Stopping Ice Dams: Super-Insulated Roofs and Snow Melt Systems
Super-insulated roofs can be an effective way to control heat loss and airflow through the roof and slow the formation of ice dams. These assemblies typically consist of closed-cell spray-applied urethane foam insulation installed between the roof rafters, typically creating R-40 to R-60 assemblies. These assemblies do not rely on ventilation to keep the roof surface temperature near that of ambient outside air; rather, they increase the thermal performance of the roof structure, thereby reducing heat loss through the roof. The closed-cell foam also acts as an air barrier, limiting the amount of conditioned air that reaches the underside of the roof deck.
By significantly reducing heat loss and airflow through the roofing system, these assemblies can reduce energy consumption. Payback of energy savings compared to the initial installation cost will vary depending on the building design and climate.
The insulation must be installed continuous with wall insulation to prevent air leakage at the wall/roof transition.
Note that snow is an insulator; heavy snowfall acts as an insulating blanket above the roof deck. This blanket can decrease the effectiveness of super-insulating by adding R-value above the deck, thus raising the temperature of the roof deck and melting the bottom of the snow pack.
It is also worth noting that closed-cell foam products have low vapor permeability. Where roof leaks occur, the foam can delay drying and promote deterioration of the roof deck over time.