Can I attach a vapor barrier directly to plaster, then install cedar tongue & groove?

Question

I have a 1900 house that's never been insulated, and I'm putting in a new bathroom and kitchen. I plan to blow in cellulose insulation in most of the house walls.

In the bathroom, I have a small space that is 9' above the shower. The ceiling is lath and plaster, but I want to install cedar tongue & groove on top of it. I already put an exhaust fan and recessed light up there. Because it is boxed in by a large beam, I am concerned about moisture build up. I am going to put a vapor barrier in the whole bathroom ceiling. Above the tub is the only space with lath and plaster.

Can I simply staple the vapor barrier to the plaster? Then nail the cedar on top of that? It would be a hassle to remove the lath and plaster just to install a vapor barrier.

Answer 1

That should be fine. In theory the vapor barrier keeps anything behind it below the dew point (depending on weather), so it doesn't much matter what material that is.

You might just use a sealing paint, though. This would eliminate the possibility that the existing paint acts as a second vapor barrier, introducing the possibility of moisture being trapped. Otherwise, consider drilling a few holes for ventilation.