©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
During snowy winters, many northern homes are plagued by ice dams. If your house suffers from wet ceilings during the winter, you may be ready to call up a contractor. Be careful, though: since most contractors don’t understand the causes of ice dams, they often suggest the wrong solution.
Ice dams form when a home’s escaping heat warms the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and melts the underside of the snow layer on the roof. Water trickles down the roof until it reaches the cold roofing over the eaves, where it freezes. After a while, the ice at the eaves gets thicker and thicker, forming an ice dam. Eventually, water backs up behind the ice dam. If the water reservoir is large enough, it can back up under the roof shingles and damage ceilings. (Image #4, below, depicts the complicated shape of a typical ice dam more accurately than the simplified drawings in Images #2 and #3.)
The four possible solutions to ice damming are:
While the first two of these solutions can reduce or eliminate the problem, the last two solutions are the equivalent of waving a white flag and admitting defeat.
Most ice dams are caused by flaws in a home’s air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both.. If escaping indoor air finds its way to the underside of the roof sheathing during the winter, the heated air raises the temperature of the sheathing. That’s bad.
Ice Dam Basics 
One energy expert who made a career of correcting ice-dam problems was the late Tony Woods , a Canadian physicist and founder of CanAm Building EnvelopeExterior components of a house that provide protection from colder (and warmer) outdoor temperatures and precipitation; includes the house foundation, framed exterior walls, roof or ceiling, and insulation, and air sealing materials. Specialists in Mississauga, Ontario. After diagnosing problems in hundreds of homes, Woods knew from experience that most ice dams were caused by air leaks.
I interviewed Woods a few years ago for an article in Energy Design Update. “You can’t say to the consumers, ‘Will you please look in your attic?’ — because they don’t want to look into the attic,” Woods told me. “Who does the consumer go to when he has a problem? It’s always the roofer. The roofer will replace any rotten wood and try to talk the consumer into buying a new roof.”
Woods recalled one home that was plagued by leaks every winter. “The lady had called three roofers in just two or three years,” Woods told me. “The first roofer persuaded her to replace the roof.” The following winter, however, her ceiling was once again wet.
“The second year, she brought in a second roofer. He said the first roofer hadn’t ventilated properly, and he built a second roof over the first roof. But she still had the same problem. Then, when the third roofer came on, I was called to look at the house. There were pot lights galore. … When we said that the problem is not the roof, the lady burst into tears.”
Unless you have a blower door handy, the only way to locate air leaks into an attic is to crawl up and look around. “Most of the sealing can be handled effectively with two-component polyurethane spray foam,” Woods told me. “Usually you have to move some of the insulation aside to expose the top plates. You can seal ducts by spraying, in most instances. Occasionally we will cut pieces of drywall to box the can lights. Also, more than 50% of exhaust fans are not connected to the outside, so we extend the exhaust ducts.” Although Woods often advised homeowners that they needed more insulation, he told me that “most of our work is stuffing and foaming holes.”
Once you’ve plugged your air leaks, check your insulation levels. The latest version of the International Residential Code requires R-49 ceiling insulation in climate Zones 6, 7, and 8. (That area includes northern Idaho, northern Utah, Wyoming, South Dakota, Minnesota, Wisconsin, northern Michigan, upstate New York, Vermont, New Hampshire, and Maine.)
In these areas, ceilings need a minimum of 14 inches of fiberglass batts, cellulose, or open-cell spray foam. If you’re using blown-in fiberglass, you’ll need about 20 inches to achieve R-49.
In climate zones 4 and 5 — an area that includes northern Arizona, northern New Mexico, and Tennessee — you’ll need a minimum of R-38 insulation in your ceiling. That means at least 11 inches of fiberglass batts, cellulose, or open-cell spray foam, or about 15 inches of blown-in fiberglass.
In homes where there isn’t enough room to get R-38 or R-49 at the perimeter of the attic, the best thing to do is to install as much closed-cell spray polyurethane foam as the space permits. In some cases, it may be necessary to install additional rigid foam insulation on top of the existing roof sheathing. (That can only be done if you are replacing the roof.)
Two more points about attic insulation:
Once you’ve done your best with steps 1 and 2, it’s worth considering step 3: ventilation.
In the world of ice-dam prevention, ventilation is capitulation. By recommending ventilation, a builder is saying, “I wasn’t able to include enough insulation to prevent the roof sheathing from being warmed by escaping building heat. So I guess I’ll use another method to cool the roof — I’ll ventilate the underside of the roof with exterior air.”
Although it is an admission of failure, this type of roof ventilation often makes sense. It’s a kind of insurance.
When using roof ventilation to address ice dams, remember:
In the past, code-minimum insulation requirements were woefully inadequate, and almost every cold-climate home leaked enough heat to generate ice dams. Many building inspectors, noting that attic insulation was code-compliant, falsely concluded that the only way to stop ice dams was by improving ventilation.
In recent years, building codes have raised the bar on attic insulation. These days, if your ceiling is airtight and is insulated to the latest building code requirements, ice dams are far less likely.
Wayne Tobiasson, a researcher at the Cold Regions Research and Engineering Laboratory in Hanover, New Hampshire, has for many years advised builders to use attic ventilation to address ice dams. Recently, however, Tobiasson has modified his advice. “I have backed off of some of my statements about ventilating in cold regions,” Tobiasson explained at the 2010 Westford Symposium on Building Science. “As we increase the amount of insulation in roofs, there is less and less need to ventilate.”
Although so-called hot roofs (roofs without any ventilation) can work well, it’s important to note that ice dams can still form on a hot roof if the snow is deep enough. (Very deep snow acts like insulation. If your roof is covered with two feet of fluffy snow, the bottom of the snowpack is insulated from cold outdoor temperatures. That raises the chance that melting will occur.)
So, if your climate is very snowy, you probably want to stick with a cold (ventilated) roof.
For more information on this issue, see All About Attic Venting .
There are lots of roofers out there with a sure-fire cure for ice dam problems: just cover your entire roof with Ice & Water Shield.
Run, don’t walk, away from such a roofer.
These are the guys who have completely thrown in the towel. They have no plan to seal air leaks into your attic. They have no plan to improve your insulation level. They’ve given up on ventilation. In fact, they have no plan whatsoever to prevent ice dams. This is what they’re saying: “Yup, your roof is going to get an ice dam. Maybe a big one. We can’t plug your energy leak, so we’ll just let the ice dam develop. The Ice & Water Shield will probably keep your ceiling dry.”
That said, Ice & Water Shield is relatively cheap insurance. Used properly — extending from your eaves to a point that is 3 feet higher than the plane of your exterior wall — Ice & Water Shield will limit damage from ice dams that form due to unusual weather conditions or some idiot who disturbed your attic insulation.
But remember: ventilation and rubberized membranes should be the last weapons in your arsenal. First, seal your air leaks and beef up your ceiling insulation.
[Author's postscript: The topic of ice dams, like many other building science topics, is multifaceted, and a blog-length article can't pretend to be comprehensive. There's always more to say on the topic, and the discussion that follows in the comments below is informative. Particularly pertinent are Bill Rose's observations on roof geometry (Oct. 1 comment) and the Oct. 12 comments by John B. and Brian T. concerning the heat flow from a masonry chimney to the roof sheathing in an unconditioned attic. Keep those comments coming!]
Last week’s blog: “Radiant Barriers: A Solution in Search of a Problem”