Finishing a Basement with Prior Moisture Issues
A family member recently had their unfinished basement ‘waterproofed’ after experiencing some major health issues due to mold exposure that originated there.
They are now thinking about whether they could at least partially finish the space in a way that would not further encourage mold or mildew issues. From what I’ve seen, this often involves lining the interior of the walls and floor with an airtight layer of insulation, then framing/subfloor, then finishes.
Are there other aspects to consider, for a basement that had moisture issues that have since been mitigated?
Some of the interior waterproofing work included things like a sump pump, interior perimeter drains, and flashing the bottom of the walls. Do these create special considerations/obstacles? Are there things which would be a good idea to do or avoid under the circumstances?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Interior perimeter drains can complicate things since they imply that water can still come through the wall. Interior drainage systems tend to be used in retrofit situations, and they work by carrying away water that makes it through the wall before that water can build up as puddles on the floor. That means the wall can still get wet at times. Exterior sytems tend to be very costly to put in after construction, but they keep water from getting through the wall in the first place.
My guess is that in certain situations, those walls will get wet, and the water will run down to the perimeter drain. That means you need to deal with that water flow at times. The easiest way to do this is probably to put a layer of dimple mat against the wall, then put rigid foam over that. The dimple mat then provides a drainage plane to allow any water seeping through the wall to get to the perimeter drain where it will be carried away. After that's in place, you can do things more normally with the rest of the project. I would be extra careful to avoid mositure prone things though, which would mean avoiding any use of batts in the walls, and avoiding any carpet on the floors.
Bill
Thanks, Bill. I really don't know much detail as to why the perimeter drain was installed. I did check with my family member and she said,
"On the exterior, they dug out everywhere around the house except along the driveway, sealed the foundation and repaired any cracks, installed a barrier to keep moisture flowing away from the house (not sure what they used though), and back-filled with a slope away from the house. (An exterior drain was against city ordinances.)"
I think you're right on about the dimple mat. I know Martin H. has mentioned that, in theory at least, concrete walls can stay damp and be happy forever. But I would guess that it's probably a little bit better if they can drain. I'm sure given their health conditions, erring on the side of caution would be preferable.
Standard advice for basements that have been wet is to fix the external drainage first. This means gutters move the water away from the building, and grading keeps ground water away. If this hasn’t been done yet, take another look.
Mold and mildew can form in basements even if there are no external water leaks. Water vapor in the air tends to condense on cold surfaces, so basement walls and floors that are in contact with subsurface earth temperatures can be an issue. This is the primary reason for insulating under the slab in new construction, rather than for energy savings – it’ll keep the floor warm and avoid moisture. Insulating the walls, after making sure they are airtight, should help. Bill gave some specific advice regarding dimple mat installation.
Carpets in the basement should usually be avoided because they get wet if there ever is any water, and tend to insulate the floor, making it colder and even more likely to condense vapor. And they have lots of surface area to grow funk.
Air sealing in the rim joist area will help reduce moisture, especially in the summertime when more moist air tends to get pulled in at the lowest level of the house.
My personal preference for finishing basements avoids finished ceilings. I’d just spray paint everything white, or possibly black if you’ve got taller ceilings and good lighting. You always end up wanting access to that ceiling space later, and drywall or drop ceilings make it much harder.
I just mentioned it in reply above, but I'll add it here as well. I asked them about exterior drainage and they said the company did this:
"On the exterior, they dug out everywhere around the house except along the driveway, sealed the foundation and repaired any cracks, installed a barrier to keep moisture flowing away from the house (not sure what they used though), and back-filled with a slope away from the house. (An exterior drain was against city ordinances.)"
So it sounds like there was some efforts to go after the issue on both sides of the wall.
Does it make sense to put rigid insulation down on top of the slab? They have ~7' to the joists. Not spacious, but plenty of headroom to spare. I think I saw an article where someone insulated the floor, put down a subfloor and then a floating floor over that. If they did that, should they be taping the floor insulation to the wall insulation?
Interesting idea about painting it all white up top. I agree - closing off access to all the utilities would just complicate things!