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Community and Q&A

Basement smells musty – even though it’s dry

Mill_house | Posted in General Questions on

I live in a house in climate zone 7 that was built in the late 40s.  The outside walls have been waterproofed and insulated (i don’t know with what kind of foam board – except that previous owner said it was rated for below grade and that it was only approx 2 inches thick.)  There is a sump pump too.  The inside basement walls have been finished with 2×4 stud walls, fibreglass insulation, poly and drywall.  the drywall has yet to be taped, mudded etc.  I know this site does not encourage the use of poly in basements.  
The stud walls are not sealed at the top and the joists are set in the foundation – they do not sit on top of the foundation wall.  The outside grade is approx 2+ feet below the joists and there are no bushes around the house.

I can smell the mustiness in the basement mostly in the summer time.  The basement does not have carpet or furniture.  It’s concrete floor on one side and old linoleum tile over concrete on the other.  I have not seen signs of water pooling or accumulating anywhere.  I’ve removed a few pieces of drywall, cut the poly and removed the fibreglass to see if there was something happening.  It all looked and felt fine. 
Someone suggested that I keep a journal of the outside temp and humidity, the inside basement humidity and the smell of the basement on a scale of 0-5.  And see if I notice any patterns.
One thing the person suggested that could be causing the problem is that because the stud walls are not sealed at the top, warm humid air from outside may be getting in behind the walls and causing an odour.  The person suggested I might want to have the rim joists sprayed with foam and extend the seal down to the top of the stud wall essentially closing that portion of the concrete wall behind the stud wall from any outdoor air.  

Another smell issue I’ve noticed is above ground on the other floors.  This one doesn’t seem so hard to figure out though.  The house has the original stucco and I’m not sure what’s behind the stucco.  I can guarantee it’s been applied directing to the sheathing though.  I have noticed – in the places I’ve removed the plasterboard on the inside – that there are signs of previous water damage.  Some of the damage is a result of the little overhang I have at the back door not being flashed and other signs are because the windows have not been properly flashed.  So this is easy to figure out.  I’m also guessing that the stucco is doing very little in terms of keeping water / moisture out.  What I’m wondering here is can an old house be turned around and given a new lease on life.  Meaning, if the stucco is removed, house wrap applied, rainscreen and some kind of siding applied, windows flashed and the inside walls are allowed to dry to something that’s in the acceptable range of moisture for wood framing, will the smell go away?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Mill,
    You can have enough mold growth on your basement walls or basement slab to have a musty smell even when the mold is invisible and the concrete looks dry. If it smells musty, it is musty.

    For an uninsulated slab, the solution is to install a layer of polyethylene, and then a layer of rigid foam, and then OSB or plywood subflooring, above the slab. For instructions on how to to this, see "Fixing a Wet Basement."

    It's possible, of course, that your basement walls are part of the problem. If I were you, I'd start by addressing the slab. If that doesn't solve the odor problem, you could attack the basement walls -- get rid of the fiberglass and the polyethylene, and install some closed-cell spray foam.

    1. Mill_house | | #5

      For an uninsulated slab, the solution is to install a layer of polyethylene, and then a layer of rigid foam, and then OSB or plywood subflooring, above the slab. For instructions on how to do this, see "Fixing a Wet Basement."
      When I read your response, I interpreted it as the poly, rigid board and plywood are all going above the slab. When I read the article, the poly and rigid board are below the slab with the plywood on top. I suspect the article has it laid out correctly.

  2. GBA Editor
    Martin Holladay | | #2

    Mill,
    Q. "If the stucco is removed, housewrap applied, rainscreen and some kind of siding applied, windows flashed and the inside walls are allowed to dry to something that’s in the acceptable range of moisture for wood framing, will the smell go away?"

    A. Almost certainly. But smells can be tricky -- you want to be sure that you know the origin of the smell before you jump to conclusions.

    1. Mill_house | | #6

      Smells are tricky. I agree. I've gutted the top floor of my cape cod style house and "the smell" is still there. I had someone out to the house with a moisture meter. We took measurements at various places and the readings all came back from 9% to 11%. When he was leaving the house he remarked "it just smells like old wood". My thoughts exactly. And I want that smell gone.
      I am about to have the roof done. New shingles etc. Because I can only think that the wood smells like that because of water issues, I'm thinking about having the roof totally covered in ice and water shield (peel and stick). I'm even thinking of having the horizontal boards removed and plywood put on. These thoughts may be extreme...but I don't know what to do. Any thoughts? And have you heard of people complaining of "old wood smell" before? Is it possible for everything to be fine and there to be a smell? I was hoping to fix things, not wind up back at square one in terms of it smelling.

      1. Mill_house | | #7

        Anyone ever heard of this old wood smell phenomenon? Is there a fix?

  3. Jon R | | #3

    Air sealing is never perfect - so you don't want cavities that support mold (like your basement walls or even plastic sheeting on the floor). But the fix is expensive. I have the same basement situation (but zone 5) and with good dehumidification there is absolutely no mold smell.

    Positive building pressure and dehumidification can help with above grade summer mold smells.

  4. Expert Member
    Dana Dorsett | | #4

    "One thing the person suggested that could be causing the problem is that because the stud walls are not sealed at the top, warm humid air from outside may be getting in behind the walls and causing an odour."

    That's a very likely scenario. Sealing up the the joist penetrations to the foundation wall, the seams of the subfloor to the foundation, and the top of the studwall to the foundation is the FIRST thing to do. Also seal up along with any side-vented appliance exhaust penetrations, and installing a best in air-tightness class backdraft preventer on the clothes dryer, etc. that connect the basement to the outdoors.

    Sealing up any plumbing, flue, or electrical chases that extend from the basement to the attic, and any penetrations of the top floor ceiling/attic floor is the OTHER FIRST thing to do, since the air leaks at the bottom & top of the building envelope are what establishes the amount of stack effect infiltration drive.

    The less ventilation there is in summer, the lower the humidity load in the basement will be, and since the smell primarily a summertime phenomenon, outdoor air is the prime suspect. With the studwall cavities sealed up ground moisture could still potentially cause problems behind the vapor barrier, but you won't be able to smell them very easily. But everything else comes after the air-sealing of the basement, and the attic floor, and any channels that connect the basement to the attic.

    An uninsulated slab is almost always below the summertime outdoor dew point, and it, as well as the bottom of any moisture-susceptible material resting on it will take on moisture from summertime air. Rugs, cardboard boxes, etc resting on the slab can easily be the source of the mold/smells. Shelving for storage is preferable to stacking boxes on the floor, and floor coverings other than fiber would be preferable if you aren't going to insulate the top of the slab.

    1. Mill_house | | #8

      Hi Dana, I've taped several pieces of poly to the basement floor and duct taped them. They've been on the floor for approx. 2 weeks. No sign of moisture. We have had an unusually dry summer, but for the past two weeks we've had rain. I'm glad there's no moisture accumulation on the plastic. But I suppose there could still be a spot somewhere on the basement wall that I haven't found yet that may be wet.

  5. Expert Member
    Dana Dorsett | | #9

    Have you measured the temperature of the plastic, and the RH/dew point of the indoor air? At 90%+ RH cool air near the slab there will be no visible moisture, but mold can still take off.

    The slab temperature may not be uniform every where too- cold spots are where the adsorption action is.

    1. Mill_house | | #10

      The RH was measured with a hygrometer one time and it indicated 60%. I don't know how to measure the temp of the plastic.
      If I'm understanding this entire thread, it sounds like mold (that is creating the musty smell) may be invisible but still present. Which is kind of baffling because I thought the mold area needed to be treated in order to kill it prior to taking on these other fixes ie) sealing top plate of wall frame/rim joist area, etc. But this doesn't appear to be the case. Rather I can go ahead and seal the top plate/rim joist with foam? Does the bottom of the wall frame need to be sealed to the concrete floor with some kind of sealant?

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