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When a roof is properly vented, the shingles will often be close to the outdoor temperature, so winter snow won’t melt on the roof until the outdoor air temperature rises above freezing.
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However, ice damming—which can lead to mold, mildew, or rot—can sometimes occur with improperly vented roofs in the winter. If the attic insulation is packed tightly at the eaves, heat from the living space slowly moves through the insulation and warms the roof deck slightly. The snow directly over the warm part of the roof melts and runs down the slope. When it hits the part of the roof above the overhang (which is colder) it refreezes, forming an ice dam on top of the roof. This can take place under a blanket of snow, so you may not see it, but the dam can allow melted snow to back up under the shingles, and leak into the attic or living space. Sometimes, ice damming is accompanied by large icicles hanging down from the eaves.
The solution is to insert special baffles between the insulation and the roof deck. This allows air to circulate under the deck, keeping it cold and, thus, preventing the heat from the living space from melting the snow. When baffles are in place, heat from the living space still slowly enters the attic, but it dissipates into the attic air, and is carried out through the vents.
From: The Healthy House, 4th edition, published by The Healthy House Institute.
For more information about proper ventilation, visit the Home Ventilating Institute (HVI).
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