vapour barrier. Then 1.5″ R7 Rigid Foam Insulation. 99% sure this sits between the rafters, not underneath them. Then Roxul insulation (exact size/R value unknown). Then an air gap, vented from the soffit with exhaust at the ridge.

The house is in a cross pattern, so there are four valleys, plus a shed roof off the master bedroom that looks to be constructed with shallower rafters. I’m guessing they are 2x8s.

I live in central Ontario, so Zone 6.

The plan, set forth by the Insurance Company’s engineer, is to leave the existing rigid insulation and vapour barrier, clean the cavity and replace the roxul with new, keeping the air barrier, resheath and replace with new steel.

I would like to upgrade the insulation with some SPF foam and in consultation with an engineer and the contractors have come up with the following plan and would very much like to have some input before moving forward as I only have one chance to do this right:

1) Use Valley-Vents throughout valley and other airflow restricted areas to promote/ensure airflow into the rafter cavities.

2) Spray 2″ of closed cell foam on top of existing R7 rigid foam which sits on the poly vapour barrier.

3) Add 5.5″ of R24 Roxul on top of spray foam giving approx. R44 total and leaving approx. 2.5″ of air barrier between top of Roxul and sheathing.

4) The shed roof, where there is less room to play with, has a drywalled ceiling, and we’ll be pulling everything out and spraying a minimum 6″ of SPF (strapping rafters if needed to get the space) while leaving the needed air barrier to allow airflow through to the upper, 12-12 roof.

So that’s the plan now, but I’m having concerns and second thoughts, such as:

a) Is leaving poly vapour barrier and rigid foam a bad idea? Am I better to go to extra expense of removing rigid foam, cutting away the poly and spraying 3 inches of SPF directly onto the T&G?

b) Is this whole plan/combination/construction method a good idea at all. Will I have condensation issues? Everywhere I read I see people spraying foam from the inside, onto the underside of the roof sheathing. I can’t do that as everything has to be done from the top down.

Any help, advice or reassurance would be greatly appreciated. Work is starting at the end of September so we’re into the nitty gritty for permits and such so I have a small window to make any changes to the plan.

Aaron in Wilberforce, ON

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  1. GBA Editor
    Martin Holladay | | #1

    Your roof configuration, with four valleys and insufficient soffit vents, is not a good candidate for a vented approach. I think that DCI, the manufacturer of the Valley Vent, oversells the product, and exaggerates the amount of air flow that the Valley Vent can handle. (Remember, the Valley Vent is a tapered piece of plastic that tapers from 1/2 inch maximum thickness -- not much -- to zero.)

    So, start with this assumption: You'll be designing your roof assembly as an unvented assembly.

    Since you've got 11.25 inches to play with, and you are aiming for R-49 insulation in your climate, here's one way to proceed:

    1. Leave the existing 1.5 inch thick polyiso in place -- let's call that about R-10. (Calling it R-10 is a little generous, but close enough.) Your remaining space is 9.75 inches.

    2. Install 2 inches of closed-cell spray foam. Let's call that about R-13. Your remaining space is 7.75 inches.

    3. Install 7 inches of open-cell spray foam. That's about R-26. So it you add it all up, you have R-49.

    With this approach, you don't have to worry about venting -- which is good, because you aren't going to have effective venting with those 4 long valleys.

  2. Berning | | #2

    Thanks Martin,

    Could I use 7.25" of R-28 Roxul instead of the open cell? Getting the other type of foam will be an issue...

  3. GBA Editor
    Martin Holladay | | #3

    No, you can't use mineral wool in an unvented assembly (unless you install an adequately thick layer of rigid foam above the roof sheathing, or an adequately thick layer of closed-cell spray foam above the mineral wool).

    I suppose you could install the mineral wool at the bottom of the rafter bays, with or without the polyiso, and then spray the closed-cell spray foam (at least R-25 of spray foam -- in other words, a minimum of 4 inches of closed-cell spray foam) above the mineral wool.

    For more information on this type of roof assembly, see Combining Exterior Rigid Foam With Fluffy Insulation. (Although the article talks about rigid foam, not spray foam, the required ratios of foam-to-fluffy are identical when closed-cell spray foam is used.)

    You may also want to read this article: How to Build an Insulated Cathedral Ceiling.

  4. Berning | | #4

    Could 2x4 sleepers across the rafters provide enough airflow into the valleys to make the polysio/sprayfoam/roxul/air-barrier a viable solution?

  5. GBA Editor
    Martin Holladay | | #5

    Yes, I suppose adding 2x4s at 90 degrees to the rafters would work.

  6. RobInNorCal | | #6

    Berning, you say you're going to replace the steel - meaning the exterior roofing, I assume - so wouldn't it be cheaper and more effective to put 4-6" of foam on top of the sheathing before you re-roof? You could use the roxul under the sheathing in that case to get to your R49 target.

  7. Berning | | #7

    Unfortunately I'm at the mercy of the insurance company. Any extra's beyond putting it back together exactly the way it was circa 1991 comes out of my pocket and the contractors (hired by the Ins. Co.) are charging insurance company prices... so far the insurance company is paying $240,000 and I'm on the hook for $42,000. I've found a local spray foam contractor that is giving me an affordable price on the 2" of closed cell and I'm sure if I buy a pile of 2x4s they'll install them as sleepers; they already agreed to temporarily place them so the spray-foamer can navigate around the roof, so they can just do it a little more carefully and leave them in place before putting the decking down.

    To further delve into your suggestion Rob, are you suggesting foregoing the spray foam underneath and putting that money into solid foam between the steel and the sheathing? Fill the rafter bays with roxul and leave an air barrier underneath the sheathing? What sort of foam would I need?


  8. GBA Editor
    Martin Holladay | | #8

    The method that Rob Hunter is suggesting is explained in this article: How to Install Rigid Foam On Top of Roof Sheathing.

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