Ice damming is the formation of ice at the roof eaves. It can damage roofing and lead to roof leaks.
Ice damming occurs in cold climates and seasons. Sunlight warms the rooftop during daylight hours, causing snow and frost to melt, flow down to the cold eaves, refreeze, and form icicles. This process is called the freeze-thaw cycle. When ice builds up on the eaves over time, water from newly melted snow and frost can become trapped behind the ice. The water can back up under the shingles and possibly into the home, damaging roofing, sheathing, framing, and drywall, paving the way for roof leaks. If a roof isn’t constructed with the right materials to weather the freeze-thaw cycle, it will be prone to ice damming.
Heat buildup within the attic exacerbates ice damming; while the sun is heating the exterior side of the roof, heat buildup within the attic heats the interior side of the roof. Insufficient insulation and poor air sealing allow heat to build up within the attic. As heat builds up within the attic, it causes snow on the rooftop to melt and contribute to ice damming. Ice damming is also caused by leaky ductwork, poor attic ventilation, and inadequate roofing. However, even if the builder insulates, air seals, seals ducts, ventilates, and installs roofing materials perfectly, a home may still experience ice damming due to the natural freeze-thaw cycle. The goal is to minimize, rather than eliminate, ice damming.
Cause 1:
Insufficient insulation
Insufficient insulation in the attic floor allows warm air from the living space to move up into the attic and build up within the attic space. As heat from the warm air builds up within the attic, it causes snow on the rooftop to melt and contribute to ice damming. Several common mistakes during installation can lead to an insufficient amount of insulation in the attic bays. For instance, an insulator may inadvertently leave some areas of the attic entirely uninsulated. Batts may be installed with gaps, leaving portions of the attic bays uninsulated. Or, the attic may be left with areas where the insulation layer isn't sufficiently deep or dense. Eaves and mechanical areas can be challenging to insulate, so they’re often insufficiently insulated. When an HVAC system is heating the home, poorly insulated attic bays allow heat from the living space to move up into the attic and build up within the attic space. As heat builds up within the attic, it causes snow on the rooftop to melt, potentially leading to ice damming.
Solution
Install insulation at the correct depth and density in all attic bays. Pay special attention to eaves, making sure the insulator walks all the way into the attic to insulate them. It's also important to check mechanical features, such as wires, ducts, and mechanical platforms, to make sure there's a sufficient amount of insulation under them. Finally, check challenging areas, such as slopes, steps, bays, bows, and gables, to make sure they're sufficiently insulated.
Cause 2:
Poor air sealing
Unsealed penetrations in the attic floor allow warm air from the living space to move up into the attic and build up within the attic space. As heat from the warm air builds up within the attic, it causes snow on the rooftop to melt and contribute to ice damming.
Solution
Completely seal all penetrations in the attic floor, such as lights, registers, and chases, to prevent warm air from moving up into the attic.
