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Community and Q&A

Waterproofing a Cinder Block Foundation Wall

marleyandbowie | Posted in General Questions on

Hi everyone. My basement flooded last weekend, resulting in about 6-8″ of standing water in the basement and probably a little less in the dirt floor crawlspaces. I can already smell mold forming, and who knows how much was there to begin with as I haven’t ever rummaged through the crawlspaces since we bought the house 3 months ago. We have a mold remediation company coming in next week to treat the entire area – floors, walls and ceilings. They are proposing to vacuum, scrub and then seal all surfaces with a Fiberlock product. Is it ill advised to water seal the inside of CMU and concrete foundation walls? Would it be better to just disinfect the inside of the walls and seal from the outside by digging down to the footers and applying a waterproof membrane? Would sealing on the inside but not the outside result in “waterlogged” CMU blocks in the event of another flood? I’m very new to all this but if anything it makes more sense to me to perforate the bottom of the CMU walls and provide a drainage route to the sump pump, rather than trap all the water in the blocks.

I like the Fiberlock line of products and it makes sense to seal the wooden subfloor and joists, but I’m nervous about treating the walls and concrete floor.

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  1. Patrick_OSullivan | | #1

    How did the water get there in the first place?

    To properly deal with basement water issues, you need a 'threat model'. Water from a burst pipe is a lot different than water intrusion via the foundation.

    1. marleyandbowie | | #2

      The water came in through the bottom of the foundation walls, and up through the sump pump well. The sump pump failed, resulting in a rapid accumulation of water up to about 8"

  2. the74impala | | #3

    It sounds like you have the solution you need, minus a secondary pump with a battery backup.

  3. GBA Editor
    Kiley Jacques | | #4

    This is a good article for you to read:
    Fixing a Wet Basement.

  4. walta100 | | #5

    In my opinion most basement water problem start before the house was built. If you want a dry basement you need to buy a lot with enough natural change in elevation so that your basement can drain to daylight. This generally means you are building on a hill and you will not have a baseball field in your backyard. Many people look at that big flat yard a as a desirable feature in a home, Me not so much I look and say where does the water want to go.

    Call me paranoid but I do not buy property next to the creek or down by the lake.

    Is there a point on your property with daylight lower than your foundation to drain to?

    My opinion any sealer applied the interior is unlikely to keep water out in the long term so long as the water level on the exterior is higher than your basement floor.


  5. onslow | | #6


    You are correct about sealing the outside of the block being better than inside. Block walls will fill with water. Perforating the blocks on the bottom course and confining drainage to the sump might be of limited value if the sump pit erupted with water. You need to analyze both the source of the water and final destination as best you can before committing to any remediation plan. I have seen homes that basically recirculated the sump water in a circle since no provision was made for dispersal. That said it is not unheard of to have two battery backup sumps to ensure better fail-safe. It might be the simplest course of action until you can get a clear idea of why you flooded.

    I will have to counter Walta's notion that building on a hill is any form of guarantee of dry basements. My second home was built into a hillside steep enough to allow a basement door at grade on the down hill side. The rest of the yard continued on down lower for another 6' . The original 30's construction was dirt floor later covered in a thin concrete slab. This was always damp and by the time we left after 25 years, erupting in spots of massive crystal formations. The block walls were prone to filling up with water due to poor or non-existent drain lines. A major deficiency along one wall was a damaged downspout drainline that emptied water into the wall cavities instead of a ground cistern. Complicating matters was forced removal of the cistern in the 70's, which compromised the parking area behind the house. Just generally a mess.

    The relevance to your situation is your mention of dirt crawl spaces combined with a basement. The presence of standing water in the dirt crawl space, which I suspect is higher in elevation than the basement infers surface water and/or downspout sources. If it was purely from a rising water table the sump pit and basement level would flood before the crawlspaces. That said, it is not like ground water levels are strictly level. Soil variations can direct flow in unexpected ways. It was not at all surprising in the high clay area we lived to see water flow along the layers down hill only to surface at the first cut into the hill, ie: my basement floor. Even good drain pipes at the base of my foundation would not eliminate that type of water flow. I currently live where rock layers carry water in seemingly odd ways. I had to account for this when designing my basement completely separate from the footing drains.

    You have not said what your terrain looks like or if torrential rains occurred. Homes built in the 40's til 90's (at least) often have one very common issue with how storm water is handled. For years downspouts and foundation drains were connected to the sewer lines. You may find that the downspouts and footing drains are connected to your sump pit deliberately or by default thanks to crushed rock or gravel under the slab. Or the drain lines that were once connected to the sewer lines have been cut off due to recent code enforcement. The prior owner may not have had the option of connecting to a separate storm water system. It would take more detail to know your situation. The sudden flooding from the sump pit is suspicious.

    Peeling down the exterior of the walls to install foundation drains and seal the exterior of the block may not provide any relief if you can't put the water somewhere lower than your basement floor. On that point Walta is correct about hills be better for drainage than flat plains.

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