GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Excessive moisture in Minnesota

Z Lessin | Posted in Mechanicals on

We live Northwest of Minneapolis (6a/boardering on 7a) in a 2 story + full basement house (2,500 sqft) built in 1998 (2×6 walls/R19 fiberglass, vinyl siding, Tyvec, Built Right Sheathing, so-so all vinyl windows) that has winter moisture problems (condensation on windows, mold, freezing). During our pre-sale inspection, we noticed moisture on the 2nd floor windows and found that two upstairs bath fans were not attached to bottom of the roof deck, the insulated ducts/flanges were laying in the insulation (thankfully the flanges were taped closed). We attributed the moisture to this and shortly after purchasing the house, reattached the ducting.

A couple years have past and we have continued to have excessive moisture on our windows all winter. I recently started running my furnace fan 24×7 to help address the issue (and was horrified when i received our last electricity bill – we have a non-variable speed furnace fan) and have been keeping our blinds open during the day, which seems to have helped the window moisture substantially.

BUT – earlier today, i was cutting in a junction box on inside of an exterior wall in a stud cavity above an existing outlet and moved the insulation to find moisture/frost on the backside of the wall sheathing. Thankfully i couldn’t see any mold or material deterioration.

I’ve spent the last 8 hours reading (tons on GBA!) and am more confused than when i started with all of the opinions available to begin better regulating the moisture and ventilating our house. My wife and I have both had concerns about indoor air quality and are open to spending money to correct the problem(s), but we would really appreciate some advice on where to start, who to trust and what options would work best in our climate.

A couple other details:
– The two upper floor bath fans are the el cheapo $19.99 units. Would upgrading to something like a higher end Panasonic with rigid ducting (insulated, of course) be a good place to start?
– The fans are currently on toggle switches. I’ve found Leviton makes a moisture sensing ‘switch’ – are these worthwhile? Or maybe just a timer switch?
– One of the upper baths is in the center of the floor – about 50 sqft. Post shower, with the door closed, the room will still be moist/mirror fogged up, 30 minutes later with the fan running. I’m thinking i may need to undercut the door to get better air flow into this room in addition to upgrading the fan.
– There are a total of four people living in the house
– No existing HRV/ERV
– Given what i’ve read (Martin H. is outstanding) about connecting an HRV/ERV to our existing ducting and the ‘already built’ condition of our house, is an HRV even an option for us? Getting new duct work to the second story bathrooms/living spaces would require substantial disruption/re-working of our current layout. Or is this a case where doing SOMETHING is critical even if its not done perfectly?

I apologize in advance for being so long-winded – I’d really like to get moving in the right direction sooner rather than later.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Bill Dietze | | #1

    Z L,
    I say that the first thing to do is purchase a hygrometer or three, place them around your home. Measure your indoor relative humidity and report back! That will tell the folks here on GBA a lot about your situation. The cheap units at the box stores aren't super accurate, but they'll give to the basic information about your issue. Is your relative humidity 30%, 45%, or super high for winter at, say, 60%?

  2. Z Lessin | | #2

    Thanks Bill...that's one detail I forgot to add :)...I ordered two from Amazon earlier today and will report back when I having some readings.


  3. GBA Editor
    Martin Holladay | | #3

    You may want to read this article: Preventing Water Entry Into a Home.

    If your indoor relative humidity is elevated, the first step is determining the source of the moisture. Either of these is possible:

    1. The indoor moisture is the result of ordinary human activities like showering and watering house plants. In this case the solution is to install a better ventilation system.

    2. You have a damp basement that is contributing to your indoor RH problem. To determine whether this is true, and to read about remedies, see Fixing a Wet Basement.

    Your instincts are correct: If you have a damp house, the first step is to verify that your bathroom exhaust fans and range hood fan are working properly. At a minimum, perform the toilet-paper test. (Can your exhaust fan hold a piece of toilet paper tight to the grille while the fan is operating? If not, something is seriously wrong.)

    Ideally, the fan ducts will be inspected. See if air is flowing out of the exterior terminations when the fans are running. You may want to hire a home performance contractor to measure air flow through the fans.

    A Panasonic exhaust fan is definitely a good investment, especially if your house is having moisture problems.

    For more information on bath fans, see Bathroom Exhaust Fans.

    Once you've followed this advice -- installed a few hygrometers, checked the performance of your bath fans, and performed the taped-polyethylene test on your basement slab and basement walls -- report back.

    In the meantime, choose the bath exhaust fan which appears to operate best and leave it on for 24 hours a day. That will lower your indoor RH.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |