How to Dry Out a Damp Crawlspace
LESLIE: Ron in Tennessee, you’ve got The Money Pit. How can we help you today?
RON: Yeah, I have a home with a crawlspace and I have had some moisture under there. And the builder, when he built it, he ran the runoff from the roof down into the French drains. I diverted that and it’s helped a lot but it’s still moist. And I’m asking if these encapsulated systems, where they trench the perimeter of the inside of the crawlspace and seal off the systems with a dehumidifier and a sump pump – how they work and if that’s a solution to these kinds of problems.
TOM: Alright. So first of all, the roof drains were going where before you capped them off?
RON: Down in the French drain.
TOM: Yeah, that’s not too smart, huh?
RON: No, it wasn’t. I diverted that and it helped a lot but it’s still moist under there.
TOM: Alright. So, now that you’ve got the roof drains disconnected from the French drain, are those drains extending out away from the foundation perimeter?
RON: For sure.
TOM: How far out do they go?
RON: Oh, 20 feet?
TOM: Oh, OK. Well, that’s a good thing.
Alright. So the second thing that you could do, easily, is make sure that the soil that surrounds the foundation perimeter is sloped away. Most of the time, that soil settles after the house is built and becomes flatter or even inverted. So you want to make sure you have a pitch where the soil is running away from the foundation, dropping about 6 inches over 4 feet. You can plant something on that grass or mulch or stone after but make sure you have good, solid drainage.
Now, let’s talk about the vents in the crawlspace. You need to have enough vents, so probably one or two on each wall. You need to make sure that the crawlspace floor has a vapor barrier on it.
What’s the crawlspace floor now?
RON: It’s vapor barrier only.
TOM: It’s vapor barrier? So it’s completely covered in plastic?
TOM: OK. And then, the other thing that you could do is you could add vent fans to the walls and have them wired onto a humidistat.
TOM: So that when the moisture builds up inside the crawlspace because the humidity is high, the fans will come on and draw the drier air in from the outside.
TOM: So those are things that you could do now, without spending a whole lot of money, to try to dry that space out.
TOM: Now, the idea of encapsulating the crawlspace is not a bad approach and many homes are starting to be built that way today. But that literally means sealing everything off 110 percent.
TOM: So since you’re kind of closer to being able to improve the grading, improve the drainage, double-check that vapor barrier to make sure it’s really solid and it’s thick and covering every aspect of that crawlspace floor. Make sure if it overlaps, it overlaps about 10 feet. Make sure it’s up against the foundation walls and then get good ventilation – cross-ventilation – in there using some vent fans wired to humidistats. You may find that that gives you the rest of the moisture reduction that you – that was left over after you rerouted those drains.
RON: Alright. Sounds good. Thank you.