26 Mar

Why the Ice? Hot Roof vs. Cold Roof

It has been a cold winter, one that is hopefully over. As a result a lot of people have been experiencing more icicles or ice damming on their roofs than usual as a result of the sun melting the snow on roofs. This happens even though the air outside is still so cold which causes the water to re-freeze before it hits the ground or rolls off the roof causing the icicle or ice dam.

RoofsThe terminology is a bit awkward because of the ventilation which occurs under the roof. A hot roof has no ventilation under the roof or air circulation (the roof is cold) while a cold roof has ventilation under the roof (and is hopefully cold but in most cases is hot). What affects the roof the most is the space which is conditioned, or where the air barrier is under the roof.

In a cold roof situation, you try to air seal the ceiling of your house so no warm or heated air (the conditioned air) escapes into the unconditioned or cold air attic space. In a house with a hatch or a stairway to an attic, this process is virtually impossible to seal the floor under the attic ceiling 100 percent because of the hole going to the attic.

The fix for this and keeping a cold roof situation is to insulate the air (walls and ceilings) where the conditioned hot air meets the unconditioned cold air. You can do this by insulating the stairway and I prefer doing the walls with spray foam and then building an insulated doorway at the top of the stairs. If you can’t do spray foam, then in the next best case scenario you can dense pack cellulose the walls and install the same insulated hatch door.

In the hot roof situation, where the roof is cold and hopefully the conditioned space in the attic is warm, what you do is move the thermal barrier and also the air barrier to the underside of the roof. Here and depending on what your philosophy is for keeping ice dams and icicles off your roof you want to spray foam the underside of your roof and the gable ends of your attic to make the attic now warm. You want to stop all air movement.

If the ceiling above the living space is the highest in your house, cannot be sealed off, and a floor exists in your attic this might be the only way to seal off the roof area of the house from the roof surface – causing the snow on the roof to melt and create icicles and ice dams. After you put about a 2-inch layer of foam on the underside deck of the roof you can dense pack cellulose the rest of the roof to save on the amount of money its costing you to insulate your roof (with cellulose you get more R value for your buck).

Remember, though, always insulate from the edge of the soffits up to and including the ridge vent, sealing off all the vents and stopping the air from moving. If you are getting thermal bridging – which is where you can see the location of rafters in the roof (from the outside of the house where the heat is leaking) – you can always install foam board on the rafters before you drywall the attic. This new “hot roof” won’t allow the snow on your roof to melt, thus stopping the icicles and ice dams and fixing your problem.

Dr. Spray Foam (Mark Gugino)