Ice Dam It: Don’t Get Mad, Get Insulation (and Air Sealing)

December 2015
by Paul Eldrenkamp

Whenever we suffer a severe winter, our phones ring off the hook with calls from people with ice dams who are none too happy about it. By this point though damage is already done. We'd much rather people call us before they get ice dams, because the best way to deal with them is to take measures to prevent them from occurring in the first place.

People have been building wood-frame houses with gable roofs in this neck of the woods for almost 400 years now. Why do we still have problems with ice dams? Wouldn't you think we'd have solved this basic problem by now?

For the first 300 or so of those 400 years, we were using roofing materials that didn't have problems with ice dams. Thatch, for instance, doesn't leak any more with snow and ice on the roof than it does with rainwater pouring down. It leaks a little bit either way, but no more. With slate, the snow slides off before ice dams even have a chance to form. Cedar roofs are pretty robust because the wetter they get, the tighter they fit—it's harder for water to find a way through. Asphalt shingles, on the other hand, have a granulated surface which snow clings to pretty tenaciously—not at all like slate. The snow just sits up there. Unlike cedar, asphalt shingles don't fit any tighter when they get wet; they can even shrink a bit when it's cold outside. And unlike thatch, they're very thin—any water that gets through the first 1/8" is basically already on the wrong side of the shingle. So asphalt shingles—the most prevalent roofing material these days—are one of the least effective barriers against ice dams of any roofing material that's been used in our region. That's strike one.

Strike two is that we're heating our houses more thoroughly these days. We expect second-floor bedrooms to be about as warm as the first floor kitchen and living room. That has not always been the case. We're trying to maintain the upper floors of our houses in the mid- to high-60s. There's a lot more heat close to the roof.

And here's strike three: There aren't that many people in the remodeling industry who understand the difference between insulating and air-sealing. What causes the snow to melt on roofs fast enough to cause ice dams is convective heat loss, not conductive heat loss (remember the three modes of heat transfer from middle school physics?). Meaning that heat loss through air leaks is the big problem. Cellulose or fiberglass sitting loose in your attic or rafter bays may be somewhat effective against conductive losses through the ceiling plaster, but may be doing very little indeed to stop heat loss resulting from air leaks through all the penetrations in a ceiling (pipes, wires, light fixtures, heating and cooling ducts, etc).

So we have a roofing material that's really good at keeping snow on the roof; we have warm upper floors of our houses; and we don't do a good job of keeping that upper-floor heat from escaping through the roof assembly to melt the snow into water that then runs down to the eave and freezes, creating the classic ice dam (not so much an ice dam as an ice "lens", but that's a different article).

What to do? Basically, there's two approaches. One is to limit your attic to one purpose and one purpose only: to hold insulation. Air-seal the attic floor, fill the attic with cellulose, and then add some good roof vents. This works really well to keep your roof cold, ward off ice dams, and save you a lot on your heating bills—and it can be pretty inexpensive. But you can't use the attic for storage, you can't put your air conditioning equipment up there, you can't even put the Christmas tree holder up there, you can't put anything up there except insulation.

The problem with this approach is that you'd be amazed at how many homeowners in metropolitan Boston have their attics packed to the gills with stuff they can't live without. The idea of not using the attic for storage is, for many, physically unbearable. So forget about this idea, in many cases. (It's not unusual for us to look at an attic insulation upgrade where the cost of the insulation is about $2000 but the cost of the moving and storage to be able to actually put the insulation where it belongs is about $4000.)

The other approach is to put the insulation in the rafters. This can work really well, too, but it has to be done meticulously. Often it's most effectively done from above and below—meaning that a good time to insulate is when you're re-roofing. In my opinion, every roof replacement project should be treated as an insulation upgrade project, too.

The way to think about this is to ask yourself: Where is the boundary between indoors and outdoors in my house? If your attic is full of stuff, the boundary is clearly (at least, in my opinion) not at the attic floor, but at the rafter plane. So put the insulation in the rafters, but make sure you do a really thorough job of it: use enough insulation and the right kind; test the work with a blower door and infrared camera; fix any flaws you find; and test again, until you're sure it's all done right. It's a lot of work, but you only have to do it once—and then you can sleep easy no matter how much it snows.