New floor framing over soil-2 vapor barriers?

Wayne Perry

I'm renovating an old farm house. At some point they created a room out of a porch, then much later someone added a small attic to it. The attic and newer roof are fine as is the side of the original house it's attached to, but the floor and one outside wall were rotted to the point of having to be completely removed. An engineer advised us to build a stem wall/footer around the perimeter of the room, replace the outside wall and frame a new deck for the floor.

We have the stem walls, we replaced the outside wall, now I'm dealing with the floor system. The stem wall surrounds soil (remember, this was originally a porch), so we have put down 6 mil. plastic over the soil and up all sides of the cavity created by the stem wall, stapling it to the inside edge of the new sill plates, and sill of the original house.

My question is, when I put in insulation between the floor joists, should I put in another vapor barrier? Essentially, it seems that the 6 mil. I have already would be enough, but then remembering the rule of thumb that the vapor barrier goes on the warm side of walls and floors, I'm confused. Any advice?

chrisBC

Personally I would insulate the new concrete walls with rigid insulation

paintdrying

One vapor barrier on the ground.As mentioned, ridge insulation on the walls solves many problems

carpdad

Assuming you have 2x10 floor joists, most insulation you can put is r-30 fiberglass. This will leave the floor cold in winters, although your area may have milder winters. Using foam boards would give you more insulation, warmer floor and less to think about moisture condensing on plywood, joists or insulation, but foam boards cost more and takes more work. Spray foam or caulk any gaps and joints to stop air flow. Then cover the whole thing with 1-1/2" foam board.

If you want fiberglass insulation only, at least cover the bottom of joists with same foam board and fill in the joints and gaps with spray foam. This keeps the joists warmer.

Vapor barrier on ground would be an insurance, but I would do a few things differently.
1. Use thicker, longer lasting material than 6 mil plastic tarp on smooth ground or bed of sand.
2. Attach the barrier to the concrete, not to the wood. First good quality (polymer caulk or construction adhesive, ones that says never dry out. Don't use silicon.) bead of caulk along the top edge of concrete, pressure treated 1x3 cleat, screwed, another bead of caulk then barrier then another cleat to hold the barrier. The caulk is to stop the air/moisture flow from ground to wood parts.

Insulating the ground and walls (not the floor) is good idea, although this is based on heat from above keeping the crawl space warm, and keeping the floor warm. Floor and walls need to be insulated really well to do this, and I think you can get more insulation value by insulating the floor joists.

Wayne Perry

Great advice! Just what I needed, so thanks! Now, it's time to get to work!