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Community and Q&A

Insulating roof with 2×6 rafters in Eastern Massachusetts

Kazio Grzyb | Posted in Energy Efficiency and Durability on

We are renovating our 110 year old house located just north outside Boston. Planning on upgrading the insulation of the top floor (under sloped roof). Currently a living space with what looks like 50 year old mineral wool insulation. Most of the lath and plaster covering the underside of the roof will go.

The gable roof’s rafters are actual 2×6″ 24″ o.c. Not vented. No exterior insulation, nor there will be any anytime soon (roof is 3 years old; asphalt shingles over 6′ of Grace Ice and Water Shield at the eaves, with double lapped 15# felt above). I am planning on 2″ of closed cell spray foam directly to the roofing boards, so ~R12-R14, followed by 3.5″ of Roxul batts, so R14.

I fully realize it does not meet the current code, but due to headroom issues I am not able/willing to fur out the rafters. It will be miles better than what it is now: moist interior air condensing on the inside of the deck. No rot, the deck manages to dry out.

While substandard, will the stack up be reasonably safe mositure-wise? Should I add interior vapor retarder (Membrain)?

I’ve been considering retrofitting venting, but then I’ll lose at least an 1 inch of space for vent baffles, and retrofitting the offsite vents will be difficult on this house. Adding ridge vent would not be a big deal. So I have pretty much decided to keep it unvented.

I’ll appreciate any feedback very much!!

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Kazio,
    Q. "While substandard, will the stack up be reasonably safe mositure-wise?"

    A. Yes. In your zone (Climate Zone 5), for this type of roof assembly, you need to have at least 41% of the total R-value of the roof assembly coming from the spray foam layer. Your stack-up passes the test. More information here: "Combining Exterior Rigid Foam With Fluffy Insulation."

  2. Kazio Grzyb | | #2

    Thanks, much appreciated!

    I also found this paper linked on your site, which was very helpful:

    https://buildingscience.com/sites/default/files/migrate/pdf/BA-1001_Moisture_Safe_Unvented_Roofs.pdf

  3. Expert Member
    Dana Dorsett | | #3

    Using an HFO-blown closed cell foam is more expensive than HFC blown foam and is ~30-40% more expensive, but it it's higher R/inch (giving you more dew point margin) and a LOT greener due to the very low impact HFO1234ze blowing agent compared to the industry standard HFCs, which have a global warming potential on the order of 1000x CO2.

    At 2" the vapor permeance of the closed cell foam layer is about 0.4-0.8 perms, depending on the product, which is an adequate drying path for the roof deck, but is also a Class-II vapor retarder sufficient to keep wintertime moisture accumulations well bounded. When it's time to re-roof you have the option of adding continuous exterior insulation to bring the thermal performance up. In eastern MA there are multiple vendors selling reclaimed roofing foam at 1/4-1/3 the cost of virgin stock foam. Since the assembly already drys toward the interior through a Class-II vapor retarder any amount of exterior foam is fine- it doesn't have to be the IRC prescriptive R20 for dew point control at the roof deck.

    To hit code-min performance on a U-factor basis, with ~R28 in the rafter bays usually only takes 2.5-3" of continuous roofing polyiso (R14- R17), even though that is well-shy of the R49 it would take to get there on an R-value basis. But ANY amount of thermal break over the exterior of the rafters delivers a disproportionate amount of performance relative to adding more R between the rafters where it's performance is undercut by the thermal bridging of the framing.

  4. Kazio Grzyb | | #4

    Thanks, Dana. I am aware of HFO-blown cc from your prior posts and I will certainly look into it.

    Will be another 30 years before a new roof is needed, so exterior insulation will have to wait.

  5. Kazio Grzyb | | #5

    Howdy. So 2" inch of closed cell got installed along with 4" of open cell. The rafter bays were so crooked and unevenly spaced that installing mineral wool batts would have been nightmare, so we went with open cell to fill the rest of the cavity.

    Does that change anything with respect to vapor management? Like would Membrain be good idea? I suppose the 2 inches of closed cell should take care of protecting the roof deck.

    Thanks!

  6. GBA Editor
    Martin Holladay | | #6

    Kazio,
    In Zone 5, at least 41% of the total R-value of the roof assembly needs to come from impermeable layer of rigid foam.

    In your case, you have a total R-value of about R-27.8,
    and 41% of 27.8 would be R-11.4,
    but in fact you have about R-13 of closed-cell spray foam, which is more than R-11.4,
    so you are good.

  7. Kazio Grzyb | | #7

    Thank you!

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