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Community and Q&A

Ice dam solution? Problem not caused by excessive heat loss in conditioned space

schreib77 | Posted in General Questions on

It appears that after reading “Prevent Ice Dams With Air Sealing and Insulation” in GBA, 2013 Martin H., I have a problem that is not the norm for ice dams, kind of an outlier.

LOTS of drifted snow, Minnesota roof with only 3:12 pitch, 10 ft of gutter for this small rooflet, NE side of home, gutter plugged with leaves stopping water flow– bad things for ice dams. Roof leaked into closed space above porch, through porch cedar ceiling.

On the positive side: ice and water barrier up six feet from edge of roof, new home, 2 to 3″ sprayed foam above all conditioned spaces with balance from blown fiberglas, low slope area is small(< 100 sq ft), mostly over a front porch with no conditioned space below, home blows 1.0 on blower door test so real tight home, soffit vents under entire lineal distance where ice dams reside, open air porch below extends 8 ft under roof from eave. The other side of the house, SW facing, has no ice dams OR snow. It melts very quickly with all the sun exposure. Here is my conclusion minus a solution: I figure that the sun is melting the heavy snow pack and it is simply running down and making an ice dam. So, instead of being heated from the home below it is warmed during sunny days from above while building ice dam at night. The sun however is at a very low angle, nearly parallel to this roof even at noon so I view this as a barely plausible explanation! My only other explanation might be that with all the sun exposure on the OTHER side it warms the entire attic during the day and even the “cold” NE side gets heated enough so that, in fact, it IS heated from below. Your ideas on the cause and solution are much appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    There are a few odd details here. You're in Minnesota. It's odd that you are proud of the fact that on one side of your roof, all the snow melts quickly. What you want in Minnesota is a roof where the snow never melts. Melting snow (or a roof where you can see the roofing) is not a good sign.

    The best roof has 12 inches of snow and stays that way.

    If the problem roof faces northeast -- generally a cool, shady orientation -- it's unlikely that solar melting is the origin of the problem.

    The first thing to do is probably to permanently remove the gutter.

    If you have a 6-foot-wide band of Ice & Water Shield, and you still have water coming through your porch roof, you've got a very big ice dam.

    This is hard to diagnose without a site visit. If you can provide more information or photos, it would be helpful.

  2. Jon_R | | #2

    A picture would help.

    With vents just below the ice area and sun heating the upper portions of the roof, you are going to have a significant temperature differential (causing high up melting and low down re-freezing). You need to reduce this differential. I'd start with measurements and blocking the vents below the ice area. If that isn't enough, then perhaps a fan (only during critical periods) to partially equalize these two temperatures.

    Worst case, you can add surface heat to prevent refreezing (eg smart heat cables).

    Elaborating on measurements: if you have high/low ΔT at night, then look into house heat entering the attic. On a sunny morning, then direct heating of the roof. Sunny afternoon, then indirect heating of the NE roof via the attic air.

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