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What Causes Ice Dams and Icicles?

December 20, 2012 / Posted by in Hints & Tips

Here in northern Michigan we sometimes get ice dams.  They are thick bands of ice along the eaves of buildings that can cause thousands of dollars of structural damage every year.  Some results of ice dams are water-stained ceilings, damaged roof shingles, sagging gutters, and peeling paint.

Proper air sealing, insulation, and attic venting are the best ways to eliminate the problem.

Heat and warm air leaking from the living space below melt the snow, which trickles down to the colder edge of the roof and refreezes.  Every inch of snow that accumulates on the roof adds to the problem.  This keeps more heat in the attic, which in turn makes the roof even warmer and melts more snow.

Contrary to popular belief, gutters do not cause ice dams.  However, as gutters fill with ice, they often detach from the house bringing the fascia and downspouts down too.

Roofs leak water onto attic insulation.  Wet insulation doesn’t work well and even after it dries, the R-value is not as high.  The lower the R-values, the more heat loss.

Water often leaks down inside the wall, where it damages wall insulation and causes it to sag, leaving voids at the top of the wall.  Moisture gets trapped in the wall cavity between the exterior plywood sheathing and the interior vapor barrier.  With time, your framing members may decay.  Mold and mildew may form on the surface of the walls.  Paint, both exterior and interior may blister and peel.

Wall paint doesn’t usually blister or peel while the ice dams are visible.  Paint peels long after the ice dams have disappeared.  Water from the leak infiltrates wall cavities.  When building materials get wet it raises the humidity inside the wall.  The moisture tries to escape through the walls and as a result, wall paint begins to peel.

The way to stop ice dams from forming is to keep the entire roof cold.  This means you need to block all air leaks leading to the attic from the living space below, increasing the thickness of insulation on the attic floor, and have a continuous soffit and ridge vent system in place.

Shoveling snow and chipping ice from the edge of the roof is dangerous to you and can damage your shingles.  Work to prevent the ice dams and you and your roof will be healthier and safer.

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