Icicles on the edge of a roof are caused by snow melting off above a warm attic, running down the length of the roof, and freezing again on the cold roof edge.
Usually, the melted snow runs down underneath the remaining snow cover and freezes at the roof overhang, which does not have the warm attic space beneath it.
A more serious result of this kind of runoff is the creation of an ice dam. In this case, instead of freezing into icicles, the ice forms a ridge on the roof that causes water to back up behind it. Often this backed-up water will work its way under the shingles and leak into the house.
Both icicles and ice dams can be prevented by providing good ventilation to the attic.
Inlet air vents along the soffit and an open space between the attic insulation and the underside of the roof are essential. This allows cold air to move along the underside of the roof and up through the gable end or ridge vents , keeping the entire roof area at approximately the outdoor temperature.
This type of ventilation also helps keep the attic cool in the summer and will prolong the life of shingles.
It should be noted that without this ventilation, even an attic with R-30 insulation can lose enough heat to cause icicles and ice dams. Still, we agree with the R-30 recommendation for attic insulation.
The vapor barrier should face down toward the living area on the first layer of insulation. Any additional layers of insulation should not have a vapor barrier. When adding additional insulation, be sure not to block off the ventilation from the soffit area. A space of two inches between the insulation and the roof is recommended.
For more information on ice dams and vapor barriers, send a stamped, self-addressed envelope to: Energy Conservation Service, PO Box 100-C, Clifford, PA 18413. Larry Wilson Laurie Graham, Energy Conservation Service Clifford. Pa