Slab Moisture
Greetings.
We built a log house with a framed addition on a floating slab, and are having some issues with condensation around the perimeter of the slab. We are in Southwest Wisconsin.
In the log house section of the slab, we wanted wood floors. So I put down 6 mil poly (taped), then 2×4 floating cedar sleepers, then rigid foam between the sleepers, then tongue and groove subfloor, then wood floor. I’m still a little anxious about that, but after talking with some builders, decided that was the best approach for us.
Now I’m trying to figure out what to do in the framed addition, which serves as a mudroom and storage. We’d planned to just paint the concrete, but with the condensation, which comes and goes, we are second-guessing that. Any ideas? Would the set-up we did in the log house section be our best bet? Or something else? Are we doomed?
Thanks,
Joe
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Replies
Joe, did you insulate the slab perimeter?
Yes, the outside perimeter is insulated with rigid foam, 1 foot down and 3 feet out. We also have rigid foam beneath the whole slab -- though the concrete guys did not put rigid foam beneath the perimeter footing. They also forgot a vapor barrier. I think the rigid foam does a pretty good job as a vapor barrier for most of the slab, and I've wondered if the missing foam/vapor barrier under the footing could be what's causing the moisture. I guess that's all pretty pertinent info -- thanks for the clarifying question.
Joe, it is likely that the un-insulated footings are keeping the slab perimeter cool enough for condensation to form. 70°F air at 50% relative humidity will condense on a surface that is about 50°. 80° air at 40% RH will condense at 54°. Those temperatures are probably in the range of your soil temperature, and with concrete at R-0.08/in, it doesn't provide much resistance to heat flow, so the surface of your slab where uninsulated will be about the same temperature.
To solve the problem, you can reduce the indoor relative humidity with a dehumidifier or air conditioner.
Did the builder place the slab on washed, crushed stone or on gravel that included fine particles? Do you know where your water table is? If the soil under the footing is damp, you could also be seeing wicking, though it usually doesn't show as liquid water on concrete.
If you are still under construction, is the space currently unconditioned and exposed to outdoor air? If so, day/night temperature cycling can chill the slab. The extra mass at the edges would keep the slab cooler longer in the day, and mid-day humidity could condense on the slab. Once the building is closed and conditioned, this would go away.
With your insulated foundation and slab, this shouldn't be a permanent issue. If you want a guaranteed fix, though, a layer of insulation on top wouldn't hurt. I would prefer a continuous layer of EPS with the poly film on top of that (you want to stop moisture coming from above), and a plywood/finished floor floating on top of the whole thing.
Thanks for your help, Michael and Peter.
I'm pretty sure they put the foundation on washed gravel, though not 100 percent certain. As to the water table, I don't know that, either.
I have a lot of finishing work to do, but the building is no longer totally under construction. It's enclosed, insulated, etc., so not exposed to outside air. But we also don't have air conditioning, and as we're off-grid, can't run a dehumidifier. So the room isn't exposed to outside air exactly, but the inside air might correlate to outside air more than other houses? We leave all our windows wide open. In the winter, we will be blasting a wood cook stove.
The moisture does seem to correlate with humidity in the air. If we're in a prolonged hot and/or damp spell, the moisture is much worse. At times when it's dry, there's no moisture.
We're considering linoleum tile for ease of cleaning in the mudroom. Do you think we could do the set-up you suggest, Peter, and top it off with linoleum?
Thanks again for your help. This website is an amazing support to us owner builders!
彼得有很好的温度cycli点ng. Linoleum should work well over plywood (or Advantech) on sleepers on a vapor retarder on EPS. Or you could double up the Advantech and let it float on the the poly and foam.