Is there a vapor barrier under the slab?

What is the current rh and how long has the slab been down?

 

Haven't measured yet as concrete is green (just poured last week!)

I was going to wait a month for it to cure a bit and test at that point. Wasn't planning on doing the actual moisture barrier/underlayment install until that point assuming the moisture level is OK.

 

I've been laying hardwood for fifteen years. For my home I full trowel spread bona 880 a moisture barrier/adhesive. Tapconed the plywood to the concrete so the adhesive had a chance to bite. Used different underlayments over plywood before hardwood in a couple rooms.

One room was adhesive/3/4 ply / aquabar

One room was adhesive/ 1/2 ply (do to some door issues)/ 30 lb felt.

The 1/2 inch ply plus 30lb felt feels much more solid and is a quieter floor to walk on. The felt really breaks up the hollow feel of wood in contact with concrete.

As a rule I shoot 1.5 inch staples.

Highly recommend felt over the plywood.

The glue will help prevent moisture but also will pull the plywood tight to reduce voids and hollow points.

I'm six months in and the floor is flat, even at a door that gets a ton of rain and overflow from a hip valley.

 

1/2 ply over the concrete, and I assume (?) 3/4 flooring, and you use 1.5" staples. Do any staples rebound off the concrete?

 

In the past I've read a guy on here who swore by mopping roofing tar on the slab, then overlapping layers of 3/4 ply, then hardwood.

I personally reddid a floor in a 100 year old basement apartment back in 2016 where another contractor had installed directly over rotten joists in contact with bare earth. EVERY SINGLE FASTENER rusted through to the top of the wood. Floor looked leopard print.

We fixed the joists, laid down 6mil, coated the back of the plywood with roofing tar, 15 lb felt, then hardwood.

That was three years ago and no call backs. Knock on wood.

 

Bona also makes an easy to apply concrete moisture primer that you roll on.

 

What did you do before the concrete went down for the moisture coming up????

 

In my case it is a dry basement (new construction - vapor barrier and insulation below slab), but, I'm overly cautious (don't want call backs!) and needed a nailing surface for the hardwood anyway.

However, after looking at the numbers and talking to the designer, it looks like we'll likely be going with "luxury vinyl", which I'll probably install floating and put "floor muffler" under. Have to wait for the basement to cure some more though. Useful information for the next project though.

 

No, 3/4 ply wood would be preferred but I have done probably 30 floors with half inch over concrete with the 1.5 staples and have had no problems.

 

The kitchen is where you can get in trouble. In front of the dishwasher specifically.


Mike.
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[emoji631] [emoji631]

 

I followed Fine Homebuilding Mag. suggestions. A floating floor not mechanically attached to the slab.
Roll a vapor barrier like Visqueen so it comes up the wall. The idea is any water vapor won't be trapped and can wick it's way out. It will be covered with molding.Two layers of plywood or sheathing glued with Liquid Nails alternating direction. Then staple flooring.
I've only done this in my home but have had no problems with it but I'm an hvac guy. Condensation can form on a floor where the temperature is below dew point. A coating won't help that. Below slab insulation is best.
I did that in a home I built in 1978 and moisture in the basement was not a problem.
Good luck.