Metal building roof foam – condensation zone calculations
Working on a 40×80 metal building project in Tennessee (CZ 4A – mixed humid). Owner wants to convert it from agricultural to a climate-controlled shop/garage.
Plan is to spray closed cell foam directly on the metal roof deck and walls. I am concerned about the condensation plane.
With a metal roof, the roof deck gets extremely cold in winter. If I spray 2 inch of closed cell on the interior, is the dew point going to end up within the foam layer, or is 2 inch sufficient to keep the dew point inside the building?
Running the numbers with an online dew point calculator suggests I might need 3-4 inch on the roof to fully control condensation in CZ 4A. Can anyone confirm this from field experience?
Also: what is the typical adhesion like on painted or galvanized metal? Do you prime first?
CZ4A metal roofs – I use a rule of thumb of R-15 minimum (about 2.5 inch closed cell) for walls and R-25 (about 4 inch) for the roof deck. At 4 inch closed cell, the dew point stays outside the foam layer in CZ4A even on worst-case winter nights. For adhesion on galvanized metal: no primer needed if the metal is clean and dry. On painted metal it depends – fresh acrylic paint is usually fine. Old chalky paint needs light sanding first. Always do a pull test before committing to the full roof.