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Community and Q&A

Eliminating musty odor under a low-sloped (flat) roof

Steverts | Posted in Energy Efficiency and Durability on

Hi Folks,
I have read all the info that I can find about low slope flat roofs, insulation and the possibility of venting.
But I am still unsure of how to proceed with my house problem.

I have a 6 year old house in Chicago with a low slope flat roof. I have musty odors drifting down from the space between the roof deck and my second floor ceiling during hot weather. (There are a couple areas that have small openings). I have opened up parts of the roof and am fairly certain there is no mold. I have spend considerable effort and money making certain there are no leaks (there had been a few in the past.)

I know that the insulation over the second floor ceiling was not done perfectly, but there is no way, without ripping up much of the ceiling, to re insulate. I also realize that there is some moist air getting into the space through the couple openings.

So, I feel something to try might be to put in a couple roof vents and try my best to seal up the openings.
Would putting in a couple powered vents at one end or the roof and a couple non powered vents at the other end be able to pull the musty air out? And what problems might this create?

Or does anyone have other thought on this?

Thanks Much



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  1. GBA Editor
    Martin Holladay | | #1

    Your reference to "a couple of openings" is vague but suggestive.

    Every house needs an airtight ceiling -- or a ceiling that is as close to airtight as you can make it. If you know of air leaks in your ceiling, the first step is to seal the air leaks. Candidates include plumbing vent penetrations, electrical penetrations, and bad weatherstripping at the access hatch.

    The hardest leaks for you to seal will probably be the cracks between the partition top plates and the partition drywall. But perhaps you already sealed those. (They are usually sealed from the attic side.)

    Once you have done your best to seal the ceiling air leaks, wait a few weeks to see if you have solved the odor problem. If you haven't, the next step would be to install a supply ventilation system that slightly pressurizes your house. For more on ventilation systems, see Designing a Good Ventilation System.

  2. Steverts | | #2

    Thanks for your input.
    My apologies, I was vague about the air leaks because it seemed too difficult to explain where they are, etc. Unfortunately, it will be impossible to seal them all properly because there is not enough access to the area under the roof.
    Because of the un avoidable "air transfer" between this area and our living space, it was suggested to me to try putting a supply and return up in the "under roof" area to get some air moving and keep it dry.
    So far, during hot days here, the musty odor is being pulled into the house...which is what I DIDN'T want.
    (Maybe after a few weeks this will dissipate???)
    I will read up on supply ventilation systems. We now have a main system for the first floor and basement and a separate system for the second floor. The home has a fairly open floor plan. Are you suggesting converting the second floor system to supply only? Or introducing a supply into the "attic" area?
    It seemed to me that a powered exhaust vent on the roof to pull out the musty air could be an answer. But I am a lowly musician who is slowly learning about houses through trying to fix mistakes done by the builder.

  3. GBA Editor
    Martin Holladay | | #3

    You should hire a home performance contractor who is familiar with pressure diagnostics. For some reason, your house is a negative pressure with respect to the attic. My guess is that this is due to an imbalance in your forced-air heating and cooling system, but there could be other reasons. If you can adjust your forced-air system so that your house is slightly pressurized instead of depressurized, this problem would probably go away.

    Your description of your HVAC system is vague. You wrote, "We now have a main system for the first floor and basement and a separate system for the second floor." What type of "system"? Are you talking about heating ducts connected to a furnace? Or ducts that provide both heating and cooling?

    Or are you talking about a ventilation system?

  4. Steverts | | #4

    Hi Martin,
    I will try to find someone in Chicago who does "pressure diagnostics". So far, i have not found anyone who includes that description in their website information. Plenty who say "diagnostics".
    It can be frustrating trying to find contractors to diagnose a problem. Usually they just want to sell what THEY do and the methods that they use.

    The "systems" I was referring to are ducts that provide both heating and cooling. So you are guessing that I might have too many returns in the system?
    Your input is greatly appreciated Martin
    Thank You

  5. GBA Editor
    Martin Holladay | | #6

    When you are talking to home performance contractors on the phone, ask if they perform blower-door tests or Duct Blaster tests. (A Duct Blaster test is used to test duct systems for leakage.) A contractor who performs both types of tests should understand pressure diagnostics. (I hope.)

    If a residential forced-air heating and cooling system has some ducts that are outside of the home's thermal envelope -- for example, in an attic or a crawl space -- and some of the supply ducts that are outside of the envelope are leaky, then the house can become depressurized with respect to the outdoors. That can exacerbate an odor problem like yours.

    This type of problem can be fixed by leak detection, leak sealing, and balancing.

    Just because you have a forced air heating and cooling system, doesn't mean that you have a ventilation system. If diagnostic work doesn't locate an obvious duct problem, your home might still benefit from the installation of a supply ventilation system.

  6. KathleenOsh | | #7

    Hi Steve,

    My husband and I have an identical problem on our remodeled Chicago flat-roof two flat. I was reaching out to see if you were able to reach any conclusions from your investigation into the issue. We've similarly tested for mold (none) and water leakage/damage (none). I'm at my wits end to be honest. The smell mostly dissipates the moment a window is cracked about 6", but during the hot summer months with the AC pumping, that's not ideal for us.

    Thanks in advance for your guidance,

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