Unfinished basement playroom – addressing moisture issues
Please help me sort out the vast amount of information I’ve consumed in researching this issue!
I have a below-grade poured concrete basement in a house built in the 1970s. We live in New England. Our basement is bone-dry in the winter when the soil is frozen, but it gets very humid (and somewhat musty-smelling) in the summer, so I assume there’s quite a bit of water vapor getting through the walls. We constantly use a dehumidifier in the spring/fall/summer seasons. We already addressed issues like rain spouts, grading, etc. French drains and water-proofing the basement from the outside are unfortunately not in the budget.
We’d like to create a play space for the kids. Our goals are to brighten the walls with paint and put down some area rugs on the floor – not planning to insulate, and not planning to put in drywall/ceiling. We are not sure whether any attempts to reduce vapor coming through the walls/floor will prove futile, and so far, I’ve read about two somewhat conflicting approaches to this issue. Can you advise me which one would be better in our situation?
1. Use a permeating concrete sealer on the walls/floor to reduce some of the hydrostatic pressure; then use paint on the walls and, if we want, epoxy on the floor. Proponents of this approach state that the permeating sealer won’t fail (as opposed to a surface sealer such as dry-lok) and because there’s less moisture getting through, it’s OK to use concrete paint that will bind to the sealer.
2. Don’t use any sealer because walls need to breathe and dry to the inside. Proponents of this approach suggest dehumidifying the air inside the basement, and don’t really suggest a good way to brighten up the walls. I was thinking of using mineral-based paint or limewash because both of those appear to be breathable.
So which is it? Do concrete walls really need to breathe and will I wreck them and any adjoining wood components by sealing the concrete from the inside? Anyone have any experience with this? Thank you for your thoughts!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
>"Our basement is bone-dry in the winter when the soil is frozen, but it gets very humid (and somewhat musty-smelling) in the summer, so I assume there’s quite a bit of water vapor getting through the walls. "
The more likely cause of the summertime humidity is outdoor air leakage or ventilation with outdoor air, not water vapor coming through the walls. The mid-summer outdoor air dew point averages in New England are in the mid 60s or higher, which is higher than the temperature of the slab, and close to the temperature of the basement. Cooling that outdoor air to basement temperatures results in a higher relative humidity. Air with a dew point of 65F brought into a 70F basement has a relative humidity of 84%, which is WELL into the humidity levels that support mold and fungus growth.
Limiting outdoor air leakage and ventilation into the basement by air sealing would reduce the amount of mechanical dehumidification necessary.
INSULATING the basement walls would raise the wall temperatures high enough to limit the amount of mold/fungus growth on the walls.
Rugs on an uninsulated basement slab in New England is a TERRIBLE idea. The floor will be more comfortable under bare feet, but the rugs insulate the slab from the room, making the basement slab a few degrees cooler than it would be otherwise. That makes the relative humidity at the bottom of the rug extremely high, and more likely to grow mold on the bottom of the rug.
A lot of moisture in basements comes from warm humid exterior air entering the basement. If you don't have bulk water, air sealing is actually your first line of defense. Rim joists leak a LOT of air.
After air sealing, see if you are reasonably able to use a dehumidifier to maintain the dew point above the wall and slab temperatures. Any rugs you put down will make the slab below cooler, which can be a problem. In theory a closed cell mat will prevent condensation below.
To check for moisture coming through walls/slab tape down plastic sheet and check for moisture after a few days.
Hi Liz -
It is important to determine where your basement moisture is coming from. There are ways to test (blower door for air leakage and moisture meters for concrete) but the taped down plastic sheet test does not work; too many false negatives. See this blog: https://www.greenbuildingadvisor.com/article/extending-the-reach-of-a-moisture-meter.
In terms of carpet/rugs in basements, see this GBA blog: https://www.greenbuildingadvisor.com/article/carpet-in-basements-the-issues-solutions-and-alternatives.
Peter
Hi folks, thanks for your responses. I should say, we did do a good bit of air sealing the basement around the perimeter and joists, so we have addressed that source of moisture as much as we could. We can tell from the efflorescence on the walls that there is at least some moisture getting through the concrete. Unfortunately insulating basement walls isn’t in our budget either :) Of the two options - sealing concrete floors and walls or leaving them alone - which would you recommend?
If you can find a foam recycler with with dow thermax or similar seconds you can insulate with foam board for a very reasonable price ($30 3"x4'8'). This foam is rated as a thermal barrier and doesn't require drywall. Although, I would probably hang white faced masonite in the kids area to at least 4' high for kid impact resistance.
>" I would probably hang white faced masonite in the kids area to at least 4' high for kid impact resistance."
I saw a basement playroom retrofitted with 2" reclaimed roofing foam & OSB through-screwed to the foundation with 3" masonry screws. The OSB painted up well, and was stout enough to use as a soccer ball rebound surface. (They even painted the outline of a diving goalie and several targets on the wall for practicing high/low & side shots.)
Liz, to answer your question regarding whether or not a concrete wall must breathe to the interior, the answer is no. Concrete is quite content remaining in a wet state. Many bridge supports are underwater all the time. But concrete is porous, which lets soil moisture diffuse inward. Your house, built in the 70s, almost certainly has minimal "damp proofing" sprayed on the outside, and likely has no vapor barrier layer between footing and wall, which lets ground moisture wick upward into the wall.
So, yes, you can safely apply any coating on the inside that retards inward migration of moisture. You ought to be sure that you have addressed any liquid water leakage issues. Long-term, as a means of addressing the musty smell in summer, you need to have a plan for eventually covering the concrete walls with a layer of rigid foam, with 2x4s on the flat or strapping over that, and finally wallboard or something else that protects the foam from damage and as a thermal barrier. Refer to this GBA article: https://www.greenbuildingadvisor.com/article/how-to-insulate-a-basement-wall. Finally, you can run a dehumidifier in the basement in the summer to keep the interior air dew point lower than that of the exterior air that leaks in.