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Roof assembly advice

grantmathews | Posted in Energy Efficiency and Durability on

Hi, my name is Grant and I am looking for advice on roof assemblies for a house I am planning to build this summer in Anchorage Alaska (Climate Zone 7). The house is small, approximately 900 sf of living space on top of 900 sf of garage/shop.

We are going with the REMOTE wall assembly with exterior rigid foam insulation attached to the outside of the wall sheathing. I have spent some time talking with the Cold Climate Housing Research Center and I have a framer on board who has built a couple of homes in Alaska using this approach.

However, I am a bit stumped on how best to construct the roof, specifically in regards to the insulation and weatherproofing. It is a low slope shed roof design with a 2:12 pitch. The current plan is to go with 24″ parallel chord trusses with 2 foot overhangs.

We have considered three options:

1) Blown in cellulose (18″ min) within the truss cavity with an air gap above the insulation.

Pros: cheapest option, standard construction practice
Cons: concerned that we would not get sufficient airflow (maybe a cupola?), makes recessed lighting more difficult to install and a potential issue without very careful detailing.

2) Exterior rigid foam (8″ min) with an additional layer of roof sheathing to fasten the roofing material. 

Pros: Can carry the benefits of the REMOTE wall around the entire building envelope, sheathing can dry to the inside, no penalty for recessed lighting or other drywall penetrations.
Cons: This results in a super thick fascia that looks a little strange. Added material cost for the additional layer of sheathing, no way for the top layer of sheathing to dry.

3) Closed cell spray foam insulation (8″ min) inside the truss cavity to the underside of the roof sheathing. 

Pros: tight building envelope, no penalty for recessed lighting or other drywall penetrations.
Cons: higher cost, no way for roof sheathing to dry

I’ve included a PDF that shows a cross section of the building with option 3 (the one I’m leaning towards).

For roofing material I’m considering EPDM or a fluid applied membrane.

Any input is appreciated – additional pros/cons of the options I’ve listed or other considerations I should account for. Also if anyone has any experience with fluid applied roofing I’d love to hear whether you would recommend it in this scenario.

Thanks in advance,
Grant

 

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    #1 is BY FAR the greenest solution, and you don't have to push the local contractors up the learning curve ladder. With 24" trusses it's easy enough to put 22" of cellulose in the

    #3 is by the LEAST-green (and riskiest) solution due to the extremely high CO2e footprint per R, even for HFO blown foam, andextremely limited drying capacity for the roof deck.

    https://materialspalette.org/wp-content/uploads/2018/08/CSMP-Insulation_090919-01.png

    #2 isn't any better than an HFO blown #3 if using 2lbs roofing polyiso, but is modestly better if using 1lb density foil faced polyiso or Type-VIII (1.25lbs density) roofing EPS.

    >"...makes recessed lighting more difficult to install and a potential issue without very careful detailing. "

    Recessed lighting is the dodo-bird of interior lighting. Surface mount LEDs work, and are much easier to detail than recessed cans if the 1950s lounge lizard look is still a thing in your neighborhood. But cove lighting and wall washes do a much better job at ambient light, delivering high visual acuity at lower luminosity by the elimination of glare.

    1. Andrew C | | #2

      Dana - I'm going to shamelessly steal your final paragraph above wrt recessed lighting. Beautiful summary. Thanks!

    2. grantmathews | | #3

      Thanks for the input! Any resources you would recommend for thoughtful lighting design?

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