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Community and Q&A

Ice dams in Montana

user-7022518 | Posted in GBA Pro Help on

I am posting for my sister who lives in Whitefish Montana. They just completed a renovation including a new roof and experienced ice dams the first winter 🙁 The plan is to take down the drywall and address the problem with the insulation. The insulation installer used closed cell on part of the underside of the roof but did not go up to the ridge and used fiberglass batts above the ceiling. The theory is that moisture and warm air is entering space through the ceiling and finding its way to the ridge where it meets the cold air from the soffit vents and having a condensation party.  The moisture then runs out the baffles. There is also snow melt 3 feet on either side of the ridge. I understand that closed cell has been called a “moisture trap” on this forum but the proposed solution is to extend the closed cell on the ridge. I was wondering if a product like Intello would be helpful. I should mention that there are recessed lights installed in the ceiling which could be a contributor.

Are there alternatives to the closed cell solution? They would like a permanent solution not a bandaid. I know the roofing contractor could have put insulation on the exterior of the roof but that didn’t happen so they can’t do anything about that except shake a fist at the Big Sky.

Also if someone could tell me what the actually R value of the closed cell is during a cold Montana winter (I thought it was like polyiso did not perform as well in cold climates).  Cheers, Lisa

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  1. T_Barker | | #1

    I think you're getting too complicated. The attic is too warm and the snow is melting and then freezing on the roof, especially at the eaves and the ridge.

    You probably need more insulation in the ceiling and better air sealing. Even then you may need more ventilation above the ceiling for the size of the roof.

  2. walta100 | | #2

    When you say “ice dam” the problem in this photo is what pop into my head. Your 3 paragraphs of word do not seem to relate to ice dams in any way.

    Ice dams accrue because of poor roof venting and massive air leaks. Only after you fix those problems does it make sense to insulate.

    Does your sister have a vented attic? 1 square inch for vent for every 300 square feet of roof?
    Have you had the house blower door tested?
    Does your sister have insulation on the bottom of her roof sheeting and her attic floor?


  3. GBA Editor
    Martin Holladay | | #3

    To prevent ice dams:

    1. You have to address all of the air leaks in the ceiling. Most ceilings are leaky. This work is best performed with the help of a blower door. The work is called "blower-door-directed air sealing."

    2. Once the air sealing work is done, you need adequate insulation -- in Montana, that means a minimum of R-49 insulation -- over the entire heated ceiling, not just halfway up the ceiling.

    For more information, see these three articles:

    "Prevent Ice Dams With Air Sealing and Insulation"

    "Ice Dam Basics"

    "Henri Fennell’s Advice on Cathedral Ceilings"

  4. user-7022518 | | #4

    Thank you for the helpful replies!

    Martin--yes there is R49 insulation over the entire the ceiling. I will recommend the blower door directed air sealing.

    Walter--thank you for the specific recommendation on venting and for a blower door test.

    Do you think the Intello would resolve the air sealing as well as the issue of moisture rising through the ceiling?

    Do you think completing the closed cell insulation up to the ridge where it is missing would resolve the problem ? This is what they are planning to do.


  5. GBA Editor
    Martin Holladay | | #5

    Q. "Do you think the Intello would resolve the air sealing as well as the issue of moisture rising through the ceiling?"

    A. First of all, it's important to understand that the issue of "moisture rising" has nothing to do with vapor diffusion. It has to do with air leakage (exfiltration). The moisture is piggybacking on the exfiltrating air.

    Intello can be used as an air barrier, but so can ordinary drywall. The air leaks through cracks, penetrations, and seams between different materials -- not through the drywall or the Intello. What you need is an intelligent air sealing expert, not a material.

    Typical ceiling air leaks happen at electrical boxes, recessed light fixtures, partition intersections, and through hidden airflow paths that are hard to describe.

  6. Jon_R | | #6

    > extend the closed cell on the ridge.

    Sounds like you have a vented partial attic and that shouldn't be combined with spray foam under the rafters.

    If you still have ice dams after fixing air sealing and insulation, you could ventilate the heat away. With balanced mechanical ventilation if you can't increase passive ventilation.

  7. Jon_R | | #7

    A clarification to #6: insulation between rafters is beneficial in reducing ice dams even when you have a vented attic (it can significantly reduce heat flow to the snow). But no need to use spray foam when other insulation is much cheaper.

  8. Expert Member
    Peter Engle | | #8

    Yes, either spray foam or other insulation between the rafters can help to reduce heat flow to the snow on the roof, but you're still hemorrhaging heat from the house out through the ridge.

    Money spent on sealing the air leaks through the ceiling will more than pay for itself over time, and that should be the first priority.

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