Permanently elevated moisture levels beneath basement slab: why?

Every once in a while, usually in basements, I happen onto situations in which the surveymaster picks up moderately elevated moisture levels over a large part of the basement slab. It fluctuates, but always looks a little high. It’s been dry here even for Colorado.
I’m thinking that it’s just a high water table and some moisture is wickingup to soil levels close to the basement grade. Any other ideas?
This is not in the mountains but on the plains below the foothills.

Unless there is a vapour barrier below the concrete floor you will always have water vapour migrating up.

My home sits on a knoll The down spouts discharge are at least 8 feet from the home or more . The dirt is well sloped away from the home .
I have a garage on on side that extends away from the home.
My deck on the rear is closed under neath with steel roof taking the water away 12 feet from the home .
I put in a sump pump down 20 inches below the cement it seldom runs .
Yes there is also 4 inches of gravel under the floor .
Guess what I have elevated moisture reading every where in the cement so I just bought a dehumidifier to make sure I do not get mould any where .
I think I know what to do and have tried all I can and as Ray says no moisture barrier you have dampness .
Oh yes I have weepers all around the home I also dug down and put in a dewatering sump out side .
This is like a well down below the weepers by 18 inches again seldom does it ever have water in it .

Industry is the only place I know that they put down heavy plastic before they pour the floor.
Roy Cooke sr. . RHI

The house could also be built over a spring.


If that was the case why would the (Oh yes I have weepers all around the home I also dug down and put in a dewatering sump out side .
This is like a well down below the weepers by 18 inches again seldom does it ever have water in it .) not have water in it ???
We got much rain in the last two months some days over two inches ???
Roy Cooke sr

Elementary physics and building science: water vapor moves from hot to cold, lowest point is coolest point, naturally occurring migrating water condensates or settles at basements or crawlspaces.

I once read that an average family of 4 produces over 17 pints of airborne moisture per day through breathing, bathing, cooking…didn’t know if that was per person or entire family.

Try these for further info I’m not a scientist or physicists hell I cant even speell it:

It’s a bad idea to pour a slab anywhere without vapor barrier, for a number of reasons.

Barry that Building Science Corporation is a great link, thanks. -Kent


I also have a Survymaster and the measurments in the search mode (not using the pins) will give high readings on concrete or stone. Maybe its picking up some minerals that conduct RF frequencies, I don’t know but I do not even bother using it on any flooring surface that sits directly on a concrete slab.

Just tried it here in the office with the same results, Paul. I’ve also gotten high readings (in search mode) on tile floors above poorly ventilated, tightly sealed crawspaces.
Do you ever actually use the remote probe attachments for anything? I don’t find much use for mine.

I have never used the remote probes but I carry them just in case.

I see this all the time, around here. The slab is underground and cool. When humidity in the house air hits it (in summer or winter when the humidistat is set too high) it condenses.

I have seen many new houses where the owners put down carpet, directly on the slab. Within a few months, they smell a musty smell. Sure enough, the underside of the carpet is damp. No cracks or seepage, just plain condensation.

They are always real angry that they wasted money on the carpet. I put in my report boilerplate the recommendation that the slab be covered with plastic before putting down carpet of flooring. Don’t want to get sued for their mistake.

Generally, vapor barrier is a term often used when discussing humidity (or vapor pressure)where vapor (eg water vapor) is prevented from moving between discrete spaces or volumes.

You should install a thick, cross-laminated, high-density polyethylene vapor barrier under the concrete slab when building a house. Concrete keeps out most liquid water when used as floor, but it is porous, so some moisture will always wick up and transpirer into the air. Here is the general sequence of tasks:

Excavate for footings, walls, slab, and sump.

Pour footing, Pour walls, or build concrete block walls. Include foundation vents in the walls.

.Install plastic sump in sump hole.

.Install a thick, cross-laminated, high-density polyethlene.
.Install a built-in-perimeter drain to divert water away from the wall. (Even with the vapor barrier, water will often work its way around the edges of the slab and the vapor brrier. The purpose of the perimeter drain and sump pump is to collect this water and pump it away from the house.)
.Install and apply cruched stone on top of the polyethylene. This channels water to the perimeter drain and sump.
.Pour the slab.

It is constumary to see some sweating on a concrete slab at times, which is due the temperature differrential and Dew Point Levels.

Ventilation of the basement area would help elliviate some of the sweating scenarios.

For those wishing to apply flooring materials on their basement floor, the majority of flooring manufacturers have instructions limiting installation of the flooring material if moisture emissions exceed 3 pounds per 1000 sq. ft. of concrete per 24 hours.

Hope this helps. :slight_smile: :slight_smile: :slight_smile:


Keep the basement windows closed during hot humid weather. Saw a basement the other day concrete floor was wet at wall. Hot and humid outside and basement windows were open above the area that was wet. The upper wall (drywall) was 63-65 degrees F. While the concrete floor was 53 degrees F.

I agree with all of the posts regarding moisture wicking up from the slab. However, the Surveymaster that was refered to in the original post will indicate high moisture on even the dryest of concrete.

William, do you put this in every report where the home has carpeting, just homes in which they mention replacing it, or in every report because any home can be carpeted?

I put it in my ‘watch list’ area of the summary if the house has a finished basement with carpeting or flooring (laminate wood, etc). If the basement is unfinished and the client is intending to finish it, I will put in the warning, as well as other warnings about:

  • Don’t enclose the furnace and water heater in a little room with no combustion air.
  • Likewise, when running HVAC ductwork for the finshed basement, make sure that the ruturn duct register is far enough away from the furnace so as to not compete for air.
  • Don’t enclose the service equipment panel in a closet or bathroom or laundry room, etc.
  • If adding a bathroom in the basement, make sure that the exhaust fan is vented to the exterior.
  • Put off finishing the walls for at least 3 years so that any settling cracks have a chance to form, be identified and sealed before there is drywall over them.
  • Make use that there are escape windows easily accessable in the basement.
    If I don’t warn them, up front, they tend to get pretty mad, later.

Hope this helps;

Thanks Will. I’m surprised that they get mad at you. Those are things I would expect a contractor to tell them. Must be no contractor if they take it out on you. -Kent

Amazing how people wiull hire the cheapest guy, then blame you for the bad work they do. If they wanted a good job, they have to hire good people.

Blame who you can, and who will listen to you. The ‘contractor’ won’t wall back.

Just like the people, around here, who think that I charge too mych for an inspection. They hire the cheap guy, then call me to complain when he misses something.

human nature, I guess.

Moisture detectors pick up density and give higher readings.

Take differential readings. If it’s high all over, it’s likely density.
If you have a loose toilet check away and then at the toilet and measure the differential.

You can not depend on non-contact measurements. You also can’t go around poking holes in everything!

This came this morning, thought I’d pass it on…

The Surveymaster will read down to 8% but the focus point for accuracy is between 10 and 20%

Aso, for straw bale homes or large timbers…

BLD5020 Insulated probes 240mm $45 ea

BLD5018 Insulated probes 127mm $45 ea

See accessories for your meter at

Chris Ranwell****
Global Product Manager