Subfloor between poly vapor barrier and felt

Q: First off, I want to thank you for taking the time to respond to my question. Second, I apologize for being a little vague in my original question. I am planning on a “floating” installation, a “one-piece” vinyl floor covering in the room. That being said, should I now go with the 1/4″ underlayment and if so, is it still OK to loose lay the felt? (Do you recommend 15# or 30#?)

I have heard that this may create noise problems due to the possible shifting of the felt/floor covering – is that an accurate assumption? Also, the combo of the 3/4″ subfloor & the 1/4″ underlayment will give me a 1″ thick substrate. Given that the shortest underlayment nail I’ve seen is 1-1/4″ long and I don’t want to have numerous holes in the poly thermal wrap installed under the subfloor, I am assuming that I should go with narrow-crown staples which are available in 3/4″ or 1″ length. My planned fastening procedure is every 2″ along edges and every 4″ ( both directions ) in the field. In closing, I have one final thought – will/would having the 3/4″ subfloor “sandwiched” between the poly vapor barrier underneath and the felt laid on top create a moisture build-up thus causing the 3/4″ subfloor to rot/deteriorate? I will look forward to your follow-up, and welcome any suggestions/recommendations you may have to offer. Thank you for your time.

A: I may have slightly mis read your original description. I understood initially that your floor was insulated and poly installed on the joists at the crawl space side, not the sub floor side of the joists. But as you say, there is no access to the crawl space so obviously the original thought was incorrect. Did you also overlap and tape the poly joints? I can now understand why you are applying the 1/4″ luon or even poplar which is often used with vinyl, not only because of thickness concerns but you want a smooth, defect free surface to install onto. Years ago when I was a young employee I helped a vinyl installer on a number of jobs but I’m no expert on that. I’m really a hardwood guy.

That aside, on first reading your email my immediate concern was of trapping moisture inside the sub floor structure. The plastic should prevent this. I’ve never known a sheet goods installer to lay roofing felt or any other type of membrane down first. In a nail down hardwood installation such is usually used but with 1000 nails punching holes in it, the felt or wax paper can hardly be a ‘vapour barrier’. In that application is retards moisture movement giving the wood floor time to adjust. In your case, I don’t see any reason to use it and there is the added risk of locking any possible moisture in the structure below. I think you have everything else covered. Definitely use the staples. Staple guns and rubber mallets are probably dirt cheap to rent unless you have an electric version. Just keep an eye out for any mis fires that don’t go flush with the underlay. You may have to use an underlay patching compound on the joints as a last prep before installation. Any rough, uneven edges will eventually show through the sheet goods.

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