Finishing a Basement: Moisture Management
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 (https://dmx1step.com/) 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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