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Old 07-25-16, 11:19 AM   #1
nibs
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Default Riddle me this (condensation)

Just finished the plywood cover on my new flat roof and have a question about dew forming and drying on the roof.
construction is as follows.

Box beam trusses 1 ft tall, plywood clad, with Roxul insulation, packed between the 2X4 jack studs. trusses on 16" centers.
between the trusses three layers of 3" styrofoam insulation tightly fitted with a fourth layer about 1 inch narrower, the resulting 1/2 gap filled with epscrete with a thin layer on top of the styrofoam to ensure no air gaps between insulation and plywood. The walls are open, unheated or cooled except for the shade created by the roof itself.

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?
It is not just on the new (yesterday) part but also true for last years construction which has an EPDM membrane.
My old brain just cannot seem to get around this riddle.

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Old 09-26-16, 06:45 PM   #2
AC_Hacker
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Have any photos?

Might make it easier to answer.

-AC
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Old 09-26-16, 10:41 PM   #3
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Quote:
Originally Posted by nibs View Post
J
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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Old 09-28-16, 12:01 AM   #4
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Thank you for the input. It makes sense.
I would hope that the trusses are better than R3, because they make up about 1/4 of the volume of the roof structure. If I calculate the truss as being 6" of wood and 6 inches of mineral wool insulation (Roxul) I come up with about R37 and with the rest of the roof being R60
To arrive at the truss value, I took the bottom and top chords thickness of 4.5 inches, then took the 12" on center jack studs and laid them flat for another 1.5 inches, I ignored the 3/8 plywood on each side of the box truss and added in 6 inches of Roxul for R 28 + R9 for the wood structure for R37, & the 12 inches of styrofoam between the box trusses at R 60, should give me an R value of 54 - 4 for conservatism so I think we have an R50 roof.
May have been better to go with 2X12 rafters, but we are pretty happy so far.

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