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I need some insulation

beachhouse123 | Posted in General Questions on


I am thinking about adding insulation in our NY beach house. The house is an “upside down” house that has exposed “tongue and groove” vaulted ceiling upstairs. We like the look of the ceiling with rafters and boards exposed but are not sure how to add insulation to the roof to enjoy winter. On the exterior roof we currently just have a water barrier with two layers of asphalt shingles. So there is no insulation. We have a forced air heating system but it’s incredibly inefficient considering our level of insulation.

After reading as much as I could online I think my option would be to add rigid board insulation on the exterior (which would require a new roof). Most commentators online suggest that I need R-39 insulation on the roof.

Does this combination sound correct?

1. Tongue and Groove (already installed)
2. Water Barrier
3. Rigid Board insulation
4. Roofing sheathing (OSB or plywood)
5. Roofing shingles

What would be the best brand/product for insulation? There are many.
Any idea how much a new roof with insulation should cost?
Should I talk to roofing contractors or insulation companies?
Do I need to install two layers of rigid board insulation to achieve an acceptable R value?
Will I notice any improvement with only 1 layer of insulation (maybe R-20)?


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  1. GBA Editor
    Martin Holladay | | #1

    Many of your questions are answered in this article: How to Install Rigid Foam On Top of Roof Sheathing.

    Layer #2 in your enumerated list isn't really a "water barrier." It's an air barrier. An air barrier at this location is essential.

    Between layer #4 and layer #5 you need roofing underlayment (asphalt felt or synthetic roofing underlayment).

    Ideally, you will install enough insulation to meet minimum code requirements (probably R-49, but perhaps only R-38). If you can't afford to meet minimum code requirements, you will have higher energy bills than you would if you met code.

    -- Martin Holladay

  2. Expert Member
    Dana Dorsett | | #2

    If it's continuous insulation (not thermally bridged by framing) R36-R37 would meet IRC 2015 code for US climate zones 4-8 on a U-factor basis (U0.026 = R38.4 "whole-assembly" ). The other R1.4-R3.4 comes from the othe materials- the t & g, the roofing, and the interior & exterior air films.

    That would take 7" of rigid polyisocyanurate or 9" of Type-II EPS.

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