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Community and Q&A

Dampness in CMU Block Basement Wall

Andrew_H247 | Posted in General Questions on

Earlier this year I noticed the paint and drywall in one corner of my finished “basement” was starting to bubble up. I removed a small section of drywall to investigate and found rotted wood and moldy drywall. I had a mold remediation company come in and remove all drywall around my basement. Once exposed, it was clear the CMU block wall foundation was visibly damp around the vast majority of the basement. That I am aware of, I have not had any physical water ingress into my basement, but around the perimeter of the floor, it looked like there could have been minimal ingress at some point in the past.

I had a basement waterproofing company come out and recommend waterproofing the exterior. My house is a bi-level (split level), so the “basement” floor is really only about 2-ft below grade. The rep for the company actually suggested I dig it up myself and check the footer drain and waterproof it if I was willing.

I took his advice and dug up about 80% of the wall around the basement on the outside (mainly centered around where the dampness was the worst). To my surprise, the top of the footer was actually ~46″ below grade (which makes sense since I am in Climate Zone 5A and it needs to be below the frost depth) and it was a bit more digging than I was expecting.

I waterproofed the exterior wall (also repaired a portion of a block that was spalled off and filled a hairline vertical crack that had formed), replaced the portion of footer drain that I had exposed (previously was just black corrugated pipe), checked that the footer drain was functioning, and backfilled with 18″ of #57 washed river stone followed by compacted dirt (clay) up to the original grade level. I also learned that my footer steps down twice along the length of my house. At the first stepdown, the footer drain steps down as well to stay below the top of the footer. At the second stepdown, the footer drain maintains its level and is actually sitting on top of the footer (this is how I have shown it in the attached detail image).

On the inside of the basement, I have been running a dehumidifier to dry the wall for about 6 months now. The wall looks dry for the most part, but if I check it with a moisture meter, I get elevated readings in several spots (mostly near the floor). There is also a lot of efflorescence on the blocks since it was damp for so long. 

I had 2 other waterproofing companies come out to see if I had solved the problem, or if something else was going on (also had the original company I talked to come back). 2 of them said I have a groundwater issue underneath the house and need to add an internal trench/sump system to lower the water level. The 3rd company said the wall looks dry, so it should be fine to refinish the basement.

A few concerns and questions I have:
1. Since my house is a bi-level, from my measurements, the footer is approx. 25″ farther down than the floor (see attached Foundation Detail 1 image). I feel like the internal trench/sump system would have to be down by the footer level to be effective. Otherwise, I would just get dampness wicking up the blocks like I most likely have now. I am trying to determine if this system is worth the headache and cost of installing.

2. If I go to refinish the basement without adding the internal system, would I run into moisture problems again? My thought was to install a continuous layer of XPS against the block (air tight). Then put my furring strips against the XPS on the wall with more sections of XPS in between the strips to get to a 2″ thickness of insulation. Finished with a layer of drywall and latex paint. Maybe an unfaced EPS would give me better drying capacity to the inside?

3. I don’t have much drying capacity above grade for my foundation (it was only ~6″ at most previously, less in other spots) and I fear I may have gone a little too high with the waterproofing as I only left ~1″ of exposed block below my siding. Once the inside wall is refinished, I am concerned I could have moisture wick up the wall toward my sill plate. From what it looks like, there is some type of capillary break between the sill and the block, but I don’t know how good of one it is (looks similar to an expansion joint pad material).

4. The overall grading around my house is OK. It’s a little flat in the backyard, but still sloping away. I have extended my downspouts 15+ feet from the house for the ones that dump onto the yard. My one area of concern is my driveway and front porch area. The driveway is in a recess area where it goes into my garage, and there is a retaining wall next to it where my front steps go up to the porch. The driveway had settled and rain water was flowing back toward the house/porch area when I moved in (~4.5 yrs ago). The previous home owner said he had minor water ingress in the crawlspace behind the porch and he added a small surface drain next to the retaining wall to fix the problem (I don’t think this drain really does much). I had the driveway mudjacked up shortly after moving in to help correct the water drainage but it still flows toward my retaining wall/porch for the most part. This seems to me like the only area where there is poor surface drainage and could be allowing groundwater to migrate under the house if that is what is happening. (see House Plot Detail 1 attached).

Sorry for the long winded story and excessive details. I have been racking my brain over this for 6 months now and just want to put the basement back together as best as I can. Any thoughts or comments would be greatly appreciated.

– Andrew

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  1. Bob Irving | | #1

    Concrete - blocks or solid - is porous, so the dampness on the interior of your blocks is normal, possibly slightly higher than normal if you don't have good drains, or very damp soil. Moisture like that will not damage the blocks or poured concrete. Good drains at the bottom of the footings is essential, as is good exterior waterproofing, but can be difficult in a retrofit. An interior trench around the edges with drains or a sump pump will also work. Your plan to line the interior with extruded polystyrene should work, then you can add a 2x4 wall with rock wool insulation and drywall. If you can spare the headroom, 2" of the same foam on the floor combined with two staggered layers of Advantech will keep the dampness of the floor from your living areas.

    My antique house basement (Zone 6) had a dirt floor with both dampness and occasional water and was basically unusable; I shoveled it deeper, added washed stone, excavated a drainline to daylight, and insulated the walls with a combination of foil faced polyiso and closed cell spray foam - neither of which allow moisture to pass through. Added 2" of foam - leftover pieces of EPS & Styrofoam - on the dirt floor, covered that with 10 mil poly (seams taped), and poured a 2-3" concrete floor. The result is a dry, comfortable basement which we use as dry storage and a wood shop (and my steel tools do not rust). It's unheated, the result of throwing out my gas boiler and oil furnace when we converted to minisplits, but never gets below 540, which is very doable for a shop. Huge and positive change!

    One other detail - while you are working down there, insulate and seal your sill/rim joist with closed cell spray foam. You can hire someone to do this, or DIY using "Froth Packs". With these upgrades, you 'll notice that your whole house will be more comfortable.

    So you're part way there; great! Keep moving ahead; it'll be well worth it.

  2. Andrew_H247 | | #2

    Thank you for the quick response and insight. In your opinion, would it be worth it to add the internal drain/sump system at this point since I have already replaced and waterproofed a large portion of the outside wall? My thought is since I am not getting physical water ingress that I have seen, adding this system may be redundant, but if I were to do it, now would be the time before I do any further refinishing. Also, since basically half of my foundation wall is below my basement floor, I can really see how much water there is down at the footer level on the inside of the house.

    Finally, is there any concern with a high moisture content wall with regards to the potential for freezing and spalling/damage to the CMU blocks? I guess I don't know how cold they might get on the outside after being insulated and if this could become an issue, but as I'm sure you know being in Zone 6 - it gets cold outside!

  3. Bob Irving | | #3

    One of the chief reasons for exterior drains - with a sand/gravel backfill so water flows down to the drain pipes - is to keep the soil next to the house dry so it does not freeze, expand, and blow out the block wall. So yes. It probably would be a very good idea to install an interior drain - which you should cover with the foam and poly for insulation and air sealing - and that should help keep the exterior dry as well. (I assume you'll be at/below footing level.).
    It's the same issue with poured concrete, but it requires a lot more force to damage a poured wall than that needed to break the mortar bonds on a a block wall.

  4. Rich Cowen | | #4

    We have fieldstone not concrete, but when I called in an expert to deal with cleaning up the internal moisture 8 years ago, he recommended we treat the interior of the walls. I followed his advice using BASF MasterSeal:

    which used to be about $20 per bag but has gone up in price since then. Anyway it worked quite well in preventing seepage. To address the internal moisture we put in an interior drain, a trench 18 inches deep filled with rocks on the side of the house with the most water, and a 4" pvc pipe that I was able to extend all the way to daylight in our front yard. That drain pipe is also carrying water from the gutter but it works, and all is buried under a brand new slab.

    My basement is unheated like the earlier poster's, and it gets down to around 50 degrees but with further exterior grade changes we should be able to warm that basement up to the 54 cited by the other poster. A heat pump water heater is keeping the moisture level down in the spring and summer, also a big plus.

  5. Andrew_H247 | | #5

    Thanks Bob, Rich. I have heard of the MasterSeal / Thoroseal before and had considered originally coating the internal wall with something like that or even a Drylok type paint but I wasn't sure if that would reduce my drying capacity to the inside. I figured if I had the rigid foam against the wall, that should provide pretty good moisture resistance, and I wouldn't have to go through trying to completely clean the years of efflorescence build-up off the walls to apply a coating (although I have cleaned some of it off).

    It sounds like it may be best to add the internal trench system at this point. I need to review with the contractor again to make sure he understands how deep below the floor the footer is because it sounded like during my initial discussion with him, that he was intending to put the drain just below the floor level, which I don't think would be deep enough in my case.

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