Ice Dams
Ice dams are the large mass of ice that collect on the lower edge of the roof or in the gutters as the result of melting snow. As more melting snow (or rain) runs down the roof, it meets this mass of ice and backs up, sometimes under the shingles and into the attic or the house.
Ice damming usually occurs with a significant depth of snow on the roof. If the attic temperature is above freezing, it warms the roof sheathing which melts the snow lying on the shingles. This water runs down the roof until it meets the roof overhang, which is not warmed by the attic and will be at the temperature of the surrounding air. If the air and the overhang are below freezing, then the water will freeze on the roof surface and start the ice dam.
Attic Venting
An attic with no insulation will generally not have a problem with ice dams. The heat coming through the attic will tend to melt snow off as it lands and prevent much accumulation. A well-sealed and insulated attic will generally not have ice dams. Like the example of a detached garage, this generally results in a cool roof and no great amount of melting. Ice dams are more frequent if the roof is complicated by many valleys and dormers or there is a large roof overhang.
Ice dams will first show up where there is inadequate insulation or major air leaks. One way to find these locations is to look at the roof with the first heavy frost in the fall or a light snow. Watch where the snow melts off first and find out what is under that spot on the roof. One common sight in such conditions is a horizontal melt line across the roof of a storey-and-a-half house, where the short kneewall meets the ceiling. Other places would be beneath a roof-ducted exhaust fan or over a leaky attic access hatch. The basic relief for ice damming is to seal all attic air leaks and insulate thoroughly, the same solution as for attic condensation.
Signs of inadequate ventilation include rusty nails or rust spots on insulation caused by moisture forming and dripping off nails, frost buildup during the winter, or possibly a lingering musty smell.
If you have properly sealed the attic you should not need more attic ventilation.. In winter, the cold outside air cannot hold much humidity or carry moisture away from the attic. In summer, attic temperatures are more affected by the sun and shingle colour than by the amount of ventilation.
Recent research shows that identical attics, with one unvented and the other vented to code, have much the same humidity and temperature. Attic computer models show that attics in damp coastal climates may actually be drier with less ventilation.
Building codes require attic ventilation. Ventilation may make a difference in a borderline situation. Attic ventilation is driven primarily by wind. To ensure thorough venting, have openings at the soffits and then higher on the roof at the ridge, gable end, or high on the roof surface. The requirement for attic vent sizing is nominally 1:300 (or one square foot of vent size for every 300 square feet of attic floor area). If you wish to improve your attic venting, ensure that it is as well distributed as possible.
Attic condensation and ice damming are related. Both can be caused by warm, moist air leaving the house and entering the attic. Attics will be in good shape if there are no holes, air leaks, or bypasses from the house to the attic and there is sufficient insulation to keep house heat from escaping. If you can ensure good air sealing and insulation, the attic will remain cool and dry, as if it were outside. For example, it is rare to see moisture problems or ice damming on the roof of a detached garage or unheated barn.
What To Do About a Wet Attic
There are many signs that an attic is wet. Prolonged wetness will rot out the roof sheathing. Often this is first noticed when re-shingling. If you have ceiling leaks only in the spring, it may be that ice has been forming on the sheathing all winter and it suddenly melts when a warm spell arrives. You may see water stains or evidence of mold on the sheathing, rafters, or trusses when you are inspecting the attic. You may find the insulation has been packed down or stained by water or ice. The smell of a moldy attic will enter the house under certain weather conditions, usually in summer.
The usual response is to increase attic ventilation. . In some cases, adding ventilation will actually pull more moist house air up into the attic and make the problem worse. The best way to fix a wet attic is to stop air movement, or leaks, from the house. Once this is done, the existing ventilation is usually more than enough to keep the attic dry.
It is important to stop air leaks because a heated house is much like a chimney. Both a house and chimney are containers of warm air surrounded by cold air. Both tend to draw air in at the bottom and expel it at the top. All winter, a heated house is trying to push air through the top floor ceiling into the attic. Block up those air leaks and keep the warmth in the house to save both energy costs and damage to your attic.
Air leaks are usually found at penetrations or discontinuities. Safety regulations prevent sealing of many types of pot lights in top floor ceilings. House air is dumped into the attic through them. Choose sealed pot lights or avoid them on the top floor.
Bathroom fans need to be ducted outside. Make sure that they are properly vented. If the ducts are located in the attic, ensure that there are solid metal rather than flex duct, insulated and sloped to the outside. Do not wrap the insulation in plastic as this will trap moisture.Taping the duct joints, or sealing them with mastic, is helpful for controlling leakage.
Plumbing stacks and chimneys are often sources of air leakage. Seal these where they pass through the attic floor. For metal chimneys inside a chase or for old masonry chimneys, you may need help from an expert to ensure proper sealing and avoidance of fire hazards. Seal holes made for electrical wiring and cable installations.
There will be little air leakage in the middle of sheet of drywall or in the middle of an unbroken plaster ceiling.There may be many air leaks where partition or bearing walls meet the ceiling or around the perimeter of the house where the attic floor (or top floor ceiling) meets the outside walls.
All discontinuities should be inspected and sealed if necessary. Look for bypasses.They are major air passages from any floor into the attic. Dropped ceilings in the room below will often conceal a direct connection to the attic. Concrete block party walls between row houses often move house air into the attic.
There are several ways to check for these large and unexpected leaks. The blower door tester can pressurize the house with a big fan and amplify the leakage. Searching the attic at night for lights from below can be helpful. Scanning batt insulation for dirty areas which have been filtering the air from below is also useful, although such straining seems to occur less frequently with blown insulation. Sometimes the holes are so big that you can see into the house below.
Some houses have heating or air conditioning ducts or equipment in the attic.These can be the major source of air leakage and heat loss in the attic. Good information on how to seal and insulate these devices has been published in Home Energy Magazine, available in some libraries.
Contrary to popular belief, gutters do not cause ice dams. However, gutters do help to concentrate ice and water at the very vulnerable roof eaves area. As gutters fill with ice, they often bend and rip away from the house, bringing fascia, fasteners, and downspouts in tow.
Roof leaks wet attic insulation. In the short term, wet insulation doesn't work well. Over the long term, water-soaked insulation remains compressed, so that even after it dries, the insulation in the ceiling is not as thick. The lower R-values become part of a vicious cycle: heat loss-ice dams-leaks-insulation damage-more heat loss! Cellulose insulation is particularly vulnerable to the hazards of wetting.
Water often leaks down within the wall frame, where it wets wall insulation and causes it to sag, leaving uninsulated voids at the top of the wall. Again, energy dollars disappear, but more importantly, moisture gets trapped within the wall cavity between the exterior plywood sheathing and the interior vapor barrier, causing smelly, rotting wall cavities. Structural framing members can decay. Metal fasteners may corrode. Mold and mildew can form on wall surfaces as a result of elevated humidity levels. Both exterior and interior paint blister and peel. And people with allergies suffer.
Peeling wall paint deserves special attention because its cause may be difficult to recognize. It is unlikely that wall paint will blister or peel when ice dams are visible. Paint peels long after the ice and the roof leak itself have disappeared. Water from the leak infiltrates wall cavities. It dampens building materials and raises the relative humidity within wall frames. The moisture within the wall cavity eventually wets interior wall coverings and exterior claddings as it tries to escape (as either liquid or vapor). As a result, interior and exterior walls shed their skin of paint.
Where to look for leaks
around plumbing stacks or plumbing walls
chimneys through the attic
any light fixtures from the ceiling below
electric wiring
ducting for fans or heating systems
perimeter walls
partition walls
party walls
above pocket doors
above lowered ceilings
where the side of a cathedral ceiling meets an open attic
split level discontinuities
where additions meet an older section of the house
above rounded corners or staircases
balloon frame walls