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Community and Q&A

Best strategy for waterproofing and insulating a half block/half brick basement?

MPEB | Posted in Plans Review on

I live in a 1920’s home in Central Ohio (Climate Zone 5A). The basement has an exposed French drain (ie, the weeping holes, trench, gravel and tile are there, but have not been covered with concrete) with sump pump. The basement is 2/3rds below ground, with the walls made up of half cement block (bottom portion) and half brick and mortar (top portion). The brick and mortar is partially above ground and is covered on the exterior with a stone and mortar veneer.

The (formerly mostly finished) basement flooded a few weeks back and I’m mostly starting from scratch. I’ve read a lot both on GBA and BS and want to try and make sure my strategy for getting the basement “right” is OK, with my chief confusion coming from handling the brick. My understanding is that the thermal properties of brick and mortar (particularly for an older house) would make it less than ideal for any insulation other than closed-cell spray foam. Given that, it seems like a half insulated wall is the feasible approach – ie, cover up the block with membrane and insulation, but leave the brick portion exposed.

My rough thought process is:

1) Install dimple membrane over portion of French drain (to allow drainage from wall to drain) and up the concrete block portion of the basement wall (adhered and sealed to wall).
2) Install 2″ XPS or EPS foam board insulation over membrance.
3) Pour concrete over French drain, incorporating foam insulation break between membrane/wall and new concrete.
4) Insulate rim joist with mix of foam board and closed cell spray foam.
5) Frame and install fire-rated drywall over insulation (both half wall and rim joist).
6) Install floor-appropriate dimple membrane over existing concrete slab.
7) Install flooring, fixtures and complete finish work (trim, paint, etc.).

Does this make sense? Could I simply apply membrane, foam board and drywall to the entire wall (ie, am I being overly sensitive to the brick and mortar)? Does it make sense to apply anything to the exposed interior brick (sealant or otherwise)? Any thoughts would be great. I’ve tried to educate myself before posting, but the mixed concrete/brick has me thrown a bit. Thanks.

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    Managing the exterior bulk water becomes critical if you water proof the interior side of a CMU wall, but if you're using the dimple mat so that it always drains to your interior drain you can be OK.

    That's not the same as waterproofing the foundation wall from the exterior though. If you made the CMU completely water tight on the interior you'd potentially put the foundation sill & band joist at risk, unless you jacked it up a bit and installed an EPDM or metal capillary break. So, for now skip the goop-type sealants on the interior, even though they'd be a good choice for waterproofing it on the exterior.

    The dimple mat isn't going to waterproof the interior from a bulk-water incursion but on the walls it redirects bulk water to the right place and is a vapor barrier against ground water moving through the walls. But on the floor it's value is less clear (other than it's vapor retardency.) If you're going to install a finish floor it needs at least R3 of insulation at your sub-soil temps to reduce the risk of mold on any sub-flooring, finish flooring, or even bath-mats & throw-rugs.

    Your subsoil temps rare in the 50sF (http://www.earthrivergeo.com/img/geothermal-article/geothermal-subterrainean-temperature-contour-map.jpg ), but your summertime outdoor dew points are in the 60s. Without insulation from below the temperature on the underside of the rug can be at or below the dew point of your ventilation air. Even if you're dehumidifying the basement mechanically it takes a fairly low RH to keep the dew point sufficiently above the temp of an uninsulated slab that mold/mildew won't get going under throw rugs, or even ~R1 of wooden subfloor.

    More slab-R is even better, but what were you thinking for flooring materials? Using moisture-sensitive materials for the flooring or the lower half of the wall in a flood-prone basement is risky, and probably wouldn't be covered by home owner's insurance in the event of another flood.

    When you have a choice, EPS is a much greener option than XPS, due to the differences in blowing agents used- HFC134a, with a global warming potenial of about 1400X CO2, vs pentane at about 7X CO2. In 50 years after most of the HFCs are gone the thermal performance of 1.5lb XPS is about the same as 1.5lb EPS, but the global warming impact cited is the 100 year number- it's even WORSE in shorter time frames.

  2. MPEB | | #2

    Thanks for the follow up Dana. Fortunately (tongue-in-cheek), the flooding was a system issue. During a torrential downpour, most of the homes on our block had somewhere between 6" to 24" of water come up through the floor drains (and in some cases, the toilets). Apparently it's an every 8-10 year issue. We came in on the low end of water and otherwise haven't had any bulk water issues; the sump runs periodically, but we've rarely seen water from the weep holes. We're in process on estimates for installing a backflow valve or elevated drain to mitigate the risk of system-caused flooding going forward. Apart from bulk flow, there's some humidity/moisture that feels about typical for an unfinished basement. A photo is attached.

    I hadn't thought through the floor condensation issue, thanks. We're a blank slate on flooring. We had (and liked) carpet in the basement previously and would lean in that direction. Might make more sense to do a tile/rug combo. One challenge is that I have about 4" of clearance to the basement ceiling; I'm 6'6" and our basement is "spacious" in a neighborhood of old homes. I could probably swing 3/4" XPS (sorry, my height dictates max R-value per inch) with 1/2"-3/4" tongue and groove plywood. That would get to R4-5 and still provide a modicum of headroom for me.

    Apart from the floor, does it make sense to leave the brick and mortar exposed (ie, no membrane or insulation) and unsealed, and just cover the CMU portion of the wall? If left exposed, would taking the drywall to the ceiling (ie, leaving a 3-4" gap between the brick and the drywall) as opposed to a half wall risk moisture issues?

    So, re-thinking the process:

    1) Install dimple membrane over portion of French drain (to allow drainage from wall to drain) and up the concrete block portion of the basement wall (adhered and sealed to wall). Brick and mortar portion of the wall left exposed and unsealed.
    2) Pour concrete over French drain, incorporating foam insulation break between membrane/wall and new concrete.
    3) Insulate floor with 3/4" XPS. Tape/seal insulation.
    4) Install 2" EPS foam board insulation over wall membrane.
    5) Float 1/2" or 3/4" tongue and groove plywood over insulated floor.
    6) Insulate rim joist with mix of foam board and closed cell spray foam.
    7) Frame and install fire-rated drywall over insulation (both half wall and rim joist).
    8) Install flooring, fixtures and complete finish work (trim, paint, etc.).

    Thanks again for you time. Any thoughts are great.

  3. GBA Editor
    Martin Holladay | | #3

    Matt,
    Assessing brick foundation walls is somewhat of an art. If the bricks are crumbly or spalling, or the mortar is falling out, proceed with caution. Interior insulation in such cases can lead to more rapid deterioration.

    If your bricks seem hard and aren't spalling, and the mortar is in good shape, you should probably go ahead an install a layer of rigid foam insulation on the interior -- especially if you don't have bushes on the exterior of your house that inhibit drying and solar exposure. (If you've got those bushes, cut them down.)

    If you insulate only the bottom half of your basement wall, the insulation is wasted. Most of the wintertime heat loss occurs at the upper half of the wall, and the masonry wall is an effective thermal bridge.

  4. iLikeDirt | | #4

    Aside from the discussion on how best to insulate and waterproof this basement, I think it makes sense to acknowledge that your basement will flood. Seems that's common in your area and your current drain arrangement appears to encourage it, in fact. So if you want to finish this basement again, I would recommend doing it with completely waterproof materials. Use cementboard and/or lime plaster for the wall coverings instead of drywall; CMUs instead of wood for partition walls; tile or vinyl flooring instead of carpet or wood; tile baseboards instead of wood; and instead of wood trim for doors and windows and such, just bevel the opening and ease the plaster into the frame--it's quite attractive. And needless to say, choose insulation materials that aren't negatively affected by standing water. No paper-faced batts. You can even get plastic doors with vinyl frames. The upfront cost of all of this may be higher, but at least you won't have to redo it again in 8-10 years!

  5. Expert Member
    Dana Dorsett | | #5

    As Nate points out, this basement WILL flood again, it's only a matter of when, which means a plywood floor is guaranteed to fail (and would probably not be covered by insurance.) Having to gut & refinish the basement once a decade CAN be avoided if you use the right materials.

    Fiber cement tile backers glue to the foam with foam board adhesive and ceramic tiles would probably be the best option, with the least amount of headroom loss. (There is ceramic tile products that look like engineered wood flooring that aren't too bad: https://www.google.com/search?q=ceramic+wood+tile+floor&biw=1376&bih=668&source=lnms&tbm=isch&sa=X&ved=0CAcQ_AUoAmoVChMIru2Ns_LaxgIVQ5mICh3SSQBR#imgrc=ymA_Y6fE3CmP7M%3A ) A wainscot of fiber-cement exterior siding for the lower 3-4' of wall may be a decent option for the finished wall if you pick the color & texture carefully, and cheaper than a ceramic wall. There are also vinyl wainscot in different colors & textures that would work.

    Put all your power wiring & receptacles well above the high-tide mark.

    If the slab is a 1-2" rat-slab that can easily be broken out, it may be worth digging down 8" and installing 3-4" of washed 3/4" screenings, 2"-3" of EPS then a 6-10 mil vapor retarder between the foam and the new slab. With a new slab you have stained concrete or polished exposed aggregate finish options, which could end up cheaper than ceramic floors:

    https://www.google.com/search?q=exposed+aggregate+concrete+floor&source=lnms&tbm=isch&sa=X&ved=0CAcQ_AUoAWoVChMI8uWuzvTaxgIVz6WICh3fMwmN&biw=1376&bih=668

    https://www.google.com/search?q=exposed+aggregate+concrete+floor&source=lnms&tbm=isch&sa=X&ved=0CAcQ_AUoAWoVChMI8uWuzvTaxgIVz6WICh3fMwmN&biw=1376&bih=668#tbm=isch&q=stained+concrete+floors

    Build so that the once a decade clean-up can be done with a with a garden hose and wet-vac, no wrecking bars or dumpsters required (except for the damaged furniture.)

  6. user-4310370 | | #6

    If your basement flooded due to sewer backup it is also possible to install a check valve in between the house and the street, usually with a pump that moves you interior produced water beyond the check valve if it becomes necessary. You can check with local reputable companies that do this, they dig and build a vault for it and usually come by yearly to check / clean the valve. You want one that has been around for a while and guarantees it works.

    While the other techniques will help with water intrusion none of it will stop the sewer backup and you will get water again.

  7. MPEB | | #7

    Thanks for all of the feedback. To follow Michael's comment, the water infiltration was a sewer backup issue that affected a cluster of homes on our block (as well as other clusters in the community). We've already gotten estimates for installation of a backflow valve on the main drain line and are in process on discussing both the valve and system issues with our city government. The sewer backup is the "every 8-10 year" event, per our neighbors. We haven't had infiltration through the walls or slab from ground flooding (we actually live on a hill, our what passes for one in Central Ohio) or from bulk/rain water. So part of the process for me is assessing what the practical risk of water is (post-backflow valve installation and/or city sewer work) against material/labor costs and asthetics of waterproof material. Waterproof flooring and insulation makes sense, but I might take a measured risk with drywall. I'll have to dig in to more of the waterproof material options and crunch some numbers; thanks for the materials/design suggestions. I think I have enough to get running. Thanks! (and hopefully I'm not back with any stumbling blocks)

  8. MPEB | | #8

    Ok, one follow up question (sorry, that was quick). In the "Fixing a Wet Basement" article (https://www.greenbuildingadvisor.com/blogs/dept/musings/fixing-wet-basement), it looks like the Delta-FL subfloor product was mentioned as an alternative to using rigid foam insulation for the flooring. My understanding is that the main purpose of insulating the slab is to reduce the risk of on-slab condensation when the weather is warm (ie, in and of itself, insulating the slab doesn't have as much of a benefit in controlling heat loss in contrast to insulating the walls). Am I understanding correctly that the Delta-FL type product and the insulation are targeting the same end - mitigating the risk of condensation on the slab - just with different approaches? The insulation limits condensation risk by keeping the slab surface warmer while the Delta-FL allows condensation but keeps it "trapped" (or in their parlance, the water vapor pressure below and above the slab is equalized) under the membrane. Is one of these a safer/better approach? It seems like the risk on the Delta-FL side would be if the product isn't properly taped or otherwise sealed you would have moisture intrusion. Am I missing something else? Thanks.

  9. GBA Editor
    Martin Holladay | | #9

    Matt,
    Q. "In the Fixing a Wet Basement article, it looks like the Delta-FL subfloor product was mentioned as an alternative to using rigid foam insulation for the flooring."

    A. It's not equivalent. I wrote that it is used by homeowners who "don’t want to lose the height required for rigid foam."

    Q. "My understanding is that the main purpose of insulating the slab is to reduce the risk of on-slab condensation when the weather is warm (ie, in and of itself, insulating the slab doesn't have as much of a benefit in controlling heat loss in contrast to insulating the walls)."

    A. That is correct.

    Q. "Am I understanding correctly that the Delta-FL type product and the insulation are targeting the same end - mitigating the risk of condensation on the slab - just with different approaches?"

    A. Not really. The Delta-FL limits the risk of mold under carpeting by separating the subfloor from the damp slab. But the slab can still be damp. Rigid foam is still the preferred solution.

  10. MPEB | | #10

    Back again with a follow up flooring question. Is there a GBA opinion on pebble flooring products (as an example, http://www.naturestone.com/indoor/basements/) for a basement application? I took a look around this site and a few others and didn't see much referencing this type of product. The literature suggests R-5, but I take that with a large grain of salt. I like the look/feel of it and the relative ease of installation, but I'm trying to understand what downsides there might be, if any. Thanks.

  11. GBA Editor
    Martin Holladay | | #11

    Matt,
    I have no experience with that product.

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  13. SATX_Epoxy_Floors | | #13

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  15. LockHard | | #15

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