Poly-carbonate roofing retrofit, potential issues, and best practice solutions…
PROJECT: Major energy retrofit in Jonesboro, GA (30 min. South of Atlanta).
CONDITIONS: The roof over the foyer is a wood frame structure, with (2) 2×10’s sistered and placed at approximately 4′-0″ o.c. Directly on top of this structure is a 20-year old poly-carbonate material, approximately 15 mm thick. The owner says there have never been any water leaks, but the blower door test results (11k cfm50: Vol – 50k ft3), seems to point to some major air leakage in this assembly. In the middle of the afternoon, the surface temperature on the INSIDE of the roof exceeds 140 degrees F, and the energy model (Manual J) comes up with a cooling load that is ‘through the roof’! (approx. 70k – 80k Btuh for just the foyer).
SOLUTION: We have gone round and round on a good solution, including replacing the material altogether. The cost almost of this approached in dollars what the Manual J came up with in cooling load, so we set that one aside…permanently.
The latest, and the one we all like the best is to air seal the existing assembly, then attach 3/16″ glass coated with Llumar’s “One-Way Mirror” film on the underside of the structure. Each section of glass will have a frame with a flange for attachment to the wood frame. The film is obviously reflective, and has an SHGC of 0.16. We are planning to provide a gap for ventilation around the perimeter of each panel by holding it off the wood frame.
1. Condensation in winter
2. Amount of heat within the space between glass and poly-carbonate (glass and poly can handle the heat, but what other issues?)
3. How much of a gap would be enough for proper ventilation?
4. Is air-sealing the existing assembly a good or bad idea?
If you want to see a picture of the roof, there is one in this article: http://www.daily5remodel.com/index.php?action=article&rowid=857
Here is a link to the performance specs of the solar film.
Llumar One-Way Mirror Specs
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