Ice dam risk in Hermantown, MN
Ice dams in Hermantown, MN: how often
52.9 dam days a winter
Severe risk. On an ordinary house in Hermantown, conditions for an ice dam come on about 52.9 days in a normal winter, mostly in January and February.
Hermantown averages 90 inches of snow a year and about 137 days with snow on the ground. Snow that lies that long is what turns a little attic heat into a ridge of ice at the eave. The conditions that make a dam need all three at once: snow, cold at the eave and heat above it.
What your attic does to that number in Hermantown
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 18.7 | High |
| Typical, some insulation, not air-sealed | 52.9 | Severe |
| Older or poorly insulated, warm attic | 95.7 | Severe |
Sealing and insulating a typical attic takes out about 34.2 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 95.7 days.
Preventing ice dams on a Hermantown roof
- Air-seal the attic floor: the gaps around the hatch, light fixtures, plumbing stacks and the top plates of interior walls. This is the fix that removes the cause, and it usually costs less than a season of contractor ice removal.
- Rake the bottom three or four feet of roof after any storm that leaves more than about six inches, from the ground with a roof rake.
- At the next reroof, ask for ice-and-water membrane under the shingles from the eave to at least 24 inches inside the wall line. It does not stop a dam; it stops the leak when one forms.
- Heat cable is worth pricing here even on a sealed house, along the eave and in the gutters and downspouts it drains into.
Running 60 feet of 5 W/ft cable whenever snow lies below freezing would use about 616 kWh a winter in Hermantown, roughly $105 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Hermantown winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 16.8 | 30.7 | 14.4 |
| February | 15.1 | 28.0 | 12.1 |
| March | 12.8 | 26.8 | 8.7 |
| April | 9.1 | 9.8 | 1.7 |
| May | 0.8 | 0.2 | 0.0 |
| September | 0.1 | 0.0 | 0.0 |
| October | 2.7 | 1.6 | 0.2 |
| November | 14.1 | 13.0 | 4.0 |
| December | 18.7 | 27.1 | 11.9 |
Hermantown next to nearby towns
Hermantown ranks 1 of 103 Minnesota cities here for ice-dam days, the worst in the state.
- Duluth, MN 5 mi, 38.4 days
- Superior, WI 11 mi, 38.4 days
- Cloquet, MN 13 mi, 52.2 days
- Hibbing, MN 53 mi, 49.9 days
- Grand Rapids, MN 68 mi, 49.9 days
Where these numbers come from
NOAA 1991–2020 climate normals from Duluth, 2.8 miles from Hermantown. A dam day is a day with snow on the roof and freezing at the eave while heat or sun melts the snow above it. This describes a normal winter, not this one.