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Community and Q&A

Musty smell – is it a roof leak?

jimbox | Posted in General Questions on

Recently, on Nov 4th/5th I noticed a strong musty smell in a room that happened to contain an air quality sensor (Airthings View Plus). 

I have a dehumidifier running down the hall from this room and the indoor RH rarely exceeds 40%. The smell is definitely around in the rest of the unit, but not as intense.

Above the most affected room (and covering most of the unit) there is a ‘flat’ roof of unknown construction–I assume EPDM. On top of the flat roof are granite pavers on paver spacers of unknown design (there is a gap between the bottom of the pavers and the roof). These pavers are walked on frequently by building maintenance. The exterior cladding is stucco of unknown details. The building is steel framed multifamily on top of a concrete PT slab covering a concrete parking garage, 4 stories high (5 if you count the parking garage).

The north wall of the most affected room is a sliding glass door to an uncovered balcony with similar paver setup as the roof with a tall and narrow corner window (~1’ on each wall, ~5’ height) on the NW corner. The west wall is an exterior wall with single 1.5×1.5’ window and aforementioned corner window.

We had the first rains of the season on the 28, 29, 31 Oct and 2, 4 of Nov. But the 31 Oct with 0.75″ of rain was the only day that recorded more than 1/8″ in 24 hrs.

During the previous 0.75″ of rain on 31 Oct, the indoor RH in this room rose from 35 to 39%, averaging 37.5% while it rained. The indoor temp ranged from 70 to 72F, and the indoor dew point from 41 to 45F. There was no musty smell at this time.

From the end of the rain on 31 Oct to the start of the musty smell 4.5 days later, indoor RH never exceeded 36%, and indoor dew point never exceeded 41F.

Outside, the outdoor RH cycled between ~87% overnight and ~65% during the day, until 3 Nov, where it remained around 80% during the whole day or about 12 hours leading up to the musty smell. For 16 hours prior to the musty smell, we had strong winds of 20 kt gusting to 40 kt.

During the same ~48 hour period when the musty smell began and continued to intensify:

In the first 14 hours of the musty smell, the outdoor RH was falling from ~93% to 84%, the outdoor temp rose from 43 to 59F, and the outdoor dew point rose from 40 to 54F.

Then over the next 14 hours the outdoor RH fell from 84% to 63%, the outdoor temp fell from 59 to 49F, and the outdoor dew point fell from 54 to 37F.

Inside the room with the musty smell, the indoor RH and indoor dew point remained elevated for longer than the outdoor measurements.

During the first 23 hours of this smell, the indoor RH rose from 35 to 44%, indoor temp from 65 to 70F, and indoor dew point from 36 to 47F in the room where the smell is the strongest. The indoor RH remained at an elevated plateau of 42% for about 12 hours, starting about 16 hours from the time the indoor RH and indoor dew point began rising. The indoor RH fell relatively slower compared with the outdoor RH and outdoor dewpoint.

Outdoor weather chart: (you have to click through the dropdown menu at the top left to see precip)

I’ve attached an image that summarizes all this text, except for the wind conditions. We had several periods of strong, gusty winds.


Direct link to full sized image:

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  1. Expert Member
    BILL WICHERS | | #1

    It is normal for humidity levels to rise during and after a rainstorm, so elevated humidity levels immediately after a storm doesn't necassarily mean there is a roof leak somewhere. If the indoor humidity levels roughly track outdoor humidity levels, that probably means you don't have a leak. If the indoor humidity level stays elevated long after the outdoor levels return to normal, then I'd be more suspicious.

    I don't think you can reliably confirm a leak based on humidity readings alone. I would look for signs of water stains, things like that. The musty smell is usually mold, and you might have an issue inside of a wall, but you'll need to cut some holes in the wall to test for that.


    1. jimbox | | #2

      That all makes sense, thanks. I was just confused because I thought that mold can't grow at 45% RH and lower, so if the interior stays that dry, then it makes sense to me that the musty smell must be coming from some other place than the interior.

      Also, the humidity in this case did not rise following a rain storm, but four days later

      1. Expert Member
        Akos | | #3


        Flat roof, stucco, balcony door to open balcony, corner windows.

        Any of those are finicky and have to be done right, any detail missed and can cause water leaks. It doesn't take much. For example I lived in a new build many moons ago where the balcony door sill wasn't done properly (I guess neither was the sill flashing under it). It was made from two pieces and the sections were not hammed but overlapped. The sill was not sloped properly and this seam was not caulked, first big rainstorm and water leak took out the whole flooring on that level. That was about 4" of caulk missed.

        Sounds like you have water leaking in somewhere, you'll have to find the source, seal that up then deal with any mold. Not fun.

        1. jimbox | | #6

          Thank you for your reply!

  2. Expert Member
    PETER G ENGLE PE | | #4

    Akos has it right. Any or all of those finicky details are commonly done wrong and can cause leaks. The reason for the 4 day offset is related to the time it takes mold to start growing. The general guideline is about 72 hours under ideal conditions (high humidity, temperatures near 70F). Even though the air inside the house is dry, the air inside the walls is not. If you have a leak in the walls or ceiling, the insulation can become wet and it can take some time to dry. During that time, the RH inside that cavity can remain near 100% for long enough for the mold to germinate and grow. You're going to need a local building envelope expert to start investigating the source(s) of moisture and take a look inside the walls and ceiling for any damage. The building owner or HOA might be responsible for this, as it is most likely a common element and doesn't just affect your unit.

    1. jimbox | | #5

      Thanks for your reply. I see. So do you think the musty smell coinciding with the bump in dew point may have been more of a coincidental/catalytic thing? e.g. the 0.75" rain 4 days earlier provided the moisture, and it would've molded either way, but it happened more rapidly because the temperature climbed that day.

    2. Deleted | | #8


  3. Expert Member
    PETER G ENGLE PE | | #7

    Something like that. Mold grows faster in warmer weather (to a point anyhow). And the musty odors always seem to be stronger in warmer weather as well. At least that's my own impression. But your relevant core observations are: rain, warm weather, several day incubation period, then mold smells. The problem now is finding it and figuring out what to do about it.

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