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Community and Q&A

Humidity Problem

Denis Recchia | Posted in Mechanicals on

Posted this in a few places, just trying to get it seen.

Hopefully I can get myself out of this mess I got myself into 🙂

I recently built a pretty tight house with my old fashioned dad, I think we ended up doing a pretty good job with making the house tight, but didnt include any mechanical ventilation in the home. Right now in early october we are seeing relative humidity levels around 75% with temperatures inside of around 70 deg F.

How can I fix this? Should I install exhaust fans in the two bathrooms upstairs? My Dad says open a window, but its going to be winter very soon, and recently its been raining so much the humidity is worse outside than inside.

Im moved into the house, so any mechanical ventilation i pick im sure is going to be a disaster. The home is double 2X4 walls with a 2 inch layer of closed cell foam with 10 inches of cellulose inside of it.

Thanks in advance,


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  1. David Meiland | | #1

    You built a new home without bath fans? You're going to need to retrofit them, or something similar. For now I would open windows, or run a dehumidifier if it's too cold outside. And, where are you located that the humidity outdoors is higher that 75%/70F, and/or how do you know it's higher?

  2. GBA Editor
    Martin Holladay | | #2

    David raised all the right issues and asked all the right questions.

    1. Where is the house located?

    2. How is it possible to design and build a house without bathroom exhaust fans?

    3. A house with double-stud walls and spray foam sounds like a tight house -- so one would think that the designer of the home would have included a mechanical ventilation system. More information here: Designing a Good Ventilation System.

    4. A new house often has a lot of moisture -- so-called "construction moisture" -- in the concrete, framing lumber, and drywall compound. Construction moisture is likely to raise the indoor humidity during the first year of occupancy. You may need to operate a dehumidifier for a few months to handle the construction moisture. But you still need a ventilation system!

  3. Denis Recchia | | #3

    The terrible designer was me and my ability to read this site :)

    House is in Connecticut, I know the humidity isnt normally this high, but its what my thermo/hygrometer is telling me now because of all the rain.

    I didnt think about the construction moisture, ill have to run some dehumidifiers until i can retrofit in the bathroom fans.

    If you look around for my posts, you can see what ideas I had, well, I put them all into play, I guess i forgot about the ventilation section on this site. Its going to be a pain to get them in through all the insulation, but at least its not a major unfixable mistake.

    Thanks alot guys, I won alot of arguments with my old school framer dad because of this site, I just never had ventilation in any of my old, wood stove heated home before this, and unfortunetly, didnt put it together with the tight house i built.

    Thanks again so much GBA


  4. Ken Levenson | | #4

    This is very fixable. While you'll likely need to install bath fan(s), and you may need to just run dehumidifiers for a bit - if your house is pretty airtight, and it sounds like it is, you should have a continuous low volume ventilation system with good heat recovery for energy savings and comfort. To provide high heat recovery, continuous, whole house ventilation, consider our Lunos e2 decentralized ventilation units. Perfect for retrofit situations like yours - as no ducts are required, just control wiring. See information on them here:

  5. Tim O'Brien | | #5


    I suggest that you install fans in the bathrooms that will be used for bathing. If you are located in an area with high humidity in the shoulder seasons, consider a whole house or freestanding dehumidifier. A whole house dehumidifier can provide positive pressure ventilation while keeping the moisture level down in your house.

    Please take a look at the dehumidifiers my company manufactures:

    Please give us a call or drop us an email if you would like to consult in detail about your moisture problem - we are located in Madison WI. Our dehumidifiers are designed and assembled in Madison as well.

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