First, Some Misconceptions
Sometimes contractors who try to prevent future ice dams may make things worse, often because they don't understand the problem and where to focus their efforts. For example, there seems to be a widespread belief that simply adding more insulation will cure any thermal ill, including ice damming. The truth is that a bad insulation job can be more hindrance than help. I've seen many homes with insulation crammed into soffits or blown into eaves in an attempt to prevent future ice dams cheaply. But this extra insulation may also increase the risk of ice problems by blocking the soffit vents and trapping heat and moisture in the attic.
Before looking at details that work, I want to look at some of the most common misunderstandings we've encountered over the past 20 years.
Myth #1: Ice dams happen only on older homes. Ice dams are often associated with older homes, but newer homes are not immune. Of the nearly 300 ice dams we removed last winter, about one quarter of them were on homes built within the last 20 years.
Myth #2: Ice dams happen only at the eaves. While most ice dams do form at the eaves, about 20 percent of the ice dams we deal with are in less obvious locations, including:
Valleys. I've seen ice dams grow 12 feet up valleys, where they force water under the shingles and valley flashing high up on the roof.
Shed dormers, roof pans, or other low-pitch areas. Ice dams can cover large areas on low-slope roofs and cause major leaks without ever forming those telltale icicles. We have removed ice dams that were invisible from the street yet covered an entire shed roof or pan area. In general, lower pitch roofs suffer more from ice dams than steep roofs do.
Roof penetrations. We see mini ice dams around all types of roof penetrations, including skylights, bath fans, chimneys, and plumbing vents. These are extra sneaky because they are virtually invisible from the ground.
Myth #3: Ice dams are related to gutters. The fact is, gutters do not cause or even increase the severity of ice dams. We remove ice dams from many homes without gutters every year, and in the past 20 years I have seen no compelling correlation between gutters and ice dams. Because ice weighs almost 60 pounds per cubic foot, however, ice dams can certainly damage gutters.
While gutters are often damaged by ice, they are not the cause of ice dams, as is commonly thought.
An ice dam without a gutter.
Another misconception is that electric heat cables can prevent ice problems. Besides being largely ineffective, heat cables consume a lot of electricity and can burn out quickly.
Myth #4: Size matters. An ice dam does not have to be big to cause big damage. Even an inch of ice can force water into the structure, especially on low-pitched roofs.
Myth #5: Self-adhering membranes will prevent leaks. Many owners believe that self-sticking bituminous eaves membranes - the various "ice shield" products - will protect their roof from ice damage. In fact, of all the homes we are called to visit because of water damage from ice dams, well over 90 percent already have membranes installed per code at the eaves and valleys. Not only is every nail hole a potential leak, but ice is strong enough to work its way up under the laps in the membrane. While self-adhering membranes reduce the severity of leaks, they do not prevent them altogether.
Myth #6: Heat cables are the answer. Electric ice-melting cables installed at the eaves are sometimes appropriate as a last resort, but only after you have taken the other measures described in this article. And be warned: Cables can also create problems. For instance, they can cause leaks if the ice around the cables melts but there is no way for water to drain to the ground. Heat cables can also dry out asphalt shingles, causing them to become brittle and crack, thus voiding the shingle manufacturer's warranty. Most heat cables last only a few seasons, giving homeowners a false sense of security. And of course heat cables at the eaves do nothing about ice further up on the roof.
Myth #7: Salt will do the trick. Sodium and calcium compounds may melt ice, but they also shorten the life of metal flashings and gutters and can damage landscape vegetation and certain types of pavers. Given the potential risks, I would never recommend using salts to remove ice dams.
Remediation: Air Sealing First
Although a surprising number of contractors neglect air-sealing, it's perhaps the most crucial step in preventing ice dams. Warm air will always find a way to get through the ceiling plane and into the attic. These leaks are called thermal bypasses, and the goal of proper air-sealing is to minimize them as much as possible. The good news is that most bypasses can be sealed with a trained eye, a can of spray foam, and a little elbow grease (see "Air-Sealing Attics in Existing Homes," 2/08). First you have to find them. The best way to detect bypasses - and indeed to figure out the overall thermal behavior of the house - is to conduct a heat loss study. This can involve blower-door tests, infrared cameras, and other specialized inspection equipment. For the client with an adequate budget, a blower-door test is a great first step. Fortunately, most bypasses should be obvious to an experienced remodeler even without these sophisticated methods.
In the attic, look for gaps around bath and kitchen exhaust fans, plumbing vents, recessed lights, plumbing and wiring penetrations, and attic hatches. Top plates at partition walls are another source of leaks. We usually pull back the insulation at these spots and seal them one at a time. On some older leaky homes, we completely remove the deteriorated insulation, then seal the leaks and reinsulate the attic. By placing a few 500-watt halogen lights in the rooms below the attic, we can quickly find the major bypasses by looking for light pouring through the gaps.
Recessed lights. The recessed ceiling lights in many older homes aren't IC-rated, so they can't be directly covered with insulation. These lights can throw off enough heat to cause snow-melting problems by themselves. At a minimum, we advise the homeowner to switch to compact fluorescent bulbs, which produce far less heat than incandescents.
The best remedy is to replace the old fixtures. If that's cost-prohibitive, we cover them with site-built rigid-foam insulation boxes made from 3/4-inch foil-faced insulation board. (These boxes are also available at some insulation supply shops.) For fire safety, we make the boxes big enough to leave 3 inches of air space around the fixture, then place them over each fixture and seal them to the top of the ceiling vapor barrier with standard silicone. Even IC-rated fixtures are seldom airtight, and can add plenty of heat to the attic, so we often cover them with insulation boxes as well.
Hvac ducts. Don't overlook ducts running through the attic. Many are so poorly insulated and sealed that they blow hot, moist air into the attic all winter. The simplest remedy is to seal the duct seams with foil tape, then wrap the ducts with foil-faced batt insulation.
Gaps around chimneys. Don't forget the space between the framing and the chimney, where fire codes require a separation. Here we use sheet metal, bent on site, to bridge the gap, then seal the joints with a firestop caulk.
Attic hatches. One place you can't seal permanently with foam is the attic hatch. Existing hatches can be retrofitted with rubber gaskets, weatherstripping, and layers of rigid insulation. When possible we prefer to install a prefabricated hatch with an integrated gasket, such as the EZ Hatch from Battic Door (batticdoor.com).