Attic Problems (part 2)
Thermal Analysis
In January (2011) I purchased a set of 3M TL20 temperature loggers. I placed 1 outside, 1 on the ceiling of the attic, and 1 directly on the duct (under the "insulation"), and the other was DOA*. Here are a the plots from those loggers:
Figure 1: Overall Plot
Figure 1 depicts all of the raw data I collected. The black line is simply the freezing point. Since the heater turns on/off frequently, it creates a thick band of measurements, Figure 2 is a closeup:
Figure 2: Closeup
Here you can see the changes in the setpoint of our thermostat. My house has a programmable thermostat so it's turned down when we're gone, and up when we're home. The unit also tries to predict thermal properties of the house, so it'll be a certain temperature at the time you want. However, I think this feature is inaccurate and flaky. Anyway, this plot really shows how often the furnace is cycling. Figure 3 shows a plot of one-hour's worth of data:
Figure 3: 1 hour
So why did I go through the trouble of all this? 1.) Just out of curiosity, and 2.) Because I the numbers to know when I've done enough work, without having to wait for ice dams again. Once the attic is done, I'll replot these numbers to see if the difference between attic temperature and outside temperature is "close enough" so ice dams won't form, see below:
Figure 4: Difference between outside and attic temp.
On average, the attic is 26 degrees hotter than outside, but I've recorded a maximum differential of 43 degrees F. According to some thermo calculations, if 6 inches of snow covers the roof, the surface underneath the snow will be about 18 degrees hotter than the outside temperature. So, if it's 20 degrees outside, the roof surface is 38 degrees, more than enough to thaw the snow, only to refreeze at the soffits.
The magic temperature when ice dams will NOT form, is 32-18 = 14 degrees. According to my estimates, my roof surface is melting snow 36% of the time. If I can reduce the temperature differential (between outside and attic temperature) to 10-15 degrees, I'll be melting snow only 1-8% of the time. Although this is a very tall order, it will keep my roof in good shape, prevent future leaks, and prevent me from having to rake the roof all the time. This 36% number seems pretty high though, and assumes 6 inches of constant snow (unrealistic) which would imaginarily be replaced as it thaws.
Doing some more calculations, this means my average attic temperature must go down to 20 degrees, not 38 degrees. Wish me luck :)
Of course, all these calculations are based on a lot of assumptions, for this short week in January. Your mileage may vary.