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Community and Q&A

Need Help with Converted Attic!

oldhousenut99 | Posted in Energy Efficiency and Durability on

Greetings All-

I am in the process of purchasing a 20 yr old colonial in Massachusetts and need some guidance regarding the roof assembly insulation. It is a production spec house with typical construction for the period. A later homeowner converted about 75% of the attic to living space and the remainder is just unvented and uninsulated crawl space. The underside of the roof is not insulated withe the exception I am assuming of the cathedralized roof in the living space. Also, there are very shallow (non-existent) eaves due to the fact that the style is early american colonial so it would be difficult to retrofit vents. The rafters are 2 x construction and with no insulation on the underside of the deck it’s not surprising that the asphalt shingles on the southern exposure are curling despite being less than 10 years old.

I am planning on installing HVAC equipment in the crawl space areas of the attic and have been planning on installing 6″ of CC SPF to the underside of the deck across the entire roof for an unvented hot roof assembly. My concern is that I just read Joe Lstiburek’s article on lessons learned from his barn retrofit and his admonishment about simply installing CC SPF to the underside (i.e. problem of thermal bridging). I do not have the inclination to install rigid foam to the top of the deck as this will incur gobs of carpentry labor expense reworking the eaves and rakes. Is it possible to use CC SPF in the rafter bays and then install 2″ foil faced to the underside of the rafters? I am also dealing with the issue of fire retardant as I am installing mechanical equipment in the unfinished portions of the attic. I assume that foil faced ISO would alleviate the need to paint the SPF or cover with drywall as an ignition barrier and offer thermal bridging protection. I understand that this is creating a vapor impermeable space in the assembly but I would assume it would not need drying. I would use dense pack with rigid foam but this will not get me to code with a net 7″ rafter.

Your help is greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    I'm sorry to say that the best solution is going to be more expensive than the fast-and-dirty solution.

    If your asphalt shingles are curling, you might consider installing rigid foam above your roof sheathing, followed by another layer of roof sheathing and new roofing. For more information, see How to Build an Insulated Cathedral Ceiling.

    If you decide to install spray foam between your rafters, followed by rigid foam on the underside of your rafters to address thermal bridging, I suggest that you install EPS foam rather than foil-faced polyiso, in case your rafters ever get wet. (The EPS will allow a little bit of inward drying).

    The assembly should be protected on the interior with gypsum drywall to reduce the risk of fire, in my opinion, although many building inspectors look the other way if you want to leave the spray foam exposed.

  2. oldhousenut99 | | #2

    Thanks for taking the time to respond Martin. So for MA I would need R-20 so one sheet of 3.3" or (2) 2". Do you need bituthane under the rigid? That's another expensive line item. If I use blown cellulose under the deck, do I need drywall over the HVAC equipment by code? Pretty sure with batts I wouldn't.

    Funny thing is BSC put taped rigid foam on the roof of their barn and kept cc SPF in the rafters. Seems to contradict their admonition to let assemblies dry. Wonder if they considered this?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "So for Mass. I would need R-20 so one sheet of 3.3 in. or (2) 2 in."

    A. Yes, that's what the code requires when you put rigid foam above the roof deck and air-permeable insulation under the roof deck. If you install 4 inches of rigid foam, it has to be either polyiso or XPS. If you install EPS, you would need about 5 inches.

    Q. "Do you need bituthane under the rigid?"

    A. No, although you need an air barrier somewhere in the assembly. More and more builders are using taped Zip sheathing as an air barrier -- or you can tape the seams of the existing OSB or plywood sheathing, or you can tape the seams of the rigid foam and use the rigid foam as your air barrier.

    Q. "If I use blown cellulose under the deck, do I need drywall over the HVAC equipment by code?"

    A. All building codes are subject to local interpretation, and the interpretation of codes varies widely, especially concerning fire safety issues in attics. You'll have to direct your question to your local building department.

    Q. "BSC put taped rigid foam on the roof of their barn and kept closed-cell spray polyurethane foam in the rafters."

    A. They weren't the first ones or the last ones to do it, either. As long as the roof sheathing is dry when these insulation layers are installed, the risk of this assembly is quite low.

  4. oldhousenut99 | | #4

    Thanks again Martin. I appreciate you taking the time to answer my questions. Your articles are great. It's a shame more tradesman don't read this magazine. When I speak with contractors about this stuff, 90% of them don't know what I'm talking about.

  5. Expert Member
    Dana Dorsett | | #5

    BTW: 6" of closed cell polyurethane is both expensive, and one of the least-green foam options, since it's blown with a high greenhouse gas potential agent (HFC245fa, about 1000x CO2). EPS and polyiso are blown with pentane (7x CO2) and much more benign.

    If you take the approach of foam above the roof deck, EPS has a stabler (and rising) R-value at the lower temperature ranges in MA, whereas polyiso has a sharp knee in the de-rating curves with a rapidly falling R when the mid-layer of the foam hits 25F or lower. But you can stack it with a couple inches of polyiso direcly on the roof deck, with a couple inches of EPS above that and hit true R20 performance in 4" without much derating in a MA climate.

    Also note, MA has several vendors of reclaimed roofing foam that can take substantial sting out of your wallet pain, selling at 1/4-1/3 the price of virgin stock. Insulatation Depot on Waverly Street in Framingham is the best known, but there are others. Search the local cragislist materials section for "rigid insulation" every couple of days- you'll find at least a couple of the others.

  6. oldhousenut99 | | #6

    Dana- Thanks for the input. I don't want to spend for the bituthane. Can I do this from bottom up? 7 1/4" dense pack cellulose, 1/2 OSB (existing), 2" ISO, 2" EPS, 1/2" Zip sheathing taped, shingles.

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