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Add insulation to dormer attic?

JohnACarr84 | Posted in General Questions on


I need some more advice. I live in North Alabama — climate zone 3. Most of our home is a built up cathedral roof system. However, we have 2 gabled dormers with a small attic space — framed out with 2x4s 24 inches on center. Currently, there is just 2 inches of XPS foam attached under the 2×4 rafters, sealed with spray foam at all seams (thermal camera looks like it is sealed pretty well). I believe there is a 3″+ air space behind the foam to a ridge vent.

What would be the best was to add R-value to these areas?


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  1. Dana1 | | #1

    Are the rafter bays vented (at one end or the other or both), or are they merely empty, no vents?

  2. JohnACarr84 | | #2

    They appear to be vented at both ends -- soffits at low end and ridge vent at high end.

  3. GBA Editor
    Martin Holladay | | #3

    In Climate Zone 3, the 2012 International Residential Code requires that roof or ceiling insulation have a minimum R-value of R-38. You now have R-10.

    I suggest that you install more rigid foam on the underside of your roof assembly -- an additional R-28 of rigid foam insulation, so that your total insulation thickness meets minimum code requirements. Since you'll have several layers of foam, you'll be able to stagger the seams between layers.

    Green builders usually avoid the use of XPS, since XPS is manufactured with a blowing agent that has a very high global warming potential. I suggest that you use EPS or polyiso.

  4. Dana1 | | #4

    Code min is R38 if some or all of the insulation is between rafters, but with continuous foam not thermally bridged by rafters you can get the equivalent performance at a lower R. The assembly really only needs to hit U-0.030 per TABLE N1102.1.3 in chapter 11 of the IRC. See:

    Accounting for the thermal performance of a 3/4" roof deck + shingles and the air films in the cavity you'd be there with about R29- that's total, not additional. You already have R10, to you'd only need another R19. That can be achieved with 3" of foil-faced polyiso applied to the underside of your existing 2" XPS.

  5. JohnACarr84 | | #5

    Thanks! Appreciate the advice

  6. JohnACarr84 | | #6

    Hi GBA,

    A quick follow up. I am finishing the addition of polyiso to the underside of the exist XPS in this attic. I have been searching online for an answer to this question unsuccessfully: For my own education, I was curious on what issues, if any, would there have been if i added cellulose or fiberglass to attic floor (keeping the 2" XPS on the underside of rafters) to build the proper R-value as opposed to adding 3"+ of polyiso in direct contact with the existing xps.

    Thanks guys!

  7. GBA Editor
    Martin Holladay | | #7

    Code requirements for the type of assembly specify that additional insulation on the interior side of the rigid foam must be in direct contact with the rigid foam. The worry is that if there is air leakage in the space between two types or layers of insulation, the thermal performance of the insulation will be degraded.

    That said, if your job is not being inspected by a code official, you can put insulation on the attic floor if you want. You'd want to do an impeccable air sealing job in the attic if you go this route.

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