This 1905 house is in northern Colorado and has no insulation
in the attic. I’ve sistered all of the original 2×4 (16″ center)
joists with 2×8’s, so the spacing is not standard. Thus, I’m
planning to use cellulose fiber instead of fixed size batts. The
ceiling below is plaster over wood lath.
I’ve read that gluing the subfloor (with construction adhesive)
to the joists results in a stronger floor, but I don’t know how
this works with a layer of plastic between. Do I:
1) cut plastic for each channel and glue to the edge of the
surrounding joists,
2) lay plastic over the joists and put adhesive both under and
over the plastic, or
3) forget about gluing down the subfloor?
Also, I’m not real sure what kind of plastic to use. I’ve got
a huge roll of 3 mil polyethylene (Dow Chemical Heavy Duty
Polyfilm).
4) How thick should the plastic be?
5) Can I use multiple layers of my 3-mil plastic, or will I
get condensation between layers?
6) How does one seal between different sections of plastic?
Replies
I'm gonna assume that you are putting a floor in the attic just so you can throw junk up there to store.
The plastic would go below the cellulose fiber, not on top of it (vapor barrier to the warm side). So if you want to be really thorough, you can lay a piece into each bay and on top of the lath and glue it to the joists.
Only use one layer of plastic. I don't remember how thick it should be, but someone will be along with that.
Another day, another tool.
Correct assumptions. The attic will be unheated and the plastic will be on the bottom (lath side). Do I use construction adhesive to glue the plastic to the joists?
I would use an adhesive caulk. BUT I'M JUST GUESSING. I can't believe no one has jumped in here with authoritative answers.
Rich Beckman
Another day, another tool.
I'll try posting to the Energy, Heating and Insulation discussion and see what they think.