Metal building spray foam is closed-cell against the steel, thick enough for the U-factor, not a light coat to stop drips for one season.
- Product
- Closed-cell against the panel
- Prescriptive roof
- R-19 + R-11 liner system
- Open-cell on steel
- No
- Typical buildings
- Barns, shops, arenas
Why metal buildings sweat here
A steel roof in this climate runs hot in the afternoon and hits the dew point at night. Moisture shows up on the panel, the purlins, and whatever is stored underneath. That is the reason for foam on a metal building, not a thicker batt in a house attic.
Closed-cell foam against the panel is the usual approach. It can be the vapor retarder, and it does not hold water against the steel the way open-cell can. Open-cell on bare metal is a wet panel waiting to happen. We do not use it there.
The code row is a liner system
Table C402.1.3, Climate Zone 1, metal building roof, is R-19 plus R-11 with a liner system. That is the prescriptive path. Spray foam is not that liner. Foam is allowed when the assembly meets the U-factor path for that roof. A one-inch or two-inch coat does not.
The thickness is taken from the product data and the U-factor the plans call for. We do not invent a thickness to match a liner system we did not install. Purlins and girts still bridge heat if the foam stops at the flange. The foam has to be detailed around the steel, not just sprayed until the drum is empty.
Barns, shops, and arenas
The metal buildings we actually see out here are barns, equipment shops, arenas, and small warehouses. Wellington has the barns and arenas. Loxahatchee and Jupiter Farms have the acreage shops. Parkland is mostly houses, with metal buildings on the larger lots.
An occupied office inside a metal building still needs the thermal barrier over the foam. A barn used for storage is a different exposure than a room people sit in. Tell us which one it is.