Most homeowners are surprised to learn that ice dams are the result of heat trapped in the attic. In a poorly insulated home, heat escapes through the ceiling into the attic and without proper ventilation, it remains trapped in the attic. This trapped heat melts accumulated snow on the roof, even though outside temperatures are below freezing. The water from this melted snow travels down the roof until it arrives at the much colder edge of the roof and refreezes forming an ice dam. As the snow continues to melt, additional water pools behind the dam working its way under the shingles and into the attic and wall cavities. While you may see some structural damage, the most extensive damage is often unseen. Moisture in the attic and walls creates a fertile breeding ground for toxic mold, wood rot and insect infestation.
To minimize your home’s exposure to ice dams, you must have adequate insulation in the attic to control heat loss and provide proper ventilation to remove heat and moisture from your attic. Your attic insulation should have an “effective R-value” of at least R-49. Attic ventilation requires a properly sized and balanced system of both intake and exhaust venting. The heat and moisture from bathroom exhaust fans must be vented out of the attic.