Snow melt

Walking the kids to school and back one passes all manner of roof ice and roof insulation problems. The neighborhood is marked by gutter freeze, and big honking icicles. What people tell the world about their homes, and particularly their attics through the snow melt.....if only they knew.

Frozen gutters. Extensive icicle growth.
Icicle formation requires that the outside air be warm enough to allow the escaped heat to melt the snow, yet cold enough to refreeze the water before it falls to the ground. The less effective the roof insulation, the warmer the roof, and greater the snow melt. Super insulation will minimize melt and icicles by cooling the roof surface. There will still be melt during warm sunny days, but then icicles are impossible.
January 21: neighbor

January 21: neighbor

January 22: neighbor

The lower roof melt areas here are wider than the higher ridges. Guessing, the insulation in the attic is on the floor and the area between the roof rafters are uninsulated. The heat that does escape into the attic travels to the roof and passes more easily between the rafters (the wider areas) than through the somewhat more insulating rafters (the thinner areas).
January 22: neighbor


I would guess this to be the opposite. Insulation is in the roof rafters. The rafters here are the weak link and show the greatest melt.
January 22: neighbor

Another example of attic floor insulation, shown with the wider melt areas, demonstrated with a kind of shingle lithography. The thermal properties of the roofing shingles comes right through.
January 22: neighbor

The roof area to the right of the downspout in this December 7 photo is a new addition with a cathedral ceiling, and one assumes extensive insulation in the rafters. You can see that the rafters on both sides of the skylight are doubled up as is the practice, and we can guess that there are two recessed lights, one on each side of the skylight. The roof to the left of the downspout is a cold attic insulated on the floor and sealed from the cathedral ceiling. The diagnal telltail? No idea.
UPDATE: Read the reader comment below for a brillaint sugestion for the cause of the diagnal telltail.


This is our roof in a photo taken January 21. Like a Westminster Kennel Club winner, this blue ribbon roof has a nice shiny coat of snow, and is completely house-broken with only a few stray icicles. A roof like this should have a very small range of icicle producing temperatures.