A lot of people are getting ice dams this year… a lot of people get ice dams every year.Why? In most cases it is due to a lack of insulation, air-sealing, or both. (Spoiler: If you want to read the best, and most detailed, piece on this I’ve seen, read Martin Holladay’s article on Green Building Advisor.)

If you call a roofer they will usually tell you to install “ice shield” under the shingles as a solution. As you’ll see, that’s kind of like telling you to take medication because you keep getting food poisoning at your favorite restaurant. Better, of course, is to stop eating there.

Ice dams occur when snow on the roof heats up, melts, and then freezes. Eventually the water cannot flow down past the ice and instead you basically get a puddle, which backs up under the shingles and eventually makes its way into the house. An ice shield will (usually) protect the portion of the roof with ice shield on it, but that water can then back up higher and you get a leak in a new spot. That’s not a solution.

Some roofers suggest using heating cables which warm the lowermost portion of the roof. The theory is that the snow will melt, make its way into the gutter and thereby solve the problem. As you can see in some of the photos below, that’s no solution either.Why is the roof getting warm enough to melt the snow? Obviously there is some amount of heat from the sun that may cause the snow to melt, but that’s usually not what’s doing it. The melt is a typically result of heat from the home going up and warming the underside of the roof. This melts the snow sitting on the shingles (as compared to solar heat that will

When snow appears in “stripes” that’s a sign of missing/fallen/insufficient insulation.

melt the top-most snow) which then tries to work its way down to the gutters. At this point it will often meet ice (the gutter isn’t heated) and thus begins your ice dam.

A hot roof is bad

The answer therefore isn’t an ice shield but instead keeping your roof cool. I know, it sounds strange that in the winter we should be concerned about keeping our roofs cool but that’s what you need to do in order to avoid ice dams.

Roofs, by typical design (and there are, of course, many exceptions to this), are kept cool with vents. Usually this means either a soffit vent (where the eaves hang over the edge, meeting the gutters) often in conjunction with a ridge vent (at the apex of the roof). Another frequent alternative are gable vents, in the side of the home into the attic.

Even if these aren’t blocked up, your roof is likely getting hot due to a lack of insulation, failing (out of place) insulation, air leakage or a combination of all three.

All of these images were taken recently in Arlington, MA. What you can see in most photos are bare spots where the roof has been heated up enough that the snow has melted. In many cases there are “stripes” where, most likely, a fiberglass batt installed along the ridgeline has either fallen out or was installed so poorly that hot air is leaking up and around it, melting the snow.

Fixing these isolated spots will help, but perhaps not solve, the ice dam problems. Instead what is needed is better/more insulation and, very likely, air barriers.Additional insulation will keep the heat in your home, where you want it, and slow its movement up to the roof. In that respect it doubles your savings: you keep the heat inside and you don’t need to pay someone to deal with the ice dam, ever.

Using an appropriate air barrier also slows down the movement of the warm interior air up to and out through the roof. (If you’re not familiar with the difference between insulation and air-sealing or weatherization you may want to see this previous post.)

What air leaks do you have from your home to your attic and ridgeline? Potentially hundreds, and too many to go into here. However, a few include things like attic hatches, knee walls, recessed lights, the space between interior walls (visible in the attic floor), chimney chases, electrical and plumbing holes, interior soffits, and many more.

The good news is that you can seal many of these yourself (please do NOT seal chimney chases or recessed lights yourself unless you know about the fire and code safety requirements). Until sealed, these leaks are literally sending your money into the attic. You paid for that warmth in your house – keep it there. If you don’t, you’re also paying for the people who will take care of the ice dam and then the person to repair any damage it caused to your home.

As for insulation, this is going to slow down heat transfer. Inevitably some heat will make it into your attic. You want to slow/stop that loss of heat as much as possible. However, please don’t use fiberglass batts. Many use formaldehyde as a binder which offgasses over many years (notably Johns Manville, sold at Lowes, does not) and I don’t recall ever seeing batt insulation installed properly, other than in a laboratory setting.

Instead take advantage of the MassSave rebates: 75% up to $2000. They’ll insulate your home (make sure they air seal first) saving you thousands of dollars, and increasing your comfort, for years to come.

If you’ve made it this far, take a few minutes to read Holladay’s piece, which goes into greater detail. You can also take a look at the slide show below which includes what appear to be numerous air sealing and insulation problems in local homes.

Professionals that deal with existing ice dams are making lots of money recently.