insulation question?

[Dave Charron]

So I am doing a rehab/flip on an old, old house.  House has old timbers for floor joists and has a crawlspace.  There is NO insulation in between the floor joists.  Never has been.  I am flipping this house.

Should I bother installing insulation under the floor?  What would you do?  This is a $200K house when complete.

[JD Martin]

When you have joists that far apart, you have several options:

1. Nail lath strips perpendicular to the joists and lay two battens side by side. Easy and cheap, and still creates a dead air space to prevent movement, so it works. Disadvantage is that it's not quite as good as having the insulation tight against the subfloor.

2. Install 2xs on edge to split the difference between the joists so that you have something to push the wire against. More work but permanent. You can use joist hangers to make it easier. 

3. Hang two batts side by side, run the whole length, then go back and push 4 ft+ wire up against both batts at once. 

4. Spray the joists with open or closed cell foam. Most expensive, but permanent and very comfortable. I swapped out batting for open cell in my own house in the basement, and the difference in the comfort of the house is astounding.