Ice Dams in Minnesota: Why the Fix Is in the Attic

In short: An ice dam forms when the upper part of your roof gets warm enough to melt snow while the eaves stay frozen, and in a Minnesota house the heat doing the melting comes from inside. Warm air leaks into the attic through holes in the ceiling, warms the roof deck, and the meltwater refreezes at the cold edge. Raking, heat cables, and steaming clear the ice for a while. Sealing the attic floor and insulating over it removes the heat, and that is the fix that holds.

Every January the calls start, and the question is usually the same: do we need a roofer? Most of the time the roof is doing its job. The trouble is the air coming up through the ceiling under it.

Why does my roof get ice dams?

The University of Minnesota Extension has published the clearest explanation we know of, and it comes down to two temperatures. For an ice dam to form there has to be snow on the roof, and at the same time the higher parts of the roof surface have to average above 32 degrees while the lower parts average below it. Snow melts on the warm section, water runs down under the snow blanket, reaches the cold section at the eaves, and freezes. Each day adds a layer. Once the ridge is tall enough, water pools behind it, finds a seam in the shingles, and comes into the house.

The cold half of that equation takes care of itself here. The normal January high in the Twin Cities is 23.6 degrees and the normal low is 8.8, so an eave that hangs out past the heated walls has no trouble staying frozen. The normal winter drops about 51 inches of snow on the metro, and that snow sits on the roof for weeks at a time. Snow is a blanket, and it keeps the cold air off the shingles while heat from below works on them.

So the only question that matters is what is warming the upper roof. Extension’s answer is that it is primarily heat flowing from the house. Sun can do it on rare days, but the dam that builds all winter is built by your furnace.

Where the heat actually comes from

Heat gets from your living room to your roof deck three ways. It conducts through thin insulation. It rides warm air that leaks up through the ceiling. And it radiates off warm things sitting in the attic, like a furnace duct, a bath fan housing, or a chimney. Extension says that in many homes air leakage is the major mode, and our thermal camera agrees.

The leaks have a name in Minnesota’s energy code. An attic bypass is any air passageway between a conditioned space and an unconditioned attic. In a real house that means the gaps along the top plates of every interior wall, the hole cut for the plumbing stack, the chase around a flue, the box the bath fan sits in, the wiring holes, the attic hatch, and every recessed light. Warm air at 70 degrees is lighter than the air in the attic, so it rises through those openings all day and all night. Blown insulation does nothing to stop it. Air goes through fiberglass about as easily as it goes through a screen door.

Recessed lights are the worst of the bunch, because they put a hot metal can inside the insulation and surround it with a gap. On a thermal scan from a kitchen job this past January, the can lights read 92.7 degrees at the fixture while the surface a few feet away read 66.2. Each of those fixtures is a small chimney into the attic. We walk through more scans like it in our post on what our thermal camera found in three Minnesota houses.

This is why the state code sequences the work the way it does. Minnesota’s residential energy rules say attic insulation shall not be installed unless accessible attic bypasses have been sealed. The code does not want more insulation blown over an open hole, and neither do we.

Why the roof-side fixes keep you busy and keep failing

Everything sold for ice dams at the hardware store works on the symptom, and some of it is worth doing while you plan the real repair.

Raking the roof removes the snow, and with no snow there is nothing to melt. Extension points out that rakes and push brooms can damage the roofing, and anyone who has done it from a ladder in January knows the other risk.

Heat cables keep a few channels open through the dam so water can get off the roof. They do not change the temperature of the deck above them, they run all winter, and the dam forms right around them.

Steaming or cutting channels through an existing dam gives water a way out. Extension says the channel becomes ineffective within days. It is a way to stop a leak this week.

A new roof is the one that disappoints people most. Minnesota’s residential code requires an ice and water barrier under the shingles, running from the edge of the eave to a point at least 24 inches inside the exterior wall line. That membrane is why most dams do not leak into the house. It is a backup, and a good one. It does nothing about the warm deck, so the dam forms on the new roof exactly where it formed on the old one. If a roofer tells you the roof is sound and the barrier is in place, believe them, and look up.

Powered attic fans come up every winter, and Extension is direct about them: mechanical attic ventilation is not a recommended solution to ice dams in Minnesota. It can create attic moisture problems and pull a negative pressure on the house, which drags more warm air up through the very bypasses that caused the dam.

What fixing it in the attic looks like

Our work on an ice dam house runs in a set order.

We start with the camera. From about October through April we bring a thermal camera on attic estimates, and on a cold day the bypasses show up as warm plumes on the ceiling from below. We also go up through the hatch, measure the insulation depth in several places, and look at the underside of the roof deck for frost and staining, which tells us where the warm air has been landing.

Then we air seal. The insulation gets pulled back along the top plates and the penetrations, and each opening gets closed with foam or sealant. Recessed lights either get an airtight cover built over them or get swapped for sealed fixtures. Bath fans get sealed at the housing and ducted all the way outside, because a fan that dumps into the attic is both a bypass and a moisture problem. The hatch gets weatherstripped so it closes tight. Extension’s long-term advice is to make the ceiling airtight first and then consider adding insulation, and that is the sequence we follow.

Then we insulate to the state number. Minnesota’s energy code calls for R-49 in the ceiling in both of the state’s climate zones, and a lot of attics between Coon Rapids and St. Cloud sit far below that after decades of settling. Blown fiberglass or cellulose over a sealed attic floor is the right tool in most open attics. Our attic insulation page covers how we pick between blown-in, batts, and foam.

The eaves get special attention, because they are where the dam lives. Trusses on an older house pinch the insulation down to a couple of inches right over the exterior wall, which is the exact spot you want it thickest. New Minnesota houses are required to have a six-inch energy heel for that reason. On an existing house we cannot raise the roof, so we use the material with the most R-value per inch in that pinch, which is closed-cell spray foam, and we set baffles so the soffit air can still travel up the underside of the deck. Once the heat flow is cut, Extension notes, only a small amount of roof ventilation is needed to keep the deck even.

Some roofs have no attic floor to seal. A story-and-a-half house, or a bonus room over the garage behind knee walls, has sloped ceilings with a few inches of batt between the rafters and no room to add depth. Those get foamed from the inside against the roof deck or the knee wall assembly, which turns the sloped section into a sealed, insulated roof. Our spray foam insulation page explains where closed-cell earns its place in those spaces.

The houses around here that get them worst

Three patterns we see over and over on ice dam calls.

The 1960s through 1980s ramblers and split-levels that fill Coon Rapids and the rest of Anoka County were built with a few inches of attic insulation and open bypasses everywhere. They are the classic ice dam house, and the easiest to fix, because the attic floor is wide open.

The 1980s and 1990s two-stories and split entries across Champlin and Maple Grove have deeper attics but the same unsealed bypasses, usually with a row of recessed lights in the kitchen and baths.

And the 1995 to 2010 two-stories along I-94, from Rogers up through Albertville and St. Michael, bring the bonus room over the garage. Those rooms sit behind knee walls under their own roof plane, and the dam often shows up on that one section of roof while the main roof stays clean.

If water is coming in right now

Dry it first. Extension’s guidance is to dry wet ceilings, walls, and insulation immediately, because moisture from an ice dam leads to mold, and to hold off on repairing the drywall until everything is dry. Their other piece of advice is the one we would underline: fix the interior damage and the heat loss together, or the damage comes back next winter.

A thermal scan on a cold day with snow on the roof is the best diagnostic there is, because the roof is showing you the leak pattern.

Frequently asked questions

Will adding attic insulation stop my ice dams?

Only if the attic floor is air sealed first. Insulation slows heat conduction but lets warm air pass straight through it, and air leakage is the main heat source behind most Minnesota ice dams. Sealing the bypasses and then insulating to R-49 keeps the roof deck cold, which is what stops the melt.

Do heat cables fix ice dams?

No. Heat cables melt channels through the ice so water can drain, which can prevent a leak, but they do nothing about the warm roof deck that is melting the snow in the first place. The dam forms around them every winter. They are a stopgap while you fix the attic.

Is it worth raking my roof after every snowfall?

As a short-term measure, yes, since a roof with no snow cannot form a dam. Rakes can damage shingles and the work is risky from a ladder. If you find yourself raking every storm, that is a sign the attic is leaking heat, and sealing it means you can put the rake away.

Does a new roof prevent ice dams?

No. Minnesota code requires an ice and water barrier under the shingles that extends 24 inches inside the exterior wall, and that membrane is why most dams do not leak into the house. It is a backup. The dam still forms, because the heat warming the deck is coming from the attic below.

If your roof grows the same ridge of ice every winter, send us a few photos and the age of the house and we will come look at the attic with the camera.