Ice dam risk in Cambridge, MN
How often ice dams form in Cambridge
40.2 dam days a winter
Severe risk. On an ordinary house in Cambridge, conditions for an ice dam come on about 40.2 days in a normal winter, mostly in January and February.
Cambridge averages 44 inches of snow a year and about 104 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 Cambridge
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 15.2 | High |
| Typical, some insulation, not air-sealed | 40.2 | Severe |
| Older or poorly insulated, warm attic | 71.5 | Severe |
Sealing and insulating a typical attic takes out about 25.0 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 71.5 days.
What to do about ice dams in Cambridge
- 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 451 kWh a winter in Cambridge, roughly $77 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Cambridge winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 5.4 | 28.9 | 13.1 |
| February | 10.0 | 25.1 | 10.4 |
| March | 7.4 | 17.9 | 5.2 |
| April | 4.0 | 3.1 | 0.4 |
| October | 0.5 | 0.2 | 0.0 |
| November | 4.5 | 8.4 | 2.4 |
| December | 12.3 | 20.5 | 8.6 |
Cambridge next to nearby towns
Cambridge ranks 13 of 103 Minnesota cities here for ice-dam days, behind Hermantown at 52.9 days.
- North Branch, MN 14 mi, 36.9 days
- East Bethel, MN 14 mi, 41.4 days
- Ham Lake, MN 21 mi, 39.9 days
- Andover, MN 22 mi, 39.9 days
- Elk River, MN 23 mi, 42.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Cambridge, 0.5 miles from Cambridge. 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.