What can I use as an air barrier under an old wood floor with no sub floor?
This is an old house 1860 with 1910 addition. The old wood floors have no subfloor and on first floor, that is above a dirt floor basement. Since we have access to the floor from the basement I want to put up something that will block airflow through the floor. A bit of insulation would be a plus. I think it should also allow moisture to pass through. I would rather put it between the floor joists (> 2" x 10", 2' on center, oak hard as rock) and not cover them up. I have considered homosote. Any suggestions?
Making the floor of an older home airtight is important for comfort as well as efficiency, but what concerns me more at this point is the dirt floor. All your problems are solvable, so don’t worry, but lets start at the very bottom. If you have an uninsulated dirt floor crawlspace you have an endless source of moisture that will continually evaporate from the ground and deposit moisture in your home. That has a lot of negative effects in terms of comfort, energy efficiency, and air quality. See this page first as it will probably answer all your questions –
Crawl Space Insulation Tips for Insulating Crawlspaces Properly
I think your first step is to cover the dirt with a poly membrane, and either lay insulation right on the dirt and keep that lower space warm, or insulate between the joists as you mentioned and keep it cooler. As for simply stopping air leakage through the floor there are a few options, but see if those pages present a solution first and get back to us. Also please let us know what building climate zone you are in so we know what kind of temperatures you are up against in winter.