[NTL1991]

I pulled some Romex from the basement today to install a circuit for a window A/C, and opened up a hole in an exterior wall adjacent to a window for an old-work box.

I'm pretty sure this is the first time I've made any holes in the exterior walls of my house. The piece of plasterboard I took out of the new hole surprised me: It's foil backed. All the other interior walls are similar Rock Lath type plaster board with a layer of plaster over it, but no foil.

It's 3/8" thick solid gypsum plaster base, with a 3/8" layer of plaster. Behind the gypsum is a layer of foil.

What's the foil all about? The house was built in 1948.

 

[ClarenceBauer]

Rock Lath was manufactured with a sheet of Aluminum foil laminated to the back side it was to provide insulation against heat & cold & as an vapor barrier.
It was only used on the exterior walls. All interior wall used Plain or perforated Rock Lath

 

[NTL1991]

Interesting! Thanks so much for the info. I haven't seen a single piece of perforated so I'm assuming it's all plain throughout the house.

I'm impressed by the vapor barrier properties of it considering it's age. And it sounds like the foil is a very early kind of Reflectix. The walls are uninsulated, but this house has never felt like other, older, uninsulated homes I've lived in, even with new windows. Makes a lot of sense now.