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Finishing a Basement: Moisture Management

Timn222 | Posted in General Questions on

Hi all,

I’m planning to finish my basement and am researching options for the floor and wall assembly to best manage any potential moisture issues.

Some relevant info:

House age: Late 1970s
• Location: Southern Ontario (Zone 6A)
Foundation: Concrete block
• Basement floor: Concrete
• Ceiling height: 7′ 2″
Foundation waterproofing: Some kind of tar spray on the outside
• Slab vapor barrier: no vapor barrier under slab
• Current state: Unfinished (slab/foundation exposed on the inside)
• Water issues:
    • Typically the basement is dry and not even particularly damp.
• On one or two occasions several years ago, a spring thaw caused a small amount (<1/8″) of water to puddle on the floor along the wall/footing joint on one side. Once I started consistently clearing snow on that side of the house, this has not recurred.
• Radon: we had a sub-slab depressurization system installed a few years ago to reduce Radon levels, which is working well. So I’d prefer to minimize penetrations into the slab that could compromise that system wherever possible.

Overall, my main concern is how to best create a vapor/moisture barrier with an air gap so that any rare/incidental water intrusion can evaporate to the outside.

I’ve been considering using the DMX 1-step product ( and laying vinyl plank directly on top of it, which is appealing is I will loose a minimum of headroom that way (given our ceilings are already quite low).

However I’m curious if there are better subfloor options for this scenario.

Also I’m not certain how the floor underlayment or subfloor should best be integrated with the wall assembly to control moisture/vapor (e.g., should walls be framed on top of the underlay/subfloor or not, does the foam board create the vapor barrier or do you add a separate vapor barrier on the inside of framing behind the drywall, etc.). I’ve seen a lot of different approaches in my research, and I’m not sure what is best

DMX installation recommends spray-foaming around the perimeter between the underlayment and the foundation walls if they are unfinished (and then presumably you frame walls on top of that). Another assembly I saw recommended leaving a gap between subfloor and wall and then spacing the foam board off the wall a bit to create a continuous airflow behind the foam board wall insulation and the sub floor such that moisture could evaporate behind the foam board and out of the building.

I know there’s often not one right way to do things, but I’d appreciate any input/perspectives. Thanks in advance.

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  1. Expert Member
    BILL WICHERS | | #1

    I used that product you linked to, but I used it on one wall that sometimes has some water come through. It's a pretty good product, basically a shallow version of dimple mat. I wouldn't have any worries using it as designed on a floor. Another product that is often recommended is Dricore, but it's quite a bit thicker so you'll lose more headroom.

    I would use at least 2" of polyiso directly against the walls. If you use something like Dow Thermax, you can leave the interior side uncovered. I would install the floor system first, all the way to the foundation wall, then install the polyiso above that. This will help limit the chances of the polyiso sitting in a puddle which you want to avoid. If you have any concern about deeper water on the floor, leave an inch or two gap under the polyiso, which can be filled in with strips of XPS for insulating if needed. Remember that most of the value of that wall insulation is for the portion of the wall above the frost line, since that's where the temperature differential through the wall is greatest. I'm assuming here that your water came in through the slab, or between the bottom of the foundation wall and the slab. If the water came through the wall, you may want to consider running the 1 step product up the wall behind the polyiso to provide a drainage plane (which is what I did, except that I used EPS on this particular wall instead of polyiso).

    Regardless of what you do on the walls, foil faced polyiso or that dimple mat like 1 step product will provide your vapor barrier so you don't need anything additional for that purpose.


  2. Timn222 | | #2

    Hi Bill,

    Thanks a lot for your reply - very helpful!

    You are correct that in the water incident I referred to it came through the slab/wall joint.

    However I did once have some water leaking in through the window wells when leaves clogged the drain there. So the notion of running the DMX continuously from the floor up that wall to create one uninterrupted drainage plane and moisture barrier is very appealing. It's good to know you used it on a wall successfully.

    Thanks again.

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