Preventing Ice Dams on Roofs is Important!
Take preventive measures and stop ice dams before winter.
Every year we get a couple big snow and ice storms. Inches of snow on the ground translate into inches of snow, ice, and eventually ice dams on many roofs. Ice dams cause attic flooding and water damage in walls and ceilings.
In turn, water in the attic destroys the effectiveness of insulation and helps create the basic pre-conditions for mold in the attic, ceiling and walls.
All of this can be prevented if you take the necessary precautions to stop ice dams. It’s not all that complicated and everybody can do it.
Causes
Ice dams start forming after several days with outside temperatures in the low 20 degrees. Warm air rises into the attic through cracks, crevices and insufficient insulation and warms the roof deck. The warm roof deck heats the shingles and begins to melt the snow.
The snow melts from the bottom up and the water runs down the slope of the roof under the snow. When it gets to the soffit over-hang or gutter, the temperature of the water rapidly drops below 32 degrees and becomes ice. This process continues and ice builds up to become an ice dam that stops the flow of water backing it up the slope of the roof. Eventually, the water finds a loose shingle or a crack in a valley and leaks down into the attic.
Good roofing jobs will have a waterproof shingle underlayment, which provides an additional layer of protection underneath the shingles.
Unfortunately, during a really cold winter the water can back up beyond the width of the shingle underlayment and find a path into the attic. While this is going on, the relatively warm, moist attic air is chilled by the cold roof deck. When the dew point is reached, moisture is released from the air and becomes an additional source of water damage.
Prevention
The only way to stop roof damming is to stop the warm air from entering the attic and keep the underside of the roof deck at the same ambient temperature as the outside air. This is accomplished in three ways: sealing, insulation and ventilation.
Sealing
Seal all openings rising into the attic. Bathroom fans used to be vented into the attic. They are a huge source of warm moist air. Upgrade old bathroom fans and vent them through the roof.
If you have whole-house fans or other major intrusions into attic space, enclose them with rigid foam boxes.
Use at least 3-inch foam. Make certain that the opening to the attic has a good gasket around the edges and cover the top of the attic door with 3- or 4-inch rigid foam board.
If you have warm air ducts running through the attic, seal the seams with shiny metal tape and wrap a metal insulation blanket such as Astrofoil or Reflextix around the ducts.
Also wrap insulation around any warm pipe running through the attic.
Ventilation
Proper ventilation is critical to keeping the underside of the roof deck cold. Your house should have both soffit vents and ridge or pot vents. If you do not already have rafter baffles (air channels) protecting soffit vents and directing the air up the roof, install them. Every soffit vent requires a rafter baffle.
Insulation
The attic floor should be insulated to R-49. Most of us do not have nearly that amount. To figure the amount of insulation you presently have, measure the depth in inches. The approximate R-value of fiberglass batt insulation is 3.2 per inch. The R-value of high performance fiberglass batts is approximately 3.8 per inch. Loose-fill fiberglass is 2.5 per inch. The R-value of loose-fill cellulose is 3.5 per inch.
To determine the R-value of your insulation, multiply its depth by the R-value of the insulation. If you have 5 inches of fiberglass batt insulation, multiply 5 by 3.2. This means that the present R-value is approximately R-16.
The approximate thickness R-49 fiberglass batt insulation is 15-inches; high performance fiberglass insulation is13-inches; loose-fill fiberglass is 20-inches; loose-fill cellulose is 14-inches. If you are adding batt insulation, use unfaced batts and be sure to snug the insulation up to the rafter baffles. Lay the different layers of batts across, not parallel, to one another to help seal the insulation and cover the rafters.
Winter won’t wait. The time to stop ice dams is now.
