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Community and Q&A

Roof insulation

Matt Watson | Posted in General Questions on

I was wanting to add more insulation to a 1963 house with cathedral ceilings – 2X6 with R19 fiberglass in zone 5. I wanted to add nail base on the outside but dosen’t look like that will be possible with quotes coming in about double what I estimated. What else can I do do get more R value in the roof? Some cathedral areas and other areas with flat ceilings and about 6″ of fill on top of that. Roof is vented now and I would prefer to seal it up since a lot of animals and birds are able to get into the attic area. Roof is approx. a 3X12 pitch. I can attach a 2X3 to the present 2X6 roof truss to gain some more depth and insulation. Thanks, Matt

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  1. GBA Editor
    Brian Pontolilo | | #1


    Sounds like what you'd like to do is create an unvented, insulated roof. This article will give you all the information you need to determine if this is possible and what might be the right approach for you:

    P.S. It would be great to know your name. It makes the forum a much friendlier place.

  2. Expert Member
    Dana Dorsett | | #2

    Assuming those are 5.5" deep milled 2x6...

    Adding 2.5" of 2x3 perpendicular to the rafters would yield 8" total. As long as at least 40% of the total R is closed cell foam applied to the underside of the roof deck there won't be excessive seasonal moisture buildup at the foam/fiber boundary. With 3" of HFO blown closed cell foam (R20) it would be good for up to R30 of blown fiber in the remaining 5". With 5" 1.8lbs density fiberglass that would come to another R2o, and with 3" (R12.5) of fiberglass as thermal break on the 2x6 rafters for most of the rafter-edge area it would make it to code-minimum performance on a U-factor basis (< U0.026) , despite being only a bit over R4o at center-cavity, and R9.5-ish where the 2x3s intersect the rafters.

    Alternatively, with 3" of HFO blown foam and compressing "contractor roll" R13s (kraft faced or unfaced, not aluminum) into the remaining 2.5" of rafter depth yields R10, and another set of R13 compressed between the 2x3s is another R10, for about R40 total (foam + fiber) at center cavity without the expense (but also not the perfect fit) of dense packing. With careful DIY fitting & installation of the batts the performance will be there.

    With about half the total R being foam the moisture accumulation risk over the winter is low.

    Going a bit cheaper...

    A 2" shot of HFO blown foam is installed (R14) would only be sufficient for dew point control on R21 of fiber insulation in the remaining 6". That can be done cheaply with R11 fiberglass to fill out the remaining 3.5" of 2x6 cavity, and compressed R11 batts in the 2x3 cavities. Kraft facers or unfaced is fine.

    When compressed to 2.5" the R11s perform at about R9, so at center cavity it would still be only (R11+ R9=) R20, which is effectively your upper-bound. The 2.5" of compressed R11s deliver an R9 thermal break over most of the 2x6 rafter edge area, but with only R34 at center cavity it's not quite going to duck under the current code max U0.026, but it will more than double the performance of your current roof, and for a lot less money than 3" of HFO blown foam and 5" of dense packed fiberglass.

    When there is only 2" of closed cell foam resist the urge to go with compressed R13 or batts , since that would make fiber more than 40% of the total. With more than 40% for the fiber-R it could end up collecting moisture at the foam/fiber boundary over a cold winter, potentially creating mold conditions inside the cavity in early spring.

    In my area HFO blown closed cell foam runs about $1.30-1.50 per board foot ($4.00 - 4.50 per square foot for 3", $2.50- $3.00 per square foot for 2"), but that's an installed price. Batts (or even dense-packed fiberglass) are much cheaper by comparison.

  3. Matt Watson | | #3

    Dana -Thanks for the great info, this all sounds like something I can do. For the non-cathedral area do I also have to add spray foam on the underside of the roof plywood or do I only need to increase the amount of loose fill in there? The cathedral parts will have the drywall removed and easily accessible but not so easy on the flat ceiling areas, what do I do there? This part will also be non vented, any issuers there? For the garage area with no insulation, how should I treat this when it also be non-vented? thanks for you help, much appreciated!

  4. Alan Afsari | | #4

    Additional questions about flash and batt/fill assemblies.

    Does the flash layer need to be thicker near the ridge of the roof?

    Would adding a thin layer of foam board inside the rafters installed (using air sealing installation techniques) (above the gypsum board) help the performance of the assembly?

    Btw, Dana, I can’t open the link you posted.

    Thanks, Alan

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