Mildew issues from false wall

5 years ago

last modified: 5 years ago

I'm having some mildew issues and trying to determine the best way to solve it. We built a false wall in front of a sloping exterior wall, creating a cavity approximately 4x0.5x1.0m in size. It's not air-tight to the room, and has some doors to use it as a storage space. Over winter we have seen some surface mould/mildew blooming (which we can easily wipe off).

The room is 17-19°C, 60% humidity but this cavity is 10-11°C and 90+% humidity. Even leaving the doors (approx. 1m² opening in a 4m² wall) does not help much. Dehumidifying bricks likewise don't seem to help.

My next thought is to install a heater inside the cavity, but am looking for any guidance as to what would be the best course of action. Thanks for your help!

Featured Answer

formulaross20

5 years ago

Natural convection is a poor way to move heat, and that is what you are relying on with an open door to the long, narrow cavity. Natural convective air loops within the area are essentially nil, yet, moisture is still free to diffuse towards the cold condensing surface, which it willingly does. You are living in a science demonstration! ;)

Before adding a heater, try increasing the convective heat flow to the cavity. Place a fan to move air through the zone (preferably in at one end and out the other). It may not cure the issue, but it certainly will supply heat to the area, warming the wall surface, and perhaps reduce or stop the moisture condensation. If you can't flow air in at one end and out the other of the cavity, do intentionally direct the flow either high or low to allow a return path for the air to exit the area.

I don't completely buy into the line of thought that "moisture is a problem in Germany because buildings are so well sealed". It may be part of the equation, but I doubt it's the complete picture. For example, our home is well sealed (0.4 ACH @ 50 Pa), the outside temperature is cold (24F/-4C) and yet our indoor relative humidity is 35%. So, what's the difference? What other sources of internal water vapor do you have? Poor bath/shower venting? Gas appliances adding combustion water vapor to the air? Number of people in a small home? What vapor barriers are lacking in the building structure? Exposed dirt crawlspaces or basements? Many things to look at. You might consider using a line-powered dehumidifier. Interior mold is a real health issue and you need to stop it now, IMO, whatever it takes.

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Comments (6)

Seabornman
5 years ago
You're humidity is way too high. Where are you located?
Adam
Original Author
5 years ago
Yes I know, and I want to get it down. I am located in Germany. I can open the window and get the room humidity lower (as it should be), but the cavity will still end up very high humidity because of the lower temperature.
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Seabornman
5 years ago
You must have something contributing to the humidity. It could be condensation, a roof leak, moisture coming up from a crawl space or basement, an out-of-control humidifier, something.
Adam
Original Author
5 years ago
I think I didn't describe it very well. The false wall is made of interlocking MDF v-groove boards, supported on a pine frame. However it is not sealed to the floor or roof, or between individual boards, so while it provides a thermal barrier it is nowhere near airtight. Thus the air in the room (17°C, 60% humidity) is the same air as in the wall cavity (11°C. 90% humidity). Think of it like a long, low wardrobe without a back panel and placed against an exterior wall. Without the wall I believe that area would be 17°C, 60% humidity like elsewhere.

Moisture is generally a problem in German buildings in winter, because they are so well sealed, so it builds up over time. The 60% can be brought down, just need to do a better job airing the apartment every day. However that has not helped the cold area behind the wall.
Adam
Original Author
5 years ago
Thanks for the advice, that's a great idea. I could easily install a fan at each end of the cavity to create continuous airflow. Hopefully that can bring it up warm enough that the humidity is at a reasonable level.

The apartment is 80m² for 2 people., no gas appliances, we shower with the window open (German bathrooms for some reason don't have any extraction systems!) and we are on the top floor of the building. However possibly the vapor barriers are missing, as it is an old building and I did find out the basement was not water sealed, so I doubt the rest of the building is either. I have also seen water damage from the roof in the stairwell. I'm going to try a full venting of the entire apartment at once to see if I can get the humidity lower, but thus far I haven't been able to get below about 50%, and it inevitably rises back up fairly quickly. Perhaps a powered dehumidifier is the best way.