RH in Concrete for Wood Floor with Vapor Membrane

[CC Solutions]

You have several issues that are confusing.

For one thing you need to consider the abrasive action of the floor moving over a long period of time. How will that affect the barrier?

You also must consider going outside of the recommended installation parameters. If you stay within them you at least have a shot at getting someone from the manufacturer to listen to you.

And besides those issues, you are trying to determine the effect concrete rH has on a wood floor, and technically it has NONE. The wood will be impacted by the moisture coming from the concrete as it tries to equilibrate with the surrounding conditions. If we take the MVER out of the equation (by using a very low perm rate poly sheet or an epoxy mitigation system) you really don't care what the rH is.

The relative humidity of a concrete slab is a fantastic indicator if the slab's potential for emitting moisture. There are many factors that determine the rate of emission and more factors that determine if that rate is acceptable for the flooring.

I can tell you I can treat that slab and guarantee you will have no emission problems up to 100% rH and / or 25lbs MVER. What I cannot tell you is if you can put down brand X poly and make it work on a slab at 90%rH with a high vapor drive because the slab is porous, the ambient is low rH and the temps are high. If I added in multiple unsealed overlap joints and wear from movement along with no underslab vapor retarder, you would be looking at a failure waiting to happen.

So to recap: Relative humidity is the reservoir of moisture waiting to cause trouble. The poly sheet is the barrier keeping it from your floor. Contributing players are the ambient conditions, the quality of the barrier install and the surface porosity of the concrete. Then we have the barrier wear over time factor to consider.

Besides, Koster and Ardex cost less than $1.50/sf. They soak into the concrete and wear like iron. How huge is this job that you can't consider a warranted system for so little cost?

Oh, just so you know this: Koster VAP1-2000FS has a perm rating of 0.1 lbs/1000/24 or 0.05 grains/1hr/ft2 .

That should give you some indication of your poly performance.