Ice dam risk in Evanston, WY
How often ice dams form in Evanston
32.8 dam days a winter
High risk. On an ordinary house in Evanston, conditions for an ice dam come on about 32.8 days in a normal winter, mostly in January and December.
Evanston averages 50 inches of snow a year and about 91 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 Evanston
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 16.2 | High |
| Typical, some insulation, not air-sealed | 32.8 | High |
| Older or poorly insulated, warm attic | 53.5 | Severe |
Sealing and insulating a typical attic takes out about 16.6 days a winter here. It does not move the risk band, but it shortens every dam. An older house with a warm attic should expect closer to 53.5 days.
What to do about ice dams in Evanston
- 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 298 kWh a winter in Evanston, roughly $51 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Evanston winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.3 | 25.8 | 10.3 |
| February | 9.1 | 21.6 | 7.8 |
| March | 7.0 | 13.0 | 3.6 |
| April | 4.3 | 2.0 | 0.4 |
| May | 1.0 | 0.1 | 0.0 |
| June | 0.1 | 0.0 | 0.0 |
| September | 0.6 | 0.1 | 0.0 |
| October | 2.9 | 0.8 | 0.2 |
| November | 5.9 | 4.6 | 1.3 |
| December | 9.2 | 23.3 | 9.1 |
Evanston next to nearby towns
Evanston ranks 2 of 11 Wyoming cities here for ice-dam days, behind Jackson at 50.0 days.
- North Ogden, UT 52 mi, 37.2 days
- South Ogden, UT 52 mi, 6.7 days
- Ogden, UT 52 mi, 37.2 days
- Hyrum, UT 52 mi, 24.8 days
- Farmington, UT 53 mi, 20.4 days
Where these numbers come from
NOAA 1991–2020 climate normals from Woodruff, 20.3 miles from Evanston. 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.