Insulating irregular joist spaces

[Ivan Vegvary]

Almost finished excavating my crawl space. Down to 42" throughout.

The floor joists (some 6" some 8") are placed at irregular intervals. While some are nearly 24" o.c., most of them are not. Spacing ranges anywhere from 12" to around 19". What is the easiest way to insulate (from below) these odd cavities. When I did some of the walls, I simply used my 36" paper cutter to cut the insulation and installed the small pieces 'side-ways'. If I made longitudinal cuts there would be a lot of waste with long thin slivers of insulation left over.

Somebody must have already solved this. Please don't tell me that professionally installed foam is the answer. I cannot afford that alternative.

All replies appreciated.

Ivan Vegvary

[Ed Pawlowski]

Spray in foam

[Steve Barker]

i know you said don't say it, but closed cell foam is the ONLY suitable answer for a floor in a crawl space.

[Cindy Hamilton]

We used this:

in several places recently. A lot of it went in exterior wall cavities in our small foyer, but we also sprayed it above the foundation, between the joists in our basement and crawl space. (The purpose there was to close up the voids in the top row of block, to reduce the amount of air that was pulled up from the ground into the house. It's a weirdly built house.)

My husband did most of it, but he got a friend from work to do the crawlspace portion (because DH is pretty spherical and didn't think he could effectively reach where he needed to in our crawlspace that's roughly 24" high, and less where the footers are).

It wasn't cheap, but it was cheaper than hiring it done.

Cindy Hamilton

[Pete C.]

How about installing a 6 mil vapor barrier across the bottoms of the joists, poking holes at the ends of each cavity and doing blow-in insulation? Tape up the holes after the install of course.

[RES]

Purchase the widest width of fiberglass insulation you can find. For 24" OC, it would be 22-1/2" or so wide. Sharpen the edge of a 4" or 6" drywall knife to use for both tucking and cutting the insulation. Lay it on one board and use another on top to compress the fiberglass insulation and then cut along the board, using it as a straight edge, with the sharpened side of the drywall knife. Personally, I'd cut it about 3/4" oversize for a better friction fit. Use the knife to tuck the sides into the space between the joists, since you want the entire surface to be against the floor sheathing above. You can also purchase stiff wires, (called lightening rods) that you jamb between the joists to support the insulation, hold it against the decking and prevent it from sagging.

Save the pieces you cut off, and you'd be surprised at how you'll find places they can be used, such as in corners and for tucking around windows and doors.

Nonny

[Jeff Thies]

I've got somewhat random joists. I've got it help up by fence wire, I'm zizaging.

The most important part is to have no holes in the insulation barrier. No airspaces to the floor. So stuffing is fine. I have one area where I overlapped to rolls, one for each edge.

I bought a bunch or R30 cheap at the home despot. You can pull that apart and lap it back together.

The floors are pretty tight now. We had the coldest winter in some time and the heat loss through the floor was good. If you don't have an IR spot thermometer, they are fabulous for looking for the weak spots in the thermal envelope (during temp extremes) not to mention "cool"!

Jeff