Tranquility does not need a vapor barrier (optional), unless over 7lbs moisture emission. Perform a calcium chloride test to determine if, a poly sheet needs to be installed.

If, you do not want to run a test. Then install the poly sheet. Make sure it is flat to avoid any crinkling noise.

Thanks. I don't seem to get much moisture there. If I don't put vapor barrier, would I need an underlayment?

You have to check with the flooring manufacturer. If you do, the underlayment is about 1/16" thick.

Vinyl traditionally does not like underpad. But when it is allowed (as in this case) the underpad is 1-2mm thick (as in 1.2mm = very common).

The 6mil vapour barrier = vapour barrier. It is not considered an underpad. The underpad listed allows some moisture in the slab but not much.

Moisture VAPOUR is not 'wetness' you can see. It is humidity that evaporates OFF THE SLAB and into the air. The slab can look 'dry' for 20 years but as soon as you lay rigid flooring over it (ie. cap it) the moisture issues (if you have any) rear their ugly head VERY quickly (3-6 months).

So...do the test or 'guess'. The age of the house will hint at what issues you may (or may not) have.

How old is the house?

Hi, thanks for your reply. The house is 95 years old

Wow! You have NO MOISTURE barrier underneath the slab. That means that slab is going to be as wet as the soil outside. That's how damp your slab is. If the soil is SOAKED then so too is your slab. Guaranteed.

The only way this is NOT the case is if you have paid ++ bucks for slab remediation. You can't miss moisture remediation. It comes in the form of SEVERAL THOUSANDS of dollars and takes many many days for professionals to complete.

You will 100% NEED a vapour barrier on that slab. The underpad is the option...not the vapour barrier. The barrier is needed unless you can prove the concrete has had remediation.

Right now I don't have anything on top of the concrete floor. I am going to buy luxury vinyl plank floor and trying to figure out what else to buy to place in between concrete subfloor and vinyl plank foor. Sounds like I should by a 6 inch vapor barrier. Assuming that's correct, should I also buy an underlayment as suggested by one of the reps at the store? Or is that too much padding abd/or not necessary?

The 6mil barrier is needed. That's for sure. The chance of adding an underpad is up to the vinyl manufacturer not the salesperson nor the padding manufacturer.

LL no longer has installation instructions listed on their website....which is not a good sign. I highly recommend you ask for the installation instructions for this product.

The biggest question is how much work does the concrete need. Old concrete slabs that are wet/assumed to be wet are notoriously difficult to get smoothed out and ready for vinyl plank flooring.

The underpad, if it is allowed, is going to be a very small price ($0.70/sqft). The cost for the subfloor preparation is going to be more expensive than the $1.79/sf vinyl (which is VERY VERY low....and something that low priced is questionable....go ahead and google Lumber Liquidators lawsuits or class action lawsuits).

A decent vinyl starts at $4/sf (still a low price but nothing harmful). A good LVP = $7/sf.

SJ McCarthy, can you recommend what could be considered a good LVP? Can't find anything that costs that much and I'd be willing to pay for something decent.

CoreTec is the brand name for better level vinyl. Or you can look for anything out of Europe. Lico flooring is made in Switzerland and is TOP OF THE TOP. Finding it in the USA is tough but it can be done.

lico.ch