Floor insulation dilemma

[mtowner23]

Last year I had an old addition on the house gutted and found that the sill plates, 1/2 the floor joists, rim joists and header joists all needed to be replaced (by replaced I mean, they needed to be added since they were already gone). I didn't know how to get the sill plates replaced without lifting the room and that was a bit more than I was comfortable with, so I had a local contractor do it for me.

Before the contractor laid the new subfloor, I asked him what I should do about insulating the floor. The space is completely closed (no ventilation) and it has a dirt floor. I wanted to put foam board across the bottoms of the joists and lay fiberglass batting on top, but he stated that with the space being enclosed, I shouldn't need anything. Did I follow my instincts... no. Now I'm second guessing what I had done. I still haven't laid anything on the subfloor and I'm wondering if I shouldn't get under there somehow and add insulation. Does anyone have suggestions on how to add insulation if you think it necessary? I can cut a hole in the subfloor (maybe need 2 - one on either side of the beam down the middle) and hang fiberglass insulation from below, but I don't really want to cut into the newly laid floor.

The walls of the room are 2x4 and I used R13 (again, another recommendation from the contractor instead of adding 2x2 strips to each 2x4) and R30 in the vaulted ceiling. There's a woodstove right out the door to the room and we keep that area about 80-84 throughout the winter. I don't want the new room to be a huge heat sink if I can help it.

Suggestions?

Matt

 

[WVBrady]

Your situation sounds similar to mine. My crawl space was uninsulated and had a dirt floor. When I purchased it (one year old), there was already a stain on the sills and headers where condensation had formed in the wintertime. The warmth from the rooms above had heated the air in the crawl space. When this air contacted the uninsulated wood exposed to the outside cold, condensation formed.

I covered the ground with the heaviest plastic that I could get locally (6 mil ?) and overlapped it. I probably should have taped it too, but didn't. I then stapled plastic to the sill, draped it down on the floor temporarily and stapled fiberglass batts to the tops of the header, also draping them down onto the floor. The plastic was long enough that I was able to fold it back over the fiberglass and staple it back to the top of the header, forming a sort of blanket. I later saw this in a book on insulation, except that they didn't fold the plastic back over the fiberglass.

From time to time I have checked for moisture on the headers and sills and also in the fiberglass and have found none. I did this 30 years ago. (Wow, how time flies.)

 

[BadgerBoilerMN]

Not bad Brady.

I would only add that it is nearly impossible to prevent air from migrating into an existing crawl space without spray foam, but certainly, the vapor barrier - lapped a foot is fine - and insulation at the perimeter and on the floor.

Many will try to insulate the sub-floor but unless I am radiating it, I find insulating over the vapor barrier faster and more efficient.

Climate also matters, but most crawl spaces should be air-tight to the outdoors.