Best Way to Reduce Crawlspace Moisture

LESLIE: Martin in North Carolina is on the line with a question about how to reduce crawlspace moisture. How can we help you?

MARTIN: I have a ranch-style house. One-floor ranch. And underneath it, we cleaned up real good and we have plastic all around the edges. About a 10-foot roll of plastic. So that leaves the center part still exposed to moisture. We just need a (inaudible). My wife tells me that we should have plastic all over the ground underneath the house.

TOM: Your wife is correct. Yes, you should have a vapor barrier across all that surface to reduce crawlspace moisture.

MARTIN: That’s what the builders say around here but I’ve heard that if you do not allow some moisture, your floorboards – in our situation, it’s oak flooring – it will separate and have little cracks (inaudible).

TOM: You are never going to have that dry of a crawlspace, my friend. There’s always going to be humidity and moisture that’s in the air. But you want to – right now, you have all the water that’s in the soil that’s going to come up and add to that humidity. You can never dry out that crawlspace so much that you’ll have any issues with cracked floorboards, trust me.

So, I get – sometimes these rumors have – are based in a little bit of truth. But there’s just no way you’re going to make it that tight when you want to reduce crawlspace moisture. So I would get some additional plastic Visqueen, some plastic sheeting. Lay it down so that the entire floor surface – entire dirt surface – is covered. Make sure your vents are open so that the – you will get some air in from the outside. But you’re going to get plenty of moisture in that house. You just won’t get as much – you won’t get so much that it’s going to impact the insulation and give you mold problems, OK?

MARTIN: Now, you’re saying that our vents should be open.

TOM: Yeah, the vents should be open for about 9 or 10 months of the year. If you want to close them during maybe December to January – when it’s really, really cold – that’s OK. But those vents should be open because that lets some of the drier, ambient air from the outside get into the crawlspace and take away some of the moisture, as well. That’s why crawlspaces have vents.

MARTIN: Well, we’ve been having them closed now for too long then, yeah.

TOM: Yeah. Nah, you can’t seal it up.

MARTIN: I’m glad you told me that. That’s a big help.