Ice dam risk in Sioux City, IA
How often ice dams form in Sioux City
19.2 dam days a winter
High risk. On an ordinary house in Sioux City, conditions for an ice dam come on about 19.2 days in a normal winter, mostly in January and February.
Sioux City averages 36 inches of snow a year and about 58 days with snow on the ground. Most of it comes and goes within a few days. 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 Sioux City
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 9.7 | Moderate |
| Typical, some insulation, not air-sealed | 19.2 | High |
| Older or poorly insulated, warm attic | 31.2 | High |
Sealing and insulating a typical attic takes out about 9.5 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 31.2 days.
What to do about ice dams in Sioux City
- 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.
Which to spend on first: air sealing or heat cable.
Sioux City winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 7.7 | 19.8 | 7.5 |
| February | 8.7 | 14.5 | 5.0 |
| March | 5.7 | 6.5 | 1.4 |
| April | 2.3 | 1.0 | 0.1 |
| October | 0.8 | 0.3 | 0.0 |
| November | 3.2 | 2.9 | 0.7 |
| December | 7.6 | 12.9 | 4.5 |
Sioux City next to nearby towns
Sioux City ranks 14 of 41 Iowa cities here for ice-dam days, behind Mason City at 28.7 days.
- South Sioux City, NE 3 mi, 19.2 days
- Le Mars, IA 23 mi, 24.4 days
- Vermillion, SD 34 mi, 23.8 days
- Yankton, SD 58 mi, 18.8 days
- Storm Lake, IA 62 mi, 27.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Sioux City Airport, 7.2 miles from Sioux City. 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.