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Community and Q&A

Basement – Best way to eliminate musty smell / mildew/ mold etc

user-6347067 | Posted in Green Building Techniques on

I am new to the forum. Thank you for your suggestions.
Purchased a lake house last year. The basement (see attached picture) is musty.
Looking for suggestions / products to redesign the room and clear the air.

My thoughts were to:
Purchase a dehumidifier
Purchase fan(s) to circulate air internally
Install a fan with a timer in the back wall between the 2 room to circulate air

Do I need:
a dehumidifier
a combination of fans to circulate the air
to exhaust bad air to the outside / bring fresh air inside

Here are some facts:
Total SF = 682
2 Separate rooms
Heating or AC
– No vents in the basement (heat pump is in crawl space to condition the 1st floor)
– Duct work is accessible (but not sure if the existing heating/AC is big enough to add this as living space.

Not wet or damp
Mildew and surface mold on the wall facing the front of the house (see pic)
There are crawl spaces adjacent to the left and right walls

Exterior Windows & Doors – Only 2 sliding doors

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  1. GBA Editor
    Martin Holladay | | #1

    D. Moye,
    I suggest, as Step One, that you read the following article: Fixing a Wet Basement.

    If you have any follow-up questions after reading this article, feel free to post them here.

  2. Expert Member
    Dana Dorsett | | #2

    (This is pretty much the response I gave to your PM a another forum.)

    You'll probably need to buy a room dehumidifier to manage the moisture issues in the summertime in your location. The things you have to look at carefully to reduce the moisture burden are:

    1> Bulk water intrusion: Check the grading at near the foundation on the front side where the visible mold is, and make sure that it slopes away from the foundation. Make sure any gutters are functioning properly and the downspouts are directing the water well away from the foundation. If need be, installing a surface drain "French Drain" on that edge next to the foundation to re-direct moisture away and down-slope, away from the sides of the house is in order. In severe cases it may take digging down 2' and installing an EPDM (membrane roofing) membrane sloping away from the foundation at least 2', installing perforated drain, and backfilling with 3/4" stone screenings (protected from filling up over time by landscaping fabric).

    2> Air infiltration: In the summer the average dew point of the outdoor air is well above the deep subsoil temperatures. With an un-insulated slab this means that infiltration of outdoor air into the basement will result in high moisture content of the cool slab, which takes on moisture out of the air as adsorb. (The process is similar to condensation, but does not appear as liquid water.) The biggest air leaks in most basements (even many walk-out basements) is the crack between the foundation & foundation sill, and band joist. Collectively it usually adds up to more leak area than all the exterior windows & doors leakage in the house. Sealing the band joist to the subfloor and foundation sill can be done with expanding foam or polyurethane caulk. Any gaps bigger than 1/2" should be sealed with foam. Inspect the condition of the weatherstripping of the sliders too, and replace as-necessary. If there is a clothes dryer, the type and condition of the back-drafting of it's vent should be inspected too.

    Any atmospheric drafted burners (gas water heaters, etc) are 24/365 drivers of outdoor air infiltration, due to the open flue and stack effect. When it's time to replace them, direct vented or power vented equipment allows you to seal up that flue, reducing the stack effect draw. Similarly (and this isn't obvious), air sealing any plumbing stack, electrical or flue chases that extend from the basement to the attic also reduces stack effect infiltration. Air sealing the top floor ceiling/attic floor at every electric box, light fixture etc. counts too. If you have air handlers & ducts in the attic it can be difficult to seal off the ceiling perfectly, but caulking every duct boot to the ceiling gypsum is a good start, as well as sealing every joint & seam in the ducts with duct-mastic, and air sealing all the panel seams of the air handler with a high quality foil tape makes a difference (and improves the efficiency of the system.)

    3> Ground moisture seepage: The capillary draw of concrete slabs & walls (including concrete block walls) is high. Sealing the concrete with a masonry sealer will slow, but not entirely block those moisture issues. On the slab use a sealer rated for blocking radon, (since you have an radon system), but the walls can use a number of different products.

    With a reasonably air sealed basement you should be able to keep the relative humidity of a 682' basement at or under 60% with just a single 70 pint room dehumidifier, which will usually be enough to prevent mold.

    Are the crawlspaces open to the basement (either fully open or vents?) If yes, are they vented to the outdoors, and is there vapor & radon barrier covering the (presumably dirt) floor of the crawlspaces?

  3. user-6347067 | | #3

    Thank you for the response.
    One crawl space is a cement slab and the other is dirt with a plastic covering. The plastic was put on years ago (I believe) and is not completely sealed.
    Both have fully open vents.

  4. GBA Editor
    Martin Holladay | | #4

    D. Moye,
    If your crawl spaces with "fully open vents" are connected with your basement in any way, then these crawl spaces are sources of moisture in the summer. You may want to consider converting these vented crawl spaces into unvented crawlspaces.

    For more information, see Building an Unvented Crawl Space.

  5. user-6347067 | | #5

    My 2 crawl spaces are not connected to the basement living area other than the adjacent cinder brick walls. No openings connecting the crawl space to the living area.

  6. Dana1 | | #6

    In your climate zone (4A) venting a crawlspace to the outdoors raises the humidity in the crawlspace, and increases the mold/rot risk to the joists & subfloor. In an air conditioned house, the subfloor and much of the joist will be cooler than the outdoor air's dew point and take on moisture. Outdoor air reaching the cool wood gives up it's moisture to the wood as adsorb, raising the moisture content of the wood, often to risky levels.This only becomes WORSE if you have insulation between the joists, since the wood will be even cooler still.

    Converting the crawlspaces to a sealed-conditioned crawlspace insulated at the exterior walls is still a good idea, even if it's only a secondary contributor to your basement mold problem.

    Insulating and air sealing the walls of the crawlspace keeps the outdoor air out, allowing the crawlspace air to dry toward the conditioned space above (and beside, into your basement) via air diffusion, but often requires a tiny amount of active air exchange to the conditioned space. Once the crawlspaces are sealed, in your house it would be best to ventilate the crawlspaces to the basement rather than the rooms above. A tiny ~3 watt (at low speed) Panasonic WhisperGreen on one end of the block wall to the basement, and a small grille through the block wall on the other end would be enough.

  7. Jon_R | | #7

    Regarding "do I need a fan to circulate air internally": probably not if the door is left open. But it's easy to measure the humidity in each room and add circulation if there is much difference.

    Note that you will probably have periods where the upstairs windows are open but the basement is damp. During these periods, you should minimize air flow between a dehumidified basement (or crawlspace) and upstairs.

  8. user-4053553 | | #8

    Are you sure the smell is not caused by a natural gas leak?

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