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Old 09-26-16, 10:41 PM   #3
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Originally Posted by nibs View Post
So now for the riddle;?
Are the trusses more insulated or less insulated than the roof in general?
In the morning the dew dries faster above the trusses.
Is this because less dew forms on the area directly above the trusses, or is it because the trusses get warmer faster in the morning sun?
The trusses are less insulated than the free span part of the roof. This means they both warm up and cool down faster than other parts of the roof. This is because of the "thermal bridge" created by the framing members. Wood is less insulating than insulation. At best wood is less than R-1.5 per inch. Assuming your styrofoam is R-6 per inch the best your wood could perform is equal to the styrofoam, however for it to perform at this level you also need the 3" of styrofoam to encapsulate the sides of the trusses, otherwise they pull energy or radiate energy from where they are exposed. For a quick and dirty calculation we should probably assume your trusses are actually only providing R-3 of insulation.

When dew forms it adds energy to the structure, like wise when it evaporates it removes energy. Since your trusses are in effect acting as radiator fins there is more surface area for them to pull energy from to evaporate the water. While the majority of the energy for the dew evaporation likely comes from direct solar radiation a not insignificant portion also comes from the air temperature, and other radiant surfaces below the roof.
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