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Old 09-03-13, 08:49 PM   #4
Apprentice EcoRenovator
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A bit about the building philosophy behind this. From everything I've been reading most people install roof heating systems using the air to either heat some kind of reservoir, usually a crawlspace, or as an one input to the fresh air inlet on a forced air system. The reason is that this kind of attic heating doesn't usually supply a high enough temperature to reliably heat the house directly. Xringer, I think your idea of using attic heat to warm the basement fits into the first category.

I'm taking a bit of an educated gamble here. I have several things going in my favor. One of them is that it is extremely rare for the high temp of the day in winter where I live to be below about 45 degrees. It can get down to 20 degrees at night though. Here are the temperature readings for the house with the roof painted black but no radiant insulation barrier in place.

On the hottest day of the year so far you can see that it was 108F outside temperature in the shade. There was a 26 degree differential temp between that and the 134F temp (lower right in the picture) taken at the interior roof peak. A 26 degree boost ain't much to work with.

Here's a picture of the temps after installing the radiant barrier and insulating the attic ducts.

Here, on another hot day, it is 106F outside in the shade and it is up to 149.9F at the interior roof peak! That's a 43F temperature boost for the attic heat over ambient outside temps. I should add that all these temps were taken with the roof damper open and the living space damper closed. I'm betting that in winter conditions when the positions of those dampers are reversed there won't be much change in the differential temperature.

My reasoning is that the differentiall temps on a sunny day tracks consistently so far at 40 to 45 F, regardless of overall high temps. I been looking at the temps everyday for a month and a half. I also think it might be a reasonable guess that when the roof damper is closed and air is actually being taken into the living area that it won't cool the roof more than having an open roof vent would. Again, more of a hunch. We'll see. I've got tons more work to do on the house, most unrelated to this project.

At any rate, a 40 degree temperature rise on a sunny 40F day still gives me 80 degree air into the house for 2 or three hours a day. If its 50 degrees, overcast and I get only a 20 degree differential temperature then I'm still getting 70F air coming in. So overall I think I may just wiggle through with out having the complexity of having a reservoir system to take the relatively cool air from the attic.
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