How to Reduce Moisture in a Basement Wall Insulation
LESLIE: Bill in Michigan, you’ve got The Money Pit. What can we do for you today?
BILL: Well, I’m having a moisture issue in between the fiberglass-batting insulation and the foam board – FOAMULAR foam Owens Corning insulation that I put in.
BILL: The moisture builds up in between the two layers of insulation.
TOM: So you have two layers of insulation? You have both fiberglass and foam boards?
BILL: Yes, sir.
TOM: So what did you put on the – you guys started with poured concrete walls?
TOM: And you attached foam to that or you left a gap?
BILL: I attached foam to the poured cement walls and then I have – it’s a walkout, so I had some knee walls. Well, they have some fiberglass batting.
TOM: Is there any space between the knee walls and the foam insulation?
BILL: No, sir.
TOM: So, generally, when you use fiberglass on a basement wall, there’s a special type of basement insulation that has fiberglass that’s encapsulated inside vapor barriers. Usually, it’s foil-face. Kind of looks like a big, warm blanket. And it’s designed specifically to be attached to the wall and give you as much insulation as you can reasonably achieve, in that sense. If you’ve got a lot of condensation in here, you may have another problem, which is that you’ve got too much moisture in that space. So let’s deal with that first.
The most common causes of moisture in a basement, everything from a little bit of condensation to full-blown flooding, is poor drainage conditions, not inside but outside the house. So if your gutter system is nonexistent or if it is there and the downspouts, for example, are discharging too close to the corners of the foundation or if the gutters are blocked and they overflow, those are all great sources of moisture that will find its way into the basement.
Those poured-concrete walls, as solid as they are, they’re very hydroscopic; they’re very absorptive. And that water will pull right up into that wall and show up as condensation inside. So you want to make sure that you have a good gutter system that’s getting that water well away from the foundation. And you also want to make sure that the soil around that foundation slopes away. We like to see it drop about 6 inches over 4 feet. And then once that’s set, inside the basement you may need to add a dehumidifier, as well, working inside and outside to manage the amount of moisture that’s in that space.
Now, in terms of the insulating of the walls and the finishing of the walls, generally, if you’re going to frame a wall, I would tell you to leave at least 3 or 4 inches between that and the foundation so that you do have some space back there. If it does get a little damp, it can easily ventilate and dry out. Once that wall is framed, one trick of the trade that I’ll often use is I’ll put sort of a dummy heating register into that wall – a couple down low and a couple up high – so it moves some of that conditioned air through, behind the wall, and helps to keep it a bit drier.
You know, managing moisture in a basement is always going to be a challenge but if you approach it that way, it’s definitely a challenge that you can overcome.
BILL: Well, great. That’s very helpful. Sounds like it might work.
TOM: Alright, Bill. Good luck with that project. Thanks so much for calling us at 888-MONEY-PIT.