With cold weather bearing down on the northern half of the United States, it's a good bet that most homeowners and contractors hope this winter won't deliver a repeat of the storms that caused an estimated $2 billion in property damage last year. But not Steve Kuhl. The Minneapolis design/build remodeler has created a thriving side business that sees ice and snow as a profit center.
Last year, his Ice Dam Company removed ice dams from 300 houses, and over the past six years he has retrofit 50 homes to solve ice dam problems. That experience has taught Kuhl what works and what doesn't.
Ice dams form when heat from the house gets into the attic, warms the roof deck, and melts snow in contact with the roof. Water then flows to a cold area (usually the eaves) and re-freezes. This process repeats until the resulting dam traps water on the roof, where it can leak into the home. Kuhl says that 20% of the ice dams he sees are on homes built in the last 20 years. "There's no excuse for that," he says. And contractors who address the problem often make things worse because they don't understand the impact of small details.
Ice dams aren't always obvious. Ice can grow six feet up a roof valley, where it forces water up under the shingles. On shallow roofs, ice can cover the entire space without forming icicles at the eaves. And spaces around skylights, plumbing vents, and the like tend to be poorly insulated, making them likely spots for ice to form.
Then there's attic frosting. Moisture trapped in the attic can form frost on the roof deck, which eventually melts and causes mold and rot. This insidious problem often goes unnoticed until the damage has been done.
You can minimize the chance of ice dams and attic frost by keeping the attic and roof deck as cold as the outside air. You can also help prevent attic frost by keeping moisture out of the attic and giving any moisture that does get in a way out. That means insulating and air-sealing the attic floor, and installing plenty of attic ventilation. Sounds simple, so why is it so often done badly?
Insulation and Air Sealing
One reason is the misconception that more insulation cures all thermal ills. But insulation has to be installed correctly, and even the best installation won't keep the attic cold if you don't stop warm air from leaking into the attic. Here are some common insulation and air sealing problems.
¦ Blocked vents "I've seen many homes where fiberglass was crammed into soffits or sprayed into eaves," says Kuhl. This extra insulation can block soffit vents, trapping heat and moisture in the attic. If you go into the attic, turn off the lights, and can't see light coming in through the eaves, the vents are probably blocked.
¦ Recessed lights Even with proper insulation, recessed ceiling lights can throw off enough heat to warm the roof. Kuhl recommends building rigid foam insulation boxes over each light and adhering them to the ceiling vapor barrier with silicone sealant. (Most construction adhesives will melt rigid foam.)
¦ Ducts Even insulated ducts should be wrapped with secondary insulation. Kuhl says kraft-faced batts, wrapped around the ducts and taped at the seams, do a good job.
¦ Shed dormers Some older homes have shed dormers with 2x4 rafters, making it impossible to get enough R-value from fiberglass. Kuhl recommends using closed cell sprayed urethane, which gives an insulating value of R-7 per inch and is also great at sealing air leaks.
¦ Cathedral ceiling roof penetrations Again, Kuhl fills gaps around skylights and other penetrations with spray foam, even if the rest of the ceiling has fiberglass, because fiberglass will settle over time.
¦ Leaky attic hatches The ubiquitous attic hatch leaks like a sieve. To solve the problem, install a prefabricated hatch with integrated gasketing. If that's not feasible, at least spend the time to build a quality hatch on site that includes weather stripping and rigid insulation.
¦ Chimney chases Fire codes require chimneys not be in contact with combustible framing, so most framers just leave a gap where the chimney passes through the ceiling frame. To keep heat out, this gap should be bridged by a sheet metal flange (in the case of a bare flue) and filled with expanding foam insulation
¦ Box vents Kuhl finds that homes with box vents have fewer ice dams than homes with soffit and ridge vents because box vents are less likely to clog after a heavy snowfall. Gable end vents are even better. "The gable vents you see today are usually fake. I would install real gable vents in addition to ridge and soffit vents, as it's tough to have too much ventilation," he says. One place where a vent is needed but often overlooked is where a shed dormer meets a vertical wall.
¦ Membranes There's a popular notion that ice and water membrane installed under the shingles at the eaves will prevent leaks in the case of an ice dam. Not necessarily. "About 75% of the ice dams we saw this winter were on homes with ice and water membrane installed to code," says Kuhl.
¦ Other penetrations Air leaks around bath fans, recessed lights, plumbing, and wiring penetrations. Where the insulation was seriously deteriorated, Kuhl has had the insulator remove the existing insulation from the attic so that he can see all ceiling penetrations. He says that anyplace you can see light coming in from the house below is a gap that needs to be sealed.
Many homes have minimal attic ventilation. The International Building Code requires a square foot of vent area for every 150 square feet of attic, but Kuhl recommends more. "Meeting code is like getting a D on a test." he says.
One place a vent is needed but often overlooked is where a shed dormer meets a vertical wall. (Products made for this purpose include DCI Products' SmartVent and Cor-A-Vent's Roof-2-Wall vent.) It's easy to miss but is a crucial step toward keeping ice off these roofs.
Complex roofs increase the likelihood of damming because every valley and flashing pan is a possible warm spot. And if you insulate with fiberglass batts, a complex roof requires more cuts and seams-spots where installers will most likely get sloppy and leave gaps.
Ice dams are also more likely with small soffits. That's because small soffits leave less room for insulation along the eaves, and the vents tend to be smaller and more likely to clog with insulation, cobwebs, and even multiple coats of paint.
On shallow pitches, the roof surface is crucial. Older homes often have asphalt rolled roofing. If possible, replace this with reinforced single ply roofing, which is less prone to leaks.
Of course you can follow the above advice and still have an ice dam under certain circumstances. Kuhl offers the following advice to fellow professionals: "Don't ever say you can prevent ice dams with 100% certainty. Say you will minimize their likelihood."
The only sure way to prevent an ice dam is to have the snow removed from the roof. And if an ice dam does form, don't try to remove it with a pick, as you will only damage the roof. Instead, use a steamer. "We have been using commercial steamers for over 20 years for the removal of ice dams," says Kuhl. "They're very effective and totally safe."
–Charles Wardell is a freelance writer based in Tisbury, Mass.