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Old 11-17-13, 12:14 AM   #16
Exeric
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Join Date: Aug 2012
Location: California
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Getting back to the problem... attic solar heat has a quality problem, not a quantity problem. That is, there is a huge quantity of roof surface space that the sun radiates onto. There is a lot of solar energy impinging on any roof. But it also has lower temperature than a dedicated solar panel because for various reasons it is not as efficient at capturing energy as a dedicated solar panel. This means that for a given surface area that the sun radiates onto the internal temperature of the air in an attic will be much lower than in a solar hot air panel.

I was realizing that there was a significant amount of heat leaking out on my hip roof that wasn't from leaks in the radiant barrier (besides the open roof vent). I could tell because everytime I went up there midday I'd be sweating, even after the radiant barrier was up. It was a lot less than before the radiant barrier, but it was still about 85 degrees up there even when it was signifiantly colder outside.

The problem isn't equal in all areas so I decided to just fix it at the worst areas. Those are the areas where the heat concentration is the highest.
In a gable roof the greatest heat concentration is right near the ridge. In my hip roof there is a big concentration there, but also along the hip rafters where the regular rafters meet it. One has to allow the air to "slip" past each junction point by dropping the radiant barrier a bit at that point. Every regular rafter that meets the hip rafter must channel hot air from every preceding regular rafter that intersects with the hip rafter. So by the time you get to the highest and last regular rafter junction point all the hot air from the previous lower rafter channels is meeting in that narrow junction point.

Here's a picture I drew that might give anyone a better idea. The areas in the broken lines are the areas of greatest heat concentration. It's a seat of the pants estimate and not a quantitative picture of heat concentration.

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Last edited by Exeric; 11-17-13 at 03:20 AM..
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