Ice dam risk in Nashua, NH
How often ice dams form in Nashua
24.1 dam days a winter
High risk. On an ordinary house in Nashua, conditions for an ice dam come on about 24.1 days in a normal winter, mostly in January and February.
Nashua averages 53 inches of snow a year and about 81 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 Nashua
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 15.1 | High |
| Typical, some insulation, not air-sealed | 24.1 | High |
| Older or poorly insulated, warm attic | 35.4 | Severe |
Sealing and insulating a typical attic takes out about 9.0 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 35.4 days.
Preventing ice dams on a Nashua 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 162 kWh a winter in Nashua, roughly $28 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Nashua winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 14.7 | 24.1 | 8.4 |
| February | 14.5 | 21.5 | 6.8 |
| March | 10.1 | 16.5 | 3.9 |
| April | 0.7 | 1.8 | 0.2 |
| November | 1.7 | 2.1 | 0.4 |
| December | 11.2 | 15.2 | 4.3 |
Nashua next to nearby towns
Nashua ranks 6 of 11 New Hampshire cities here for ice-dam days, behind Claremont at 34.6 days.
- Lowell, MA 11 mi, 18.8 days
- Methuen, MA 16 mi, 20.5 days
- Manchester, NH 16 mi, 22.8 days
- Lawrence, MA 17 mi, 20.5 days
- Fitchburg, MA 19 mi, 29.3 days
Where these numbers come from
NOAA 1991–2020 climate normals from Nashua 2 NNW, 3.0 miles from Nashua. 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.
- Snow-on-the-ground counts come from a second, nearby station, because the closest one does not record them.