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Poor man’s deep energy retrofit – Roof chapter

Jeff_Hudson_Valley | Posted in General Questions on

On verge of buying a foreclosure. Great potential for a deep energy retrofit. Roof still have 5 or more years on it. Current Cape Cod second floor will be taken down to studs. (Most of second floor “ceiling” is the rafters with only a small attic space.) I plan to take the insulation to the roof deck with closed cell. 5.5″ thick, between the rafters. (No head space to put 1″ or 2″ foam board under the rafters unless we raise the roof.) When we replace the roof, I want to add at least 2″ foam board on top of the roof deck to do that properly.
– I understand I shouldn’t sandwich the roof deck wood otherwise I’m opening myself up to rot.
– So, I’m thinking of stapling up plastic against the roof deck so the spray foam doesn’t stick to the roof deck, only the rafters, and when roof is replaced, we take off the roof deck and we’ll have a relatively even surface to put the foam board on, then the (new) roof deck. It’ll still have some gaps that could be somewhat sealed with 1-part. 

Do you know if that’s been done and was it an unexpected pain in the tukus, or did it work?

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  1. RussMill | | #1

    Sandwiched roof and wall sheathing is done everyday. Most frown on it. But its done without problems.

    Its like high moisture in thick cellulose walls. Many still claim it will rot osb or zip wall sheathing. It hasnt occured for most part, only in leaky situations.

    I would bite the bullet, ccspf on inside, reclaimed polyisonon exterior side of roof, 2nd layer of sheathing, paper and shingles.
    Poly sheeting is so hard to seal.

  2. Jeff_Hudson_Valley | | #2

    Thanks, Russel.

  3. Expert Member
    Akos | | #3

    The 5.5" of cc SPF doesn't buy you much insulation R value once you take into account the thermal bridging from the rafters. It only gets you an R26 roof.

    Depending on what zone you are in, a much better option is to go with 2" of cc SPF and fill the rest of the cavity with either open cell or batts. This gets you around an R25 roof for way less money.

    For the correct ratio of cc SPF to batts see:

    As Russel mentioned, provided your roof deck is dry when applied, there is nothing wrong with the foam sandwich.


    I used the wrong spreadsheet with exterior foam. 2x6 with cc SPF is R22, with 2" SPF+batts R17.5.

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