Riddle me this (condensation)

nibs · 07-25-16, 10:19 AM

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.

07-25-16, 10:19 AM	#1
nibs Apprentice EcoRenovator Join Date: Jun 2010 Posts: 206 Thanks: 1 Thanked 18 Times in 17 Posts	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.