Mold-free basement floor
Hi,
We bought a house in snowy Canada about a year ago only to discover soon after that the basement was full of Mold. A vapor barrier was installed right against the concrete wall and floor, created condensation and mold fest…
We are now at the stage where the basement got demolished, mold removed, all tests good to start rebuilding properly. Nightmare is over but we don’t want another one!
My question is relative to the flooring. The house is from the 60’s, so there is no vapor barrier under the concrete slab. I don’t have issues from water coming in from a high water table or other but I know moisture coming thru will be an issue or challenge. From what I have read and been told so far an option would be to put down tile right in the slab, its level isn’t bad for tiling but there are some cracks here and there so I’m afraid of tile cracking. I don’t want to use Ditra being plastic I’m afraid of mold underneath. I’ve also been told to do a raised subfloor with plywood but with openings every 12 feet to let air circulate and moisture out. I have read on the board here to use a fully adhered moisture barrier on the slab, this makes sense since no air gap means no mold, but how do I build the floor over that?
Final note I don’t have a lot of head room in that basement so even if no possibility of mold is the number 1 priority after what we’ve been through, the second is head room, third is cost.
Thanks in advance for helping us!
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Replies
If water condenses on the floor and keeps things moist, mold will grow on just about any floor using dirt as a food source. There are paints with moldicide (not sure if that’s a word) in them.
You might want to consider just putting down something that is easy to clean like an epoxy floor. Epoxy is relatively inexpensive as floors go, it goes down quickly, and it’s very easy to clean. Any mold could just be mopped off with some disinfectant and you’d be good to go.
Bill
Hi Bill, I am going for something nicer since there will be a family room in the basement. Some professional epoxy finish are quite nice but I thought adhesion was affected by the moisture from the slab.
You are right it doesn't take much for Mold to grow which is why we are looking for an option that has no air gap.
Consider a fully adhered coating, unfaced EPS foam, Hardibacker, tile. Do not use wood in a moisture trap.
Hi Jon, Do you have examples of fully adhered coating? Would that be a membrane like Blue skin or else or liquid. I have about 1000 sqft to cover so looking for easiest and most economical, provided it does the required job.
Jeff, mold generally needs moisture and temperatures above about 50°F to grow. When the floor is cold enough, moisture condenses. So if there is not a source of liquid water, you just need to keep the floor warm enough and/or the air dry enough that moisture will not condense out of the air.
Without insulation, the floor surface is probably about 50°F, which will create condensation if the air is 70° F and 50% relative humidity. If you have even just an inch of insulation on the floor, the floor surface will be very close to room temperature and should not allow condensation unless humidity levels are very high. Or if you keep the air at 70° and below about 30% RH you should not have a problem.
The easier of those two options to maintain is likely adding insulation. Even 3/4" will probably be enough. Over the foam you can float two layers of 3/4" subflooring, with seams staggered and overlapped, use any tile underlayment you want, with a total buildup of less than 3". If you have a perimeter drainage system you can leave small gaps between foam sheets (3/8" or less) which will allow water to drain without affecting thermal performance.
For ceiling height, one trick I've done when finishing basements is instead of lowering the ceiling below the floor joists, install panels between joists about 1/2" to 1" from the bottom of the joists. That creates a bit more headroom and some visual interest that helps disguise a low ceiling. Large, monolithic ceilings always feel lower than ceilings that are broken up with details (color and lighting differences aside).
Hi Michael, If I was to do as you suggest with a subfloor I might do vinyl planks instead of tile. Tile was really our option if straight on the slab. I assume then I would put only 1 layer of 3/4 subfloor? By the way love the idea of panels between joists, I would go with drywall unless you have other ideas.
Thanks again to all so far for the help
Jeff, you either need two layers of subfloor or you need to interrupt the foam with sleepers. Here are a couple of articles that explain the concepts. They are behind paywalls but have been influential pieces.
https://www.finehomebuilding.com/2005/03/01/the-no-mold-finished-basement
https://www.jlconline.com/how-to/foundations/a-basement-floor-without-concrete_o
In your case I would be sure to add a layer of poly sheeting OVER the foam.
I'm glad you like the idea of panels between joists. Drywall would be the least expensive material. Once you factor in labor, you might decide that cabinet-grade plywood, PDF or similar would be easier, but there's nothing wrong with drywall in that location. Hopefully most of your wires and plumbing are run through the joists rather than below the joists.
If you are certain the only source of water feeding the mold is from water condensing out of the air and condensing on the floor a large dehumidifier set to 50% RH connected to a drain should keep the mold at bay at about $25.00 a month to operate.
I am not so sure you do not have Leakey walls and floors. In the 60s basement slabs were poured directly on the dirt. Today we would have several inches of gravel and a plastic vapor barrier. Over the years the quality and quantity of water proofing on walls have improved. It is likely you have no footing drains.
Walta