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Best way to insulate and cover a 3:12 pitch roof in New England

lobsterdad | Posted in General Questions on

Hi Martin,

I am putting a 32′ x 13′ addition onto my home which is an hour north of Boston.  My architect has designed a 3 1/8 pitched non-vented “cold design” hip roof with 2 x 10 rafters, and R-38 closed cell spray foam insulation.  Sheathing is 5/8 cdx covered completely with Grace ice and water shield and 3 tab archetectual style asphalt shingles.

The roof gets morning sun only and my contractor is concerned that there is a potential for ice dams and eventual roof leaks, since the sun may cause some melting of snow in the morning and possible refreezing in the afternoon when the temperature drops, causing damage to the shingles.  Also he has seen in his 30 years experience that the self-sealing aspect on the nail holes through the Ice & Water Shield is not 100%.

The other concern I have is the possible toxicity and out gassing of spray foam due to the formaldehyde.  I have looked into Icynene as it claims to be non-toxic after drying, but I am not sure if that is completely true.

My questions are as follows:

1.  What would you recommend as the best system that would minimize the potential for leaks while  being safe from a toxicity standpoint if I end up using spray foam.  Also I have read that it is not ideal to use archetectual shingles on a 3 pitch roof as water does not shed as efficiently as regular shingles.  If I do go with spray foam, would open cell or closed cell be better?

2.  Would a standing seam metal roof be a better option altogether?  

Thank you!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Lobster Dad.

    Hip roofs and lower slope roofs are tricky, which is probably why your architect speced the closed-cell spray foam and full covering of ice and water membrane. The former to get as much insulation into a roof where venting isn't a good option and the latter to help prevent leaks due to the low slope. But R-38 doesn't even reach the current International Residential Code minimums for your climate zone and the roof sheathing will have no drying potential between two impermeable layers.

    Check out this article: How to Build an Insulated Cathedral Ceiling. You may want to consider exterior continuous insulation and fibrous insulation in the rafter bays. This way you'd have better thermal performance and a roof assembly that can dry inward.

  2. ssnellings | | #2

    The Western States Roofing Contractors Association (WSRCA) has recommended that architectural shingles not be specified or installed on slopes lower than 4:12 for over a decade due to ice and wind related water issues. For a detailed overview of the issue, check out the WSRCA's technical bulletin No. 2009-2 titled 'Cautions Regarding Laminated Shingles and Water-Shedding Roof Systems that are Specified on Lower Slopes'.

    At some point I need to do more research on this issue instead of just kicking out references to this one bulletin over and over again, but it convinced me very easily.

    So I would definitely recommend a standing seam metal roof if it's in your budget. To cover Brian's concern about permeability, you'll want to check with your contractor to make sure they aren't going to just cover the whole roof again with ice and water shield under the metal.

    Regarding the R-38 vs R-49 issue raised by Brian, if you're in New Hampshire I think they just moved to R-49 last year with the adoption of the 2015 IECC with amendments (before that, I think the lower half of the state could get away with R-38). That would help explain why your roof was designed to R-38 if you're in NH. If you're in MA, I'm not sure there's a good reason. However, no matter the reason, your architect should probably revise to show R-49.

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