slab on grade high relative humidity

[structSU10]

I have a slab on grade, place with a vapor barrier, but inside a building that has yet to have mechanical systems turned on which after 3+ months is still reading up to 95% RH.

The contractor needs to place rubber floor which requires 85% RH. They want to use a moisture vapor emissions reduction system - a one coat product that seems to effectively create a vapor barrier made of epoxy.

It says it can be used on grade, but I am unsure if its a great idea with an existing vapor barrier below. I also think that once mechanical is turned on the issue will go away, but they are up against time for building turn-over and this issue has persisted for a long time.

Any thoughts / insights into using such a product?

 

[phamENG]

Sounds like a band-aid.

1) Is the epoxy product compatible the flooring adhesive?

2) Is the installed vapor barrier actually doing its job? You're in New York, and it's November. I seriously doubt your average ambient is above 75%RH. So the moisture is coming from somewhere. You need to find the source of the moisture and address it directly. If it is coming up through the slab, you need to figure out if the vapor barrier under the slab is doing its job. If it isn't, you need to fix it. That moisture is still going to try to come up through the slab, and over time there's a good chance the bond between the epoxy and the concrete will be compromised and you'll have bigger problems after everyone moves in.

 

[dik]

make sure the product works... and check with the rubber floor supplier for compatibility. Also long term... moisture will be 'trapped' in the slab.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[structSU10]

the product literature is specific on being compatible with floor adhesives and such.

yes that is my concern as well. There is some sub slab drainage in the building but the sump it drains to is dry. they installed a 10 mil vapor barrier.

 

[Harbringer]

If this came as an RFI I would return this not reviewed. This is a issue for the architect to investigate. Getting outside of your area of expertise is a good way to get sued.

 

[dik]

I'd hardly rely on an architect to do this correctly... I used to have a drinking buddy, way back, who went into the priesthood... I used to caution him not to let his horizons be limited by his halo. From a liability point of view... you are, however, correct... architects do floors.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

Spread a moisture absorbent material on the floor and let sit for 48 hours. Check the relative humidity in the slab again. Repeat as necessary.

What method are you using to determine the RH of the slab?

data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACsAAAAWBAMAAACrl3iAAAAABlBMVEUAAAD+AciWmZzWAAAAAnRSTlMAApidrBQAAAB9SURBVBjTbZGBDcMwCAS/G9z+00Z9eCBViSyL5AxnIv2PT4X3zST8FS8mMyc/X7ozM5ijeWdDz5nQqS02G5OwbTIBP97ScXmZxLzUCdeYD3ZNLc36pH+a6WVSHeqS66277i2hh5NSZVJJvTsTzMzh0PlH8WblM/Q7QdcO/AD+swWPce8W8wAAAABJRU5ErkJggg==

 

[Harbringer]

I'd hardly rely on an architect to do this correctly...

Perhaps not but I'd hardly rely on a structural engineer to resolve a moisture issue with flooring correctly either. It is the architect job to resolve these problems or get qualified consultants to help them resolve them. This might be an issue for the project geotech or some "moisture specialist" but is sure isn't an issue for the structural engineer.

It reminds me of a Verizon commercial I saw about a year ago, the tattoo artist told the client, "Stay in your lane bro!". Words to live by.

 

[dik]

It depends on the structural engineer, I guess... I often do stuff, and expose myself a bit, to make sure a project is successful. Include informing others on the project of possible issues.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[Ron]

Agree with dik...I'm a structural engineer but I deal with a lot of building problems and have a lot of experience in doing so. The recommendation I gave above is based on a successful outcome from similar conditions but on a government contractor's new conductive floor system.

Doing proper testing is also important. While it can be expensive to do so, we sometimes do gradient relative humidity testing within the slab. This lets us know the approximate source of the moisture. Most often it is just moisture within the slab that is in excess of that needed for hydration (common when high water cement ratio mixes are used or the contractor adds water at the site). High relative humidity in the upper half of the slab can be remediated by absorptive material spread on the floor and allowed to "pull" the excess moisture out.