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Question on finishing brick-walled basement without exterior waterproofing

jonhaque | Posted in General Questions on

Some background info on the house:

It’s a semi detached, about 100 years old, multi-wythe brick foundation, and it’s in Toronto.
The basement windows are above grade.
The basement has a poured concrete floor.
The ground at the front (West) of the house never gets wet due to the front porch. Along the south there is an asphalt driveway which slopes decently – there are a couple of spots where the water ponds a few feet away and downslope from the walls. The remaining wall I can’t see because it’s covered by the back deck. Gutters and downsputs are good and drain away from the house.

The basement smells fine in the winter, but can smell musty in the summer, especially if the air-condition has not been on. This seems to be at its strongest in the late morning.
I’ve been trying to monitor for humidity by taping seran wrap in squares on the floor and walls, but haven’t managed to catch any liquid in spite of the occasional musty smell.

I would like to finish my basement, but without doing exterior waterproofing as it will blow the budget.
I have not had any water problems in the year or so that I’ve owned the house, and there have been some very severe rainstorms and snowstorms in that time. I will put in a backflow preventer valve to protect from the sewer.

What I do worry about is humidity coming through the foundation walls. There is some efflorescence and spalling, but nothing crazy, and only on two of the walls 38

From reading articles and discussions on this site I think I understand two things:
A) If I put up drywall I risk mold since there is no external water/vapor barrier
B) I should not insulate the walls as I would be exposing the brick to a harsher freeze/thaw cycle
C) I should worry about condensation on the concrete floor and moisture evaporating from the ground through the floor

For A, I was wondering if the following would make sense: put vents along the top of the walls to allow any humidity that’s come through the walls to dry to the interior and not be trapped by the drywall.

For B, is my understanding correct, or is that too extreme a view?

For C, I wonder what flooring options I have that don’t require giving up a lot of headroom as the ceiling is rather low. I was wondering if putting vinyl tiles directly on the floor or on top of some kind of foam pad would work.

I’m thinking of creating a combined furnace/storage room where the furnace, water heater and chiney are, and not finishing that room at all. Could that unfinished space allow humidity in the floor to evaporate into the room instead of being trapped under the finished floor in the other rooms?

Thanks very much,
Jon

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    You're overthinking and overcompensating a bit.

    Toronto is in IECC climate zone 5, and the freeze/thaw spalling risk isn't really all that high compared to colder climates. With reasonable surface water management the above-grade exterior bricks continue to be a very good drying path. DO try to do something about your gutter puddling problem- even it if takes digging in a near-surface drain to a dry-well a good distance away from the house. Below-grade the freeze/thaw spalling potential is negligible, but even digging down a half-meter and installing 3-5cm of EPS tight to the wall would mitigate that considerably.

    Parging the interior with a lime mortar would be protective of the brick and old mortar from the moisture movement that shows up as efflorescence.

    Installing as little as R3cm of unfaced Type-II EPS on the interior side of the brick would be sufficient to be able to install a non-strucutral batt-insulated 2x4 wall on the interior without needing interior side vapor retarders, but 4-5 cm would be better. At 5cm it's still not quite vapor tight to qualify as a vapor barrier per Canadian code, and it does allow some drying of the below grade brick toward the interior but at a controlled rate. If you want give the brick more drying capacity toward the interior you can stop the foam 25-50cm from the slab. Do NOT install 6 mil poly on the interior- wallboard with latex paint and the R5+ foam on the above grade section is more than sufficient to prevent wet insulation. If there is to be poly sheeting anywhere, it would be between the brick and foam, but it blocks the drying path toward the interior.

    If you take this approach, put at least R3 of foam between the bottom plate of the studwall and the slab, as a thermal & capillary break.

    Ideally you would insulate the slab to at least R3 (2cm of EPS), since your subsoil temps are well below your summertime outdoor dew point tempertures. With R3 under the finish flooring you have a sufficient thermal break that you could use rugs & bath mats on the floor without much risk of mold underneath it. But without the thermal break, a rug can be insulating enough that the air at the bottom side has a VERY high relative humidity. Outdoor air infiltration into a cool basement in summer is the primary cause of musty-basement smells. Insulating both the walls AND the slab is the cure.

    To prevent ground moisture migration into your finish flooring a 6-mil poly sheet between the insulation and the rest of the stack-up is important.

    After the walls are insulated keep an eye out for increased efflorescence on the exterior near ground level. If there is significant efflorescence on the near-grade exterior you can protect the brick with a parge of lime mortar there too. Sacrificial parges don't last forever, but a century or so is probably good enough(?) This has been a standard method of protecting brick from "rising damp" in Europe for a very long time.

    How deep are your roof overhangs?

  2. GBA Editor
    Martin Holladay | | #2

    Jon,
    Dana has given you good advice.

    To reduce the musty smell, the most important steps are (a) to install polyethylene and rigid foam on the floor, followed by plywood subflooring, fastened with TapCons to the slab below, and (b) to insulate the interior of your walls with rigid foam or spray polyurethane foam.

    I wouldn't worry too much about brick spalling, although Dana's advice about keeping an eye on the bricks and installing sacrificial parging if necessary is a good one.

    Remember, finishing a basement is always risky, since your basement will always be at risk for water entry. If you want to finish your basement, you have to accept the risk that your finishes might be ruined in the future.

    For more information, here are links to two relevant articles:

    How to Insulate a Basement Wall

    Fixing a Wet Basement

  3. jonhaque | | #3

    Thanks Dana and Martin for the advice.
    I will go through those two articles.
    Dana, the overhangs only stick out about half a foot.

    Do I need to use the fancy new mold-resistant drywall from Georgia Pacific?

    For the flooring, how thin can I get away with making the rigid foam and plywood ( to save headroom)?

    Thanks

  4. Expert Member
    Dana Dorsett | | #4

    With rigid insulation and 6-mil poly between the drywall & brick, and an insulated slab w/ ground vapor retarder, the basement's wallboard doesn't need to be anything special. If there is any risk of minor flooding, stop the gypsum 10-20 cm from the finish floor, and use exterior vinyl trim board for the kickboards.

    Don't go less than R3 on the floor foam, and don't use polyiso- only polystyrene. The thinnest floor would be ~13-15mm thick tile-backer board glued to the foam with foam-board construction adhesive and ceramic tile, rather than wooden subflooring with wood finished floor. Since the backer board is fully supported everywhere by the foam, it won't flex & crack ( as long as the seams of the backer are staggered with those of the foam by a good bit). It's stiffer than standard wooden subflooring spanning joists.

    If your basement slab is a thin rat-slab and easy to break out, you could dig down a bit, install some clean gravel/screenings, a vapor barrier, and pour a 7-10 cm concrete slab at about the same level as your existing slab (or even a bit lower), with either polished exposed aggregate or stained concrete as the finish floor, with no loss of headroom, and extremely flood-damage resistant.

  5. badgerboilerMN | | #5

    We demo slabs regularly, installing a vapor barrier, 2"of XPS and of course PEX since a concrete slab is a terrible thing to waste. The side walls get 2# spray foam, XPS or Polyiso depending on the application.

    I wager there will be no "drying to the inside" without active de-humidification.

    Most basements in wet climates will need a dehumidifier for part of the year to keep rH under control.

  6. jonhaque | | #6

    How does this sound for a plan?

    Walls:
    - 3-5cm un-faced type II EPS, followed by batt-insulated 2x4 wall. Do I need to put 6-mil polyethylene anywhere in there? Do I need foam as well?
    - At least R3 of foam between the bottom plate of the studwall and the slab, as a thermal & capillary break. (Is this needed if there is foam above the slab?)
    - End the drywall at least 10 cm above the floor in case of minor flooding

    Floors:
    From your suggestions and the "fixing a wet basement" article, it sounds like I have a couple of options depending on how much height I can give up:
    a) 3cm EPS, followed by 6-mil polystyrene sheet, then plywood fastened with tapcons, then any kind of finish flooring.
    b) 6-mil polystyrene sheet, 3cm EPS, ~13-15mm thick tile-backer board glued to the foam with foam-board construction adhesive. Could I use vinyl tiles rather than ceramic here?
    c) A dimpled subfloor product like Delta-FL, followed by finish flooring. Their website says "It can be covered with laminate floor systems, or with a plywood subfloor topped off with carpet or vinyl flooring." Couldn't vinyl flooring be put on without a plywood subfloor?

    Thanks again

  7. GBA Editor
    Martin Holladay | | #7

    Jon,
    Q. "[I plan to install] 3-5 cm. of un-faced type II EPS, followed by batt-insulated 2x4 wall. Do I need to put 6-mil polyethylene anywhere in there?"

    A. The answer to your question can be found in one of the articles that I urged you to read in my last comments. Here, again, is the link: How to Insulate a Basement Wall.

    In that article, I wrote: "Basement wall systems should never include any polyethylene. You don’t want poly between the concrete and the insulation; nor do you want poly between gypsum drywall and the insulation. You don’t want poly anywhere."

    You last comments make several references to "6-mil polystyrene sheet." I'm guessing that you meant to write "polyethylene," not "polystyrene" -- but I'm not sure.

    Q. "[I am thinking about installing a] 6-mil polystyrene [polyethylene?] sheet, 3 cm. of EPS, ~13-15 mm thick tile-backer board glued to the foam with foam-board construction adhesive. Could I use vinyl tiles rather than ceramic here?"

    A. Yes. Of course, no matter what type of finish flooring you install, you have to follow the flooring manufacturer's installation instructions. You can look those up online.

    Q. "[I am thinking about installing] a dimpled subfloor product like Delta-FL, followed by finish flooring. ... Couldn't vinyl flooring be put on without a plywood subfloor?"

    A. Flooring manufacturers have very specific requirements for acceptable subflooring or underlayment. To answer your question, you'll have to choose a flooring and research the manufacturer's installation requirements. Here is a sample document from one manufacturer (Armstrong): Subfloors and Underlayments.

  8. jonhaque | | #8

    I was confused about the poly in the wall assembly, but on closer rereading of the documents and the comments above I think I get it.

    Walls:
    3-5cm of unfaced Type II EPS, followed by 2x4 studs and drywall.
    If I understand the 'How to insulate a basement wall' article, I should not put any batt insulation in the wall and there should be no polyethelene or other barrier - just the rigid foam.

    Flooring:
    - Looks like if you put vinyl on sub-grade concrete you are rolling the dice according to the armstrong link:
    "Resilient flooring products, whether sheet, plank, or tile, function as moisture vapor retarders on top of the floor slab. If more moisture is rising from beneath the concrete than can be accommodated by the flooring and adhesive, failure of the installation is inevitable"

    So I need 6mm polyethelene or Delta-FL for the bottom layer no matter what I do to keep moisture from the ground at bay. And I need some rigid foam to keep the finish flooring from contacting cold polyethelene?

    Do you recommend installing a sump pump? The walls don't get wet in the sense that you would ever see actual droplets, but in some places the paint is peeling off the brick.

    Thanks again

  9. GBA Editor
    Martin Holladay | | #9

    Jon,
    In my first comment, I wrote, "Remember, finishing a basement is always risky, since your basement will always be at risk for water entry. If you want to finish your basement, you have to accept the risk that your finishes might be ruined in the future."

    Only you can decide whether the risk is worth it.

    Q. "Do you recommend installing a sump pump?"

    A. There is no reason to install a sump pump unless you have a sump.

    And there is no reason to install a sump unless the sump is connected to a French drain filled with crushed stone -- or else the water that enters your basement will have to flow over your finish flooring on the way to the sump.

    The usual reason that someone might install a sump is to handle liquid water entering the basement. If you want to invest in a French drain, a sump, and a sump pump to be ready to handle that eventuality, you should.

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