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Community and Q&A

Adding Roof Deck Insulation to Cape Cod with Closed Cell Spray Foam

user-7188984 | Posted in General Questions on

Background: I own a 1940’s Cape Code outside of Buffalo, NY (Climate Zone 5a) and I am in the process of electrifying my house. A heat pump was installed last summer, and this year I am getting solar panels installed. Before having solar installed I am planning to replace my 15-20yr old asphalt shingle roof, and this seems like a good opportunity to add some additional insulation. In the summer the upstairs bedroom gets quite warm, while the downstairs and basement are relatively cool. 

Can exterior roof deck insulation be installed with closed cell spray foam insulation already in place underneath the roof deck?

Existing Roof Information:
+/- 950sf
9/12 Pitch
3/4″ tongue and groove board roof decking
2×6 Rafters, 16″ on center
6″ Closed Cell Spray Foam

Proposed Roof:
1. 6″ Closed Cell Spray Foam (+/- R39)
2. 3/4″ tongue and groove board roof sheathing
3. Air Control Membrane
4. 3.5″ Foil Faced Polyisocyanurate (+/- R21)
5. OSB roof sheathing
6. Ice and Water Shield
7. Roof Underlayment
8. Asphalt Shingles

Note: At some point in the future I would like to have the option to add continuous exterior wall insulation. I am planning to add a small overhang (1′ to 1.5′) to the roof to allow for this future connection.

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  1. Expert Member
    Michael Maines | | #1

    As long as the sheathing is dry when the polyiso is installed, and nothing happens to allow the sheathing to get wet from the exterior, it should be fine.

    Your R-values are a bit high for foam that's been in place for a while. Closed cell foam and polyiso both age to somewhere around R-5.6/in.

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