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Community and Q&A

Old house, ice dam resolution

Syau | Posted in Green Building Techniques on

Hello There,

I have read with great interest many of articles and conversation with fellow readers. Here is a brief description of my situation.

I own half of a 1891 ballooned-framed duplex located in Saratoga Springs, NY. The building has a low slope roof with parapet front and side walls, and drainage in the back into gutter. Since it was built, the house has suffered from ice dams. In fact, when I purchased the house two years ago and performed renovation, the back side framing lumber had significant rot.

The last couple of winters I had water leaking into the interior, ruining drywall. This winter, I have decided to permanently solve the problem.

The roof is not insulated, asphalt with silver coating applied a few years ago. Attic had crawl space about 2′ in height at the front of the house, to 4″ in the back. There is some blown in cellulose on attic floor. There are gable vents on front and back attic walls of the house.

Here we the options I am considering:

1) hybrid attic, where conventional vented attic (closed cell foam 1″ attic floor for air seal, 20″ of blown in, 6″ air space), and where there is insufficient height, unvented attic ( closed cell foam to fill attic space). The back gable vents will be closed, gable vents added to side wall, and a copula built in the middle of my somewhat flat roof.

2) convert to unvented roof. Add polyiso 5-6″on top of existing roof, seal with rubber membrane. Add blown in under the roof deck ( with some kind of fabric to hold it in place) to reach total R value of at least 50. Seal attic vents. Closed cell spray foam on attic walls.

A. What do you think about 1 vs 2
B. for 2. Can I add polyiso on top of existing asphalt? Do I need to add vapor barrier? If so where?
C. For 2. Should I seal the ceiling-attic transition to remove airflow? I know there shouldn’t be vapor barrier but I am not sure about air barrier.
D. For 2, it has been recommended to add venting on top of the rigid insulation to remove hear build up from thick snow. Any ideas how this can be done? I have parapet wall on three of the four sides.

Sorry about the long email. I look forward to your thoughts.


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  1. GBA Editor
    Martin Holladay | | #1

    Q. "What do you think about 1 vs. 2?"

    A. Go with 2.

    Q. "Can I add polyiso on top of existing asphalt?"

    A. Yes, although stripping the existing roofing to inspect the roof sheathing is a better way to go.

    Q. "Do I need to add vapor barrier?"

    A. No. The new polyiso is an air barrier, and so is the new roofing.

    Q. "Should I seal the ceiling-attic transition to remove airflow?"

    A. Yes.

    Q. "It has been recommended to add venting on top of the rigid insulation to remove hear build up from thick snow. Any ideas how this can be done?"

    A. This type of roof assembly (for a low-slope roof) is an unvented assembly. For more information, see Insulating Low-Slope Residential Roofs.

  2. Syau | | #2

    Martin, Thank you for your response.

    More clarification around air sealing between ceiling and attic. Why is this necessary? I thought the unvented roof dries to the interior, if there is no air circulation between the attic and rest of living space how would that work?

  3. GBA Editor
    Martin Holladay | | #3

    If you do a very good job of air sealing on your attic walls, you're right. You wouldn't need to air seal the leaks in the ceiling, because the ceiling is (in theory) entirely within your home's thermal envelope.

    However, I'm guessing that access is difficult, and that you will find it hard to air seal your attic walls. That's why it never hurts to seal the leaks in the ceiling.

  4. Expert Member
    Dana Dorsett | | #4

    To hit R50 above the roof deck would take 9" of polyiso at a cost of about $5 per square foot.

    The 6" above the roof deck would still run ~$3 per square foot, but it's not clear that fattening that out to R50 with fiber blown in netting from inside a cramped crawl space may end up being substantially more expensive than $5/square foot for the same R value. If you have sufficient space above the roof to accommodate the full-R, get it quoted both ways. I've seen a project in recent years where even closed-cell spray polyurethane under the roof deck came in much cheaper than fiber in netting when the under-deck space while, cramped, wasn't even as awkward as your stated 2', let alone the 4" height at the taper.

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