I'm installing some 1/2 prefinished wood flooring over a plywood subfloor.
It's an old house without much crawl space and there is no insulation between joists. Maybe sometime in the future, I can solve that issue. Right now, I just want to optimize the R-value of this retrofit.
I can't add a lot of thickness, so I'm looking for something affordable, thin, and with a decent R-value in addition to the felt or tar paper as a vapor barrier. One idea was 6 mil plastic. I've seen this product rated with an R-value of .87. Doubling that would give 1.74. Not bad. The question is whether it really has that R-value. Maybe it does in an application where it traps a lot of air like a greenhouse, but would it under a floor?
1/4 cork sounds like a good idea but the R-value is only .236
The 1/4" cork is what I was going to recommend. I'm surprised the r-value is that low though. Using plastic beneath a wood floor isn't a great idea. There is a reason they make/sell/recommend/use a more expensive felt paper, rather than simply using 4 or 6 mil plastic. While the felt is a moisture barrier, it still allows the wood to breath. You'd probably never notice an issue with the plastic, but it's simply another layer that could cause an issue.
In the end, I think working on the floor side of things is way more trouble (cost) than it's worth for an outcome that will more than likely not be noticeable at all. Just install the floor over a felt paper and go down to the crawl space where you can make a real difference.