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Measuring moisture coming thru basement slab


My 2150 SFT ranch home with ICF walls and a full basement has NO vapor barrier (polyethylene layer) under the slab. Just pea gravel surrounded by drain pipes that exhaust into a lake behind the home. Currently I am running a stand-alone dehumidifier continuously and this is keeping the basement at about 35-40% RH. I also have a radon SSD fan installed a few weeks back. The fan is a RadonAway RP140 that uses 14W and is energy star rated.

I am wondering if there's a way to tell how much moisture is coming in thru the basement slab. Based on the measurement, I would like to test and see if a more powerful radon fan will help extricate more moisture thru the radon pipe so I could potentially do without the dehumidifier in the basement.

While we are on this subject, I read elsewhere moisture travels upward; I would appreciate correction on this point, if needed.

Thanks in advance.


Asked by Venkat Y
Posted Sep 3, 2014 4:19 PM ET


6 Answers

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There are several ways to measure moisture. Not all ways are acceptable by experts in the field. Why would you want to bring the moisture any lower than you already have? Any lower will generate bloody noses.
Moisture/water behavior can be studied by looking up Dew Point, capillary action and hydrostatic pressure.

There is a meter you can install into the slab which you can mechanically monitor by inserting the reader into an installed capsule. It's made by Wagner and cost upwards of $800.00. I use this model in my business.


Answered by Richard Beyer
Posted Sep 3, 2014 4:34 PM ET


First, 35-40% RH is much dryer than is needed to inhibit mold growth, etc. 60-65% would be fine, and would require a LOT less mechanical dehumidification.

During the summer months the most likely source of moisture in the basement is outdoor air infiltration, not ground water coming up through the slab (especially slab that is depressurized for radon abatement.)

You don't state your location, but in most places east of the Mississippi in the US the average summertime outdoor dew points are 60F or higher. The air in a basement that is running ~70F/60% RH has a dew point of only 56F. That means outdoor air entering the basement RAISES the humidity (!).

A basement running 40% RH/70F has a dew point of 45F, which is a LOT drier than the outdoor air (it's drier than the outdoor air even in most of the western half of the US.)

Water vapor has no preferred direction relative to gravity, the center of the earth, the center of the solar system, or the orientation of your personal chakras. :-) When all else is equal, water vapor moves from higher-humidity to lower. So the lower you keep the RH in your basement, the more water vapor it will move through the slab.

Subsoil is usually at a fairly high humidity relative to it's temperature, but when you de-pressurize the slab with a radon fan the soil proximate to your slab is constantly being replaced by air drawn from above grade. This air is usually drier than the entrained subsoil air, so depressurizing the slab tends to have a drying effect, reducing the amount of moisture that would otherwise migrate through the slab to your ultra-dry 35-40% RH basement via diffusion.

The healthy-comfortable range for human health is 30-60% RH, but if anyone in the family is allergic to dust mites it's better to hold the line at 50%, which reliably interferes with dust-mite reproduction, causing any colonies to die off. But there is no good reason to keep it as low as 35-40%, especially in an ICF house, since the materials deep in the layers of the assembly are all quite tolerant of moisture (unlike OSB-sheathed studwalls.)

Answered by Dana Dorsett
Posted Sep 3, 2014 4:52 PM ET
Edited Sep 3, 2014 4:53 PM ET.



Your right if the home was built in a perfect world. Missing the vapor barrier illustrates it was not.
"60-65% would be fine" as stated in the beginning of your comment is not accurate. By no means am I challenging your expertise ;), please view it as a simple correction because you to corrected it at the end of your lengthy explanation.

Delaware department of public health and many other state's publish the following.......
"try to maintain the home's relative humidity between 20-40 percent in the winter and below 60 percent the rest of the year."




Some facts are missing. What's the water table in relation to your slab? Is the house built on a slope with poor drainage or a flat lay of land? What soil type surrounds the property? Are there any surrounding wet lands? Does the property have an in-ground swimming pool on site which gradually loses water? What's the RH when your mechanicals are not running? What was the radon long and short term reading before it was installed? Since there's no vapor barrier below the slab the likelihood of water working it's way up the walls and through the slab is valid, even with a radon fan.

Answered by Richard Beyer
Posted Sep 4, 2014 11:27 AM ET


Thanks for the responses. I am in East central Illinois (zone 5A).

Richard and Dana,

The reason I am not satisfied with the dehumidifier and the current RH 35% is:

1. My basement still has the "basement smell" which seems to be in between a faint musty odor and stale air? I am not sure, but there sure is a "basement smell" that I was hoping the 35% RH would take care of, but it doesn't seem to be.

2. If I control the moisture entry at the source such as by using a higher powered Radon fan, then I am hoping that would fix the "basement smell" and also I could avoid running the dehumidifier altogether.

The basement does NOT have a sump and the drain pipes around the foundation footings go into a lake in the back, so I think the basement slab is at a higher level than the top of the lake. No surrounding wet lands. No swimming pool. I stopped the dehumidifier yesterday morning and the RH this morning read 43% in the basement Vs. 47 % upstairs. The AC was run for a few hours yesterday afternoon.

The 72-hour RadaLink test showed 6.3 which correlated with the longterm Safety Siren Pro 3 series electronic Radon monitor. After installing the Radon fan, the levels went down to 0.8 to 1.3.

Elsewhere on the web, I read one could tape some plastic wrap to a portion of the slab and monitor readings inside the enclosed space. Would this be useful in my situation?

Also, if I do experiment with a high powered fan for a year or two, would I be risking subsoil to be sucked up to fill the space between the gravel and the slab and just blocking much of the Radon pipe opening? When you say the air for the fan comes from "above grade" I take it, there are natural pores that go all the way to the ground outside for make up air to be brought in?

I understand there are capillary pores in concrete. Are these used for diffusion from water vapor in the air as well since I don't suspect water ever touches the bottom of the slab and instead drains away into the lake?

Are there any sealants that can be used underneath the slab to seal it? I can't use any on the top of the slab since the basement is already finished.



Answered by Venkat Y
Posted Sep 5, 2014 10:23 AM ET


You probably already know this, but it's possible to install a layer of polyethylene (or polyethylene plus rigid foam) on top of your existing slab, followed by 3/4-inch plywood or a 3-inch-thick topping slab.

Answered by Martin Holladay
Posted Sep 5, 2014 10:33 AM ET


Richard: "60-65% would be fine" is accurate, from a mold & rot perspective. ASHRAE Standard 62.1 puts 65% as the upper bound"...to reduce the likelihood of conditions that can lead to microbial growth." ( http://tc21.ashraetcs.org/faq.html )

Explosive mold growth doesn't really take of until about 70%RH, but the musty-basement smell can often creep in somewhere around 65%, especially if there is no insulation under the slab and there is cellulosic materials such as cardboard boxes in direct contact with a cool slab. The air-films adjacent to a cold slab are cooler and higher RH than the room average.

I understand that most public health departments alternately recommend either 60% or 50% the upper bound for human health & comfort, but that has fairly good margin on the musty-basement smell

Venkat: If 35% RH average basement air humidity isn't suppressing the musty basement smell you may have an acute leak somewhere keeping something too damp, and not a moisture diffusion through the slab problem.

If you have stacks of cardboard boxes on the slab, elevating them an inch with wood pallets or on an inch of foam-board insulation to raise the temperature at the bottom of the boxes, but with a 35% RH average that isn't the most likely cause.

If there is a lot moisture coming through the slab you don't need to measure it quantitatively. Tape a square foot of 6-mil poly to the slab for a week, then peel it up. With high moisture migration rate slab the section under the plastic would appear visibly darker & wet. You may want to do this in several places to see if there is an acute area of higher moisture than the rest of the slab.

Answered by Dana Dorsett
Posted Sep 5, 2014 12:08 PM ET

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