- Q.Assuming an attic is leaking
some heat that reaches the roof sheathing, causing an
ice dam, does the snow on the roof tend to act as
insulation, trapping the heat near the sheathing? Will
the melting of the roof snow be worse when there are 12
inches of fluffy snow on the roof than when there are 4
inches of snow?
Paul Fisette responds: The simple answer is
yes. Snow is an insulator. Its R-value varies,
depending on moisture content and density of the
snow granules; but on average snow has an R-value
of 1 per inch — about the same as wood.
Twelve inches of snow have roughly the same
insulating value as a 2x4 wall filled with
The worst ice dams occur when there is deep snow
followed by a period of very cold weather. To
prevent ice dams, the most important and least
expensive step is to seal all air leaks connecting
the house to the attic. It is also important to
reduce conductive heat loss from the house with
deep layers of carefully installed insulation.
Once you have minimized heat flow into the
attic, install an effective roof venting system.
Continuous soffit vents that communicate
effectively with continuous ridge vents are a good
choice for most houses. I think ridge vents that
have an external baffle are best. They provide more
reliable suction because the air stream jumps over
the baffle regardless of wind direction, creating
negative pressure over the vent.