I'm curious as to where exactly the humidity comes from in an otherwise dry, well-sealed basement? Our house is ICF, as is the walk-out basement. The two windows and one door have been carefully detailed. The part of the walls underground are well sealed, and there has never been a drop of water seen even despite last year's monsoons. Yet there continues to be a lot of humidity in the basement. I run a dehumdifier, averaging about one day in three. Once the humidity hits 65-70%, it starts to get musty, so I run the dehumidifier.

No one is living in the basement. No laundry, no bathroom, nothing going on down there. I understand humidity on the main floor, with perspiration, expiration, evaporation, cooking, laundry, etc. The main floor runs about 60% humidity with the A/C running at 76 F.

There is no underslab insulation, but a decent job was done on the vapor barrier. The house is in hot, humid South Carolina. Is the humidity coming up through the slab? The walls and slab were poured over 2 years ago, so I would think that they are done giving up moisture, but maybe not? I'm not really complaining; just curious. Will I always have to run a dehumidifier in the basement?

Question, what is under the slab? Plastic? 6mil? 10mil? Likely creeping up under the slab as well as through the foundation wall. Where else would it come from? I ordered 10mil for under my slab. I expect it to stop 99.9% of humidity. I should only get what comes up through my footing creeping up through the concrete. The clay in my area is pretty wet so we'll see. I'm interested to see what other responses you get.

Well, if there is a decent vapor barrier under the slab, and I mean right under the slab and not under a layer of "blotter sand," as some concrete subs like to have, then there still is the possibility that no capillary break was placed between footer and concrete wall, so that moisture wicks up from the footer into the wall. If that is the case, then this route is running around any "dampproofing" spray applied to the exterior of the wall before backfill. You can paint a concrete sealing product onto the interior of the wall to address this. If the slab was poured right up against the concrete wall, with no break between slab and wall, then some moisture will wick through that connection, and sealer may have to be applied to the top of the slab itself. One such sealer is GhostShield Lithi-Tek LS9500 (http://ghostshield.com/shop/lithi-tek-ls-9500/).

Bear in mind that if there is no change of air in the basement, such as through the air conditioning system, to remove moisture, than it's just a matter of time for water vapor inward migration via diffusion to accumulate. But by sealing the concrete you can retard the inward migration rate to reduce the dehumification needs.

I agree - paint the basement with moisture barrier paint and consider having a blower test done to look for any air leaks to the outside. Or to the upper floor if the AC is off. Basements are cooler than the upper floor, so even with no additional moisture, %RH will be higher.

When the temperature is 76F upstairs, what is the temperature in the basement? If it's cooler down there, the RH will be higher.

Good point about the lack of capillary break. I can see where moisture can migrate in, although it seems like a lot of moisture. The basement is about 2 degrees cooler than the upstairs.

The vapor barrier under the slab is 10 mil, and was in good shape, at least before the concrete crew came in.

I'm not sure how I would paint the interior of ICF walls. I'd have to caulk all of the little seams somehow. Eventually. I'll install drywall. Would it be good, bad, or ineffective to put up a vapor barrier on the ICF prior to drywall? Is there a target RH to shoot for to prevent mold?

Around 60% or 50% if you have carpeting or cool spots. If you really want to save energy, you can use 50% only for some small portion of the day (perhaps 4 hours) and then let it go to 80% or so for the remainder. Ie, intermittent drying is enough to stop mold (but never allow condensation).

Paint that I used for dehumidified storage was intended for lining concrete water tanks and would bridge small gaps.

If the basement is poorly airsealed, and the house is running negative pressure due to exhaust fans, you could be getting a bunch of humid air pulled in from the outside, and the RH will skyrocket as soon as it cools to earth temp.

This is IMO the cause of most basement dehumidifer needs, not ground water.

Absolutely. Preventing pressure differences can be as important as air sealing. Few people measure leakage and far fewer than that measure operating pressure differences.

The basement is very well sealed from the outside. Unless a window or door has a faulty seal, which is possible, everything else is very tight. The outside walls are ICF from slab to roof, so I doubt there's much leakage there. The door and windows have been sealed multiple times, inside and out. There are no fans running upstairs, either. Right now, we're having very low humidity outside. It will be interesting to see if the humidity in the basement slows down, which would indicate outside air infiltration, or remains the same, which would indicate ground moisture.

What you are likely experiencing is the simple effect of psychrometrics. You indicated that your live in hot, humid South Carolina and that your main floor RH is 60% with the AC running at 76F. Go to our website and exercise our Psychrometrics software.

http://www.borstengineeringconstruction.com/Psychrometrics_Calculator.html

If there is any air infiltration into your basement from your main floor or from the outside…and there always will be air infiltration no matter what you do…the dew point temp is 61.2F. This means the RH in your basement will be 100% at 61.2F and water will be condensing on your basement surfaces if they are at or below this temp. If your basement is between 61.2F and 76F, the RH in your basement will corresponding be between 100% and 60%.

If you want to determine if you will have any moisture issues associated with the building assembly layers that you are considering, go to our website and exercise our Building Assembly Moisture Analysis software:

http://www.borstengineeringconstruction.com/Building_Assembly_Moisture_Analysis_Calculator.html

If you basement is between 61.2F and 76F, the RH in your basement will be between 100% and 60%.

He reported that his basement was only a couple degrees less, which would tend to indicate the humidity was coming from infiltration rather than mixing with the upstairs air. Although, he did indicate that his humidity tolerance wasn't much higher than the 60% he tried to keep the upstairs at. If that's the case, even the two degree differential between up and down might be enough to create a sense of excess humidity

It's interesting that the temperature differential between up and down is so small for him. Yesterday, we had outdoor temperatures of 84F. My indoor temperature was about 82F with little to no ventilation in operation and the temperature in the basement was 67F, despite the fact the doors down there have no hardware installed and there are big double holes to the outside in each one.

I guess I missed where jdebree indicated his basement temp was only a couple degrees less than his upstairs 76F temp. I would have expected his basement temp to be more like 66F with a corresponding basement RH of 84.5%. Yes, if the air infiltration is from the outside air, the basement RH would be higher than that. And as the basement dry bulb temp further approaches the dew point temp, the RH will quickly approach 100%.

Posted By jdebree on 15 Jul 2014 12:33 PM
...Is the humidity coming up through the slab? The walls and slab were poured over 2 years ago, so I would think that they are done giving up moisture, but maybe not? I'm not really complaining; just curious....

Get some 4 mil clear plastic sheeting and duct tape and tape a 2'x2' piece to the concrete floor ensuring the edges are sealed to prevent any air infiltration.  Observe for a few days to see if condensation is collecting under the plastic.  That will tell you if is moisture is coming up through your slab.

Jdrbere - your problem is not air infiltration. It is not likely to be moisture from the slab. It is almost impossible for the moisture (or air) to be coming through the ICF wall.
In all likelihood, the problem is lack of infiltration. As you suggest, it take a surprisingly long time for the ICF to give up all the moisture trapped in the foam walls. It is not unusual to see high humidity in an ICF home for two or more years even in our relatively dry climate.

As the basement floor (no insulation) is the coldest surface in the house, it is natural for the humidity to condense on it. Do arkie's simple test. If there is moisture under the poly after a few days, in may be capillary action bring up moisture. If there is condensate on the poly (or the rest of the slab) then it is just condensate.

ON dry days, open wide and exchange the air as often as possible. It should bring some relief but the main problem may not go away for another year or two.

I would not put poly on the walls prior to drywall but if you can afford the time, wait a will longer with the boarding. I have seen the drywall mud filling the screw holes, swell a bit as they absorb the moisture coming out of the walls.

Posted By FBBP on 18 Jul 2014 11:02 PM
Jdrbere - your problem is not air infiltration. It is not likely to be moisture from the slab. It is almost impossible for the moisture (or air) to be coming through the ICF wall.
In all likelihood, the problem is lack of infiltration. As you suggest, it take a surprisingly long time for the ICF to give up all the moisture trapped in the foam walls. It is not unusual to see high humidity in an ICF home for two or more years even in our relatively dry climate.

As the basement floor (no insulation) is the coldest surface in the house, it is natural for the humidity to condense on it. Do arkie's simple test. If there is moisture under the poly after a few days, in may be capillary action bring up moisture. If there is condensate on the poly (or the rest of the slab) then it is just condensate.

ON dry days, open wide and exchange the air as often as possible. It should bring some relief but the main problem may not go away for another year or two.

I would not put poly on the walls prior to drywall but if you can afford the time, wait a will longer with the boarding. I have seen the drywall mud filling the screw holes, swell a bit as they absorb the moisture coming out of the walls.

Huh?

Concrete is a good air barrier, but has HUGE capillary draw. Dry concrete is a so-so vapor barrier, but wet or high moisture content concrete isn't much of a vapor retarder at all.  At 2.5" of Type-II EPS is only somewhat tighter than standard latex paint, a class-III vapor retarder.  If there was no capillary break between the wall and footing, the wall could very easily be a primary source of basement moisture, even if there is pretty good drainage at the footing.

Similarly , if there isn't a vapor barrier/capillary break such as poly or EPDM sheeting under the slab, the slab can be a huge source of moisture. 
In most of the US infiltration into an air-conditioned house adds a moisture burden in summer. Only in the dryer areas of the Rocky Mountains & further west would air infiltration provide any drying function.

To get significant drying out of opening the windows, the outdoor dew point has to be well below the temperature of the basement (or the dew point of the basement air.)  In my New England neighborhood there are many summer days where the outdoor dew points are north of 70F, and the mid-summer average dew point is about 65F.  Pulling/leaking 65F air into a 68F basement (the typical temp in my basement due to the uninsulated slab) raises the relative humidity in the basement to 90%, which is a significant mold hazard.  The "dry"  part of "ON dry days..." needs to be specified in terms of outdoor dew point, not rainfall, etc.   To get down to 50% relative humidity in a 68F basement with ventilation requires an outdoor dew point significantly lower than 49F. (Which is a bit rare in summer in the eastern half of the US.)

I only shoot for 60% RH in the basement, which could be attained via ventilation when the out-door air dew point is in the mid- 50s, but even that is well below the summertime outdoor average.  And that's why mechanical dehumidification is usually a better option than ventilation.