Ice dam risk in Rexburg, ID
How often ice dams form in Rexburg
42.8 dam days a winter
Severe risk. On an ordinary house in Rexburg, conditions for an ice dam come on about 42.8 days in a normal winter, mostly in January and February.
Rexburg averages 70 inches of snow a year and about 125 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 Rexburg
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 21.0 | High |
| Typical, some insulation, not air-sealed | 42.8 | Severe |
| Older or poorly insulated, warm attic | 70.0 | Severe |
Sealing and insulating a typical attic takes out about 21.8 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 70.0 days.
Preventing ice dams on a Rexburg 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 392 kWh a winter in Rexburg, roughly $67 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Rexburg winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 19.8 | 30.5 | 12.5 |
| February | 10.7 | 28.6 | 10.0 |
| March | 5.3 | 24.9 | 6.1 |
| April | 2.0 | 4.3 | 0.7 |
| October | 1.6 | 0.8 | 0.1 |
| November | 10.5 | 9.9 | 2.7 |
| December | 20.2 | 26.0 | 10.6 |
Rexburg next to nearby towns
Rexburg ranks 1 of 26 Idaho cities here for ice-dam days, the worst in the state.
- Ammon, ID 25 mi, 36.4 days
- Idaho Falls, ID 26 mi, 36.4 days
- Blackfoot, ID 52 mi, 20.1 days
- Jackson, WY 56 mi, 50.0 days
- Chubbuck, ID 70 mi, 20.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Ashton 1N, 23.9 miles from Rexburg. 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.