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Finishing Basement – Insulated sub floor question

bogden | Posted in General Questions on

Hello, thank you for reviewing my question / dilemma. I am in the process of finishing my unfinished concrete poured basement in my 2,550 sqft ranch home built in 2002. Based on the articles within GreenBuildingAdvisor I took the approach of insulating all of my walls with 2″ EPS glued directly to the walls. I am 70% done with this part of my project. I have utilized canned spray foam insulation between the seems of the 4’x8′ panels and tyvek tape. I have fixed all areas where I’ve had water seep in through hair line cracks in the foundation wall in the past. Go figure, after a month of having the insulation secured to one of the walls that has never had any water issues, I discovered a small puddle of water. My plan originally was to glue 2″ EPS from top of floor joist to the bottom of the floor. And for the floor to use 1″ thick rigid foam butted up to the 2″ wall EPS. Plywood over the 1″ rigid foam for my sub-floor. However, here is my dilemma, I have subscribed to the wisdom of having the thermal break and air sealing by utilizing 2″ EPS on the walls, and intended on doing the same with the 1″ rigid foam on the floor, BUT, after seeing the water seep through on a wall that previously never had any water issue I have realized that water is going to come in to the basement in some occasions. So if I did the sub-floor with gluing the Rigid foam directly on concrete basement floor then I am giving the water that gets into the basement through the walls no where to go. If enough water was to come in a particular spot and had no where to go, I suspect the water pressure could increase enough to begin penetrating the seems of the EPS, or even pop the insulation off the wall over time. Or if it does find a way to get above the rigid foam on the floor it would then be very close to plywood sub-floor and could possibly saturate bottom plate of framing. SO my solution that I have come up with mentally is to utilize a dimple plastic underneath the rigid foam, tuck it about an inch underneath the 2″ EPS on the walls. This way any water that comes in through the walls would be directed underneath the dimpled plastic. The little bit of air flow provided by the dimpled plastic would eventually allow the floor to dry or absorb the moisture through the concrete. OF COURSE this goes against the whole wisdom behind air sealing and creating a thermal break on the concrete basement floor ensuring no condensation. But I think this is the only way of ensuring water that occasionally comes in through the walls has a place to go without destroying the finished basement.
What do you guys think? Thank you, and sorry for the long worded question!

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  1. severaltypesofnerd | | #1

    What's your climate?

    I did a dimple mat slab floor 5 years back, and it has performed great. Dimple mat, EPS, ply, hardwood.

    Or there are a variety of pre-made air space basement floor products, typified by "DriCore" floor panels, with or without insulation.

    I also just tried a product for filling hairline concrete cracks (it's meant for keeping out radon gas which it turns out is primarily leaking into buildings because of cracks not other reasons).

  2. GBA Editor
    Martin Holladay | | #2

    Finishing a basement always carries risk. Rule number one for finishing basements is, "If your basement has a water-entry problem, solve the water-entry problem before you begin insulating or finishing." For more information, see Fixing a Wet Basement.

    In a case like yours, with puddles near the base of a wall, the standard solution is to install an interior French drain at the perimeter of the basement. This French drain is connected via perforated pipe to a sump. The sump drains to daylight or is equipped with a sump pump. Details are in the article I linked to.

  3. Jon_R | | #3

    > after a month of having the insulation secured to one of the walls that has never had any water issues, I discovered a small puddle of water.

    Not surprising, the moisture probably used to evaporate from the walls before it accumulated.

    True, if you direct wall water under a floor that allows water movement, then you may not need an expensive interior side perimeter drain. Distribution and evaporation or small holes in the slab might be enough to remove the water. Or a sump pump can remove larger amounts of water. If you smell mold, you can slightly depressurize the under-floor gap to prevent the odor from entering the living space.

  4. seabornman | | #4

    Keep in mind that you have effectively lowered the temperature of your foundation wall and you used an insulation that is vapor permeable. The water could be condensation and not a leak you haven't noticed before.,

  5. GBA Editor
    Martin Holladay | | #5

    The scenario you suggest is highly unlikely.

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