Ice dam risk in Ottumwa, IA
How often ice dams form in Ottumwa
11.9 dam days a winter
Moderate risk. On an ordinary house in Ottumwa, conditions for an ice dam come on about 11.9 days in a normal winter, mostly in January and February.
Ottumwa averages 24 inches of snow a year and about 38 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 Ottumwa
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 5.8 | Moderate |
| Typical, some insulation, not air-sealed | 11.9 | Moderate |
| Older or poorly insulated, warm attic | 19.5 | High |
Sealing and insulating a typical attic takes out about 6.1 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 19.5 days.
What to do about ice dams in Ottumwa
- 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.
Which to spend on first: air sealing or heat cable.
Ottumwa winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 7.1 | 17.5 | 6.4 |
| February | 5.8 | 8.7 | 2.8 |
| March | 3.0 | 3.7 | 0.7 |
| April | 1.4 | 0.5 | 0.0 |
| October | 0.5 | 0.2 | 0.0 |
| November | 1.6 | 2.1 | 0.4 |
| December | 4.2 | 5.4 | 1.7 |
Ottumwa next to nearby towns
Ottumwa ranks 39 of 41 Iowa cities here for ice-dam days, behind Mason City at 28.7 days.
- Oskaloosa, IA 22 mi, 11.9 days
- Pella, IA 37 mi, 14.7 days
- Newton, IA 57 mi, 19.6 days
- Kirksville, MO 57 mi, 7.9 days
- Coralville, IA 63 mi, 14.9 days
Where these numbers come from
NOAA 1991–2020 climate normals from Ottumwa Ind Airport, 5.5 miles from Ottumwa. 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.