I am remodeling my master suite. The original floor was 1/2" play (randomly sized pieces looks like offcuts) with OSB over the ply. When we pulled the OSB up, the builder had tarpaper between the two layers. I am not a fan of tarpaper or OSB. We will be putting T&G ply down. What should we use between the layers of ply? FYI, we will be screwing the new layer down and then carpet.
Laying T&G ply sub-floor over existing 1/2″ ply sub-floor. What underlayment should i use between the layers
subfloor
Related Solutions
I did the same thing in my last house, using DeltaFL and 4x8 sheets of OSB.
Spacing
I used spacers to leave a 1/4" gap around the outside, and then framed all interior walls on top of the subfloor.
I had laminate installed on some of it, and carpet on the rest. I can attest it's a great way to do a floor as it has the feel of an upstairs floor. I lived in that house for 4 years without any issues whatsoever.
Screws
I screwed it all down, and ended up using at least 24 tapcons per sheet. My floor was really quite level, but any minor variation meant when you walked on that spot you could hear the clicking of the plastic sheet, so for a couple weeks after I had it installed I was still finding 'clicking' spots and adding tapcons.
The downside: it's backbreaking work. I would definitely invest in a rotary hammer drill. I helped a friend do his the same way, and we had a rotary drill that time, and it could do 4 or 5 holes in the time a normal hammer drill can do one. Total install time was about 10 hours to do around 900 sqft (including everything) with 3 guys. I did mine mostly by myself working a couple hours every evening, about 350 sqft, and it took me a couple weeks.
Well technically it is plywood, it's just not very good plywood. Having said that I've seen flooring guys use a product that looked very much like luan that was no more than an 1/8 of an inch thick to get the level they needed. This was in a hospital remodel so it was definitely up to code and it went under a commercial grade thermo-sealed floor so it should be good for just about any application unless your subfloor is really atrocious. Ask around at a dedicated flooring supply store and they should be able to point you in the right direction.
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Best Answer
Nothing. If you fasten the new layer well there's no need for it. Multiple layers of wood subfloor are commonly used without internal bonding or buffering.
If you're worried about sound due to movement of the random sheets, zigzag a 1/8" bead of construction adhesive over the original plywood to bond it to the new and help span any voids due to flatness issues.