What to Do When You Get Moisture Through Basement Walls

LESLIE: Thomas in Michigan is on the line and has a question about moisture through basement walls. What is going on?

THOMAS: Well, I’ve had my – I have poured walls in the basement. And I had it recently – I had spray foam through – on the sill plates in the exterior – in the walls – exterior walls. But now I’ve developed some – a moisture through basement walls. And it looks like I’m getting a mildew. And I’m wondering, is there any product that I can put on there or insulate it? I have – outside, I have about a 2-foot exposure from the ground up to the vinyl siding. And is there a product that I can put on that or on the interior or exterior that would stop this moisture content and possibly keep the mold out or the mildew out?

TOM: OK. So, first of all, no matter what you put on those walls, outside or inside, you’re not going to make them waterproof. The way to reduce moisture through basement walls is to manage the water at the foundation perimeter by trying to keep the first few feet of soil around your house as dry as possible.

The way you do that is two things. Number one, you make sure you have gutters, the gutters are clean and the downspouts are extended at least 4 to 6 feet from the house. Most are not. Most usually turn out a few inches into a splash block and end up sort of doing a U-turn and sitting – dumping all that water right up against the perimeter there. And secondly, you make sure the soil is sloping away. If you’ve got a lot of mulch, for example, or stone or anything like that that doesn’t let the water run over it and out, that will keep water at the foundation perimeter. So, those two things are most important.

Beyond that, on the inside foundation, what you could do is apply a water seal to the foundation surface. Thompson’s WaterSeal is a good product. And that’s a product that will seal some of the porous nature of the concrete walls and slow that evaporation of the water coming from the outside.

So, manage the water and then seal the walls on the inside. You don’t have to do anything else on the outside than that. There’s nothing you need to put on that exposed foundation that would help.

THOMAS: Oh, good. I know I have gutters and I have downspouts away but I’m still getting – you know, the cold weather we have here now, it seems like since it got cold – but I have the basement heated. So I’m getting a condensation there. I’ve been running fans and a dehumidifier down there but we’re just wondering if there was something on there that I could stop that. So, you say Thompson’s WaterSeal will …?