Commercial batts fill a cavity. They do not cover an open attic, and a metal stud still conducts heat around them.
- Where
- Studs, furred walls, framed bays
- Not where
- An open attic floor
- Facing
- Unfaced unless the assembly needs it
- The steel
- Still a thermal bridge
Where a batt is the right product
A commercial batt belongs in a cavity: a metal stud wall, a furred wall, a knee wall, or a framed bay with a depth that matches the batt. It does not belong rolled across an open attic and called R-38.
A 3.5-inch metal stud with an R-13 batt is R-13 at the cavity and almost nothing at the steel. The stud is a thermal bridge. Continuous insulation, when the wall assembly needs it, is a different layer. A batt alone does not turn a metal-stud wall into the code wall.
Facing, in this climate
Zone 1 is hot and humid. A polyethylene sheet on the inside of a wall can trap moisture. We do not add an interior vapor barrier because a house in a cold climate would have one.
Unfaced batts are the usual cavity fill here. A kraft facing is used only when the assembly actually calls for it. We do not install a faced batt backward and hope the paper is on the right side.
What a batt will not do
Batts do not air-seal the top plate, the stud track, or the penetrations. The cavity still gets sealed. A batt shoved into a bay with the edges folded is not the R-value printed on the package.
An attic floor that can take a hose gets blown-in, not a grid of batts. If someone sold a batt attic as the cheaper R-38, the inches will not be there and the gaps will.