Dealing with Basement Water Intrusion
At my workplace the basement walls become saturated after heavy rains. I am not a builder but am responsible to manage the building. Right now the basement is unfinished, but we’d like to finish, as soon as we can get a reliable solution for moisture. I’d value advice from the GBA & building science community.
Here are the specifics…
1) At construction 12 years ago exterior concrete block walls were damp-proofed
2) Drain tile with pea gravel was installed outside foundation around perimeter, draining to daylight. Water comes out of the drain pipe, so it is flowing.
3) Basement ceiling is at grade on north and floor is walk-out on south, sloping 13′ over 50′ run. Surface grade is away from building on all sides for a few feet, but building is on the low side of a gentle hill extending about 500′ to the north and about the same to the west. Lots of hydrostatic pressure.
4) Downspouts go to buried flex pipe and drain to daylight 50′ downhill from building
4) There is efflorescence in the areas with a history of visible moisture after heavy rains, and along the joint where the concrete floor meets the block wall.
5) We remediated for mold two years ago, and have tested again periodically. Although the more recent testing has come back “clean…” at best, the moisture puts us at constant risk. At worst, there could be mold growing at areas not being tested, and it can begin again anytime.
6) 3 of the 4 basement walls are block drying to air. The wall which is 100% under grade is framed and insulated. Polyethylene against block, then 2X4 studs with fiberglass batts, no finished surface. None of the blocks have ever been painted or sealed from the interior.
7) The wall behind the visqueen is damp 100% of the time, but of course we never see it. The original premise of the builder was that the impermeable plastic would contain the moisture and protect against mold spores having access to moisture.
Since we’re already built, I need help on how to bring correction from this point forward. Here are my questions:
* Because there is efflorescence along the soleplate of the insulated wall, I’m guessing the lumber (although treated) may be at risk for rot or mold after 12 years of nearly constant moisture exposure. Should we pull out the entire wall of framing & insulation, clean or remediate the block professionally as needed, and start fresh?
* Is it advisable to drill out the wettest blocks and fill the hollow space with something to make them less permeable? Any recommended products or systems? Other ideas for keeping out bulk water? The basement is conditioned space, with furnace/AC and constant dehumidification set to 45% summer and 35% winter.
*If we start with clean block on all 4 sides, what do you recommend for the wall assembly?
*What do you think of de-watering system at base of wall, 2″ air gap, XPS & MGO board?
Really appreciate the help in thinking through the options.
-David James
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Replies
David,
You should probably talk to a contractor who specializes in wet basements. The best solutions require excavating the exterior of your building down to the footing. A good exterior waterproofing system includes a high-quality waterproof membrane, a dimpled drainage mat, and coarse granular backfill.
Of course, when the footings are exposed, you should verify the integrity and operation of the footing drains.
你有什么具体建议high-quality waterproof membrane?
David,
Being that your existing drain tile is flowing water, you know that it is working. However, apparently the hydrostatic pressure from that hillside is pushing water up against your walls before it gets a chance to drain down to the tile. You could dig up the foundation and improve the detailing in such a way to keep the walls dry, but that would be complicated and expensive to execute in a way that you are 100% certain that the entire moisture/dampness problem has been solved.
I would consider installing a French drain out maybe 15-20 feet or so from the foundation. That would be a trench with an independent drain tile. Install the tile with the proper pitch and line the trench with geotextile fabric all the way up the sides to the surface. Then fill the whole trench with drainfield rock to the surface. You might cap the rock with fabric at about 12” deep, and then fill rock on top of the fabric to reach the surface. That way, you would have a filter to keep silt out of the majority of the rock bed. The trench might be 3 feet wide or more at the top. It would be as deep as your footings, or possibly a little deeper.
This drain will intercept all the water coming down the hill on the surface, as well as the water moving through the soil under the surface from the pressure created by the hillside. You would have your ground around the house pitched to drain all the surface rainwater into the French drain as well.
With this approach, no water should get near the basement walls, so everything associated with them will dry out and stay dry. I think this would be a lot less technically demanding than trying to perfect a wall system that has hillside water pushing up against it.