Ice dam risk in Portage, MI
How often ice dams form in Portage
18.7 dam days a winter
High risk. On an ordinary house in Portage, conditions for an ice dam come on about 18.7 days in a normal winter, mostly in January and February.
Portage averages 41 inches of snow a year and about 60 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 Portage
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 9.9 | Moderate |
| Typical, some insulation, not air-sealed | 18.7 | High |
| Older or poorly insulated, warm attic | 29.7 | High |
Sealing and insulating a typical attic takes out about 8.8 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 29.7 days.
What to do about ice dams in Portage
- 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.
Portage winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 15.0 | 21.2 | 7.6 |
| February | 9.7 | 16.5 | 5.4 |
| March | 3.7 | 6.0 | 1.4 |
| April | 0.6 | 0.5 | 0.0 |
| November | 2.9 | 2.6 | 0.5 |
| December | 9.3 | 12.8 | 3.8 |
Portage next to nearby towns
Portage ranks 41 of 90 Michigan cities here for ice-dam days, behind Sault Ste. Marie at 48.8 days.
- Kalamazoo, MI 5 mi, 19.4 days
- Battle Creek, MI 20 mi, 19.4 days
- Sturgis, MI 29 mi, 18.7 days
- Coldwater, MI 35 mi, 17.3 days
- Elkhart, IN 40 mi, 14.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Three Rivers, 18.6 miles from Portage. 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.