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Insulation on top of roof sheathing?

user-5454807 | Posted in Energy Efficiency and Durability on

I have a 100 year old wood frame three family dwelling in Zone 4 (Long Island) on which we are going to replace the windows, siding and roofing.
My question pertains to the roof. What could we use to insulate over the plywood sheathing and underneath the GAF Timberline HD shingles?
We are using 1.25″ Rockwool Comfortboard 80 to wrap the house under the new siding (the only Comfortboard currently available locally without a 100 day special order) and as we renovate the apartments we’re using Rockwool batts in the cavities (covered with Certainteed Membrain). 
Most of the third floor has an uninsulated cathedral ceiling that we will probably lower at some point to insulate it with Rockwool batts (we really like Rockwool).
Can we use foam board on top of the roof sheathing? What type would be best? And what would be the minimum/maximum recommended thickness?
Thank you.

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  1. andy_ | | #1

    I think you might want to take a bigger picture assessment of the roof and interior before choosing a product (unless you have already and didn't mention it).
    Typically you'd want to address the air leakage and vapor drive before just throwing insulation at something. Outside foam might be a great solution, but it also might be just expensive, complicated, and possibly counter productive. You won't really know unless you take a good look at your goals, existing structure, climate, budget, or you could just cowboy it and hope for the best.

    1. bwsct | | #3

      Hi Andy,
      I'll be doing a similar project as the poster Robert.

      What should be considered for air leakage if he's doing the windows? Lets just assume its wood construction. and he said he's in climate zone 4.

  2. walta100 | | #2

    Have you looked for used foam insulation?

    I have not look lately but it was priced well below the new price.

    You will need a few layers to get to R49 and some long screws.


  3. user-5454807 | | #4

    Okay. So the wall construction is wood, 5/8" shiplap over 3/4" tongue and groove sheathing (fir, southern pine? Definitely not cedar), both of which will remain. We're removing the aluminum siding on the exterior. Windows will be Andersen 400 casements, properly flashed and sealed. Henry Blue Skin has been suggested as the building wrap, over which will be the 1.25" Comfortboard. ACG battens will be affixed through that, from which Hardi-plank Artisan Lap Siding and Certainteed Monogram XL Double Dutch lap vinyl siding will be hung (each on two sides of the building).
    We have so far renovated one of the three apartments down to the studs, sealing any gaps or holes we could find with Great Stuff foam and tightly filling the cavities with Rockwool Batts, covered with Certainteed Membrain. We will do the same with the other apartments at some point. However, the third floor is a converted attic with two doghouses so it has lots of ceiling angles. Thus my feeling we need to insulate the top of the roof as the ability to insulate from underneath will be limited even with lowering the central part of the cathedral ceiling.
    My thinking at this point is to put 2-4" (depending on construction difficulty and cost: the roof is pretty steep and high off the ground) of rigid foam between the sheathing and a new layer of plywood. Either a newly formulated (low greenhouse gas) XPS or (foil faced?) Poly-iso.

  4. user-2310254 | | #5


    Can you salvage the windows? Unless they are shot, it is hard to justify the cost of replacement? Storm windows are often a better investment.

    Did you look at Andersen's 100 Series. I had these in a previous house and really liked them. And they were much less expensive than the 400 Series.

  5. user-5454807 | | #6

    Hi Steve,

    No the windows are really shot. We already replaced a third of them when we renovated the apartment. We did look at the 100s but preferred the 400s.

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