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Community and Q&A

Ice Dams In New Construction

pamountainbiker | Posted in General Questions on

I am in the process of building a house. The house is insulated to R49 in the attic (was just insulated and inspected), I also inspected the ventilation and based on my calculations we have more than enough and the soffit vents are open. Also, my can lights and penetrations are all foam sealed. I got a variety of smaller 6-8 inch icicles and a couple larger ones this weekend. We got a late winter storm this weekend that had a mix of snow and sleet, fluctuating temps, etc. Noticed these formed, but was surprised. They even formed off my garage which is insulated but unconditioned. I noticed some of the other homes in the development also got them but not to the same extent. The large ones formed off my two roof valleys. Is this normal for new construction? The house is unoccupied and indoor temp is 65 degrees. How concerned should I be? I don’t remember getting these in prior storms this winter, but the house heat probably wasn’t on. Not sure why I’d get them on my unconditioned (north facing) garage though.

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  1. Expert Member
    Akos | | #1

    Is the upper level a full story or is it a half story with lots of jogs/sloped ceiling?

    My guess you have some larger interior air leaks in the areas you see ice dams. This the bane of half story construction, you need to find and seal these up. No amount of insulation will prevent ice dams if the ceiling/walls are leaky.

    1. pamountainbiker | | #3

      It is actually a full second story, but there is a knee wall there that is entirely spray foamed.

  2. GBA Editor
    Kiley Jacques | | #2

    I'm giving your question a bump. It seems the usual suspects, inadequate insulate and significant air leaks, are not necessarily at play here. Do you have an extra layer of protection in the form of Ice & Water Shield on top of the drip edge? I know it is recommended that roof valleys get extra waterproofing, as they can trap a lot of snow.

    1. pamountainbiker | | #4

      Yes, indeed I do have ice and water shield.

  3. charlie_sullivan | | #5

    We had some weird weather recently: unseaonably warm follow by snow, followed by rapidly dropping temperatures. I got some small icicles where I never see them on a well insulated and air-sealed building. I think it's just the roof surface warmed by ambient air causing snow melt which was then caught by the rapidly dropping temperature. Your roof is overcomplicated, and that is worrisome, but the icicles in this odd weather would not worry me.

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