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Community and Q&A

High Humidity After Home Energy Retrofit

FiveQuarterCon | Posted in GBA Pro Help on

I’m a contractor in south-central Wisconsin (Climate Zone 6). I have a client that purchased a 50’s ranch two years ago that had gone through a deep energy retrofit right before the sale. The retrofit was done by a company that is funded through federal and state grants. Since the homeowner has moved in there have been alarmingly high humidity levels. It rarely drops below 50% even in the winter. There is LOTS of water condensing on all the windows and signs of the mold and mildew growing.  The homeowner has been running stand-alone dehumidifiers which helps but they are constantly running. 

I looked around the space and found that they have installed 2″ XPS on all the basement walls and air-sealed the rimmed joist but there is no vapor barrier under the slab so you can see the moisture being drawn up. They blew in cellulose in the wall cavities and attic. The attic insulation is roughly R60. I was not able to confirm that they air-sealed the ceiling but it seems that they did based on the IR camera. 

They did not install any form of ventilation system. 

I’m looking for any advice on how to get their humidity levels under control. I thought about installing a central dehumidifier into the HVAC but it runs so infrequently that I don’t think it would be very effective. I’ve also thought that an HRV should have been install at the time the retrofit was done but I’m worried that will do enough to lower the humidity either.

Any advice on solving this problem would be greatly appreciated.

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  1. charlie_sullivan | | #1

    I would prioritize adding HRV, perhaps informed by monitorizing CO2 levels, which will likely indicate the need for it, and controlling the moisture coming up through the slab. There should be no need to run dehumidifiers once those are taken care of.

    1. Jon_R | | #6

      +1 on covering the slab. Adding a HRV will help in Winter (and make it worse in Summer). And think about other sources of moisture that could be eliminated.

      Dehumidifiers will still be nice to have when it's mild and muggy outside (like the 70F and 74+% in Madison right now).

  2. alison_feldmann | | #2

    I'm curious how you resolved this issue. I'm thinking about doing my own deep energy retrofit and want to avoid this problem. As I understand it, an HRV will not remove moisture. The exceptions are the Minotaire and CERV systems which combine circulated air just like an HRV but use a heat pump to extract the heat from the outgoing air instead of a heat exchanger. The advantage of the heat pump is that it can control the humidity of the circulating air. The Minotair system will actively monitor and control humidity. I think it can also be installed using the existing ductwork from the central air.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      A lot depends on what climate you are in. An HRV won't remove moisture from incoming air, but in many places outdoor air is dryer than that inside, so simply ventilating with an HRV will reduce humidity.

      1. Expert Member
        BILL WICHERS | | #5

        What Malcolm said is especially true in winter. If you’re in the frozen north, we most of us seem to be, an HRV is probably all you need to keep humidity levels under control in the winter. If you’re down south somewhere, you might need a dehumidifier.

        The time when humidity tends to be the biggest problem is in the spring and fall, when it’s wet and humid outside but not warm enough to run your air conditioner. I have that exact problem myself right now. Last night I even ran one zone on heat so that I could run the other zone’s air conditioner (ack! Horrors! Terribly inefficient way to dehumidify!), to let me get the humidity level down under 60%. We’ve had this happen enough that I’m going to end up putting in a central dehumidifier to deal with it.

        During the summer, air conditioning tends to take care of the humidity as long as your system isn’t oversized. In the winter, if you have a problem at all, an HRV bringing in some of the dry outside air is probably all you’ll ever need — and an HRV uses a lot less energy than a dehumidifier, even a small dehumidifier.


        1. alison_feldmann | | #7

          Hi Bill and Malcolm,
          I'm in the PNW, close to Portland, Oregon. Here we are in Marine 4 and the air is very humid from Fall through Spring, whereas Summers tend to be very dry. So, I guess most of the year we have the conditions you are dealing with right now: humid but too cold to run AC, so some sort of central dehumidifier is necessary.

          The small house (700 sq ft) I'm considering to do the DER only has baseboard heaters and no AC (which is common in the PNW because the summers are so dry). The siding and roofing needs to be replaced anyway, so I'm thinking about adding an air barrier + exterior insulation + rain screen + new siding. However, I'm worried that this will trap moisture coming up from the wet crawlspace. I could encapsulate the crawlspace but I've read that this is impossible to do very well on a retrofit, so I have to come up with some other way to dehumidify the air. The Minotair seems like a nice system for this situation, but I'll likely have to supplement the heat by adding a duct heater:

          Thanks for your thoughts!

  3. Deleted | | #3


  4. PAUL KUENN | | #8

    I use LUNOS HRVs on all my DER projects as long as they are not on a busy street with lots of road noise. Super energy efficient, quiet and easy to retrofit quickly. My houses worked on are all under 0.5 ACH with 6-8" of exterior insulation here in the Appleton, WI area. First buy them a cheap Hygrometer to measure indoor humidity and get them to lower it. Make sure they are running all fans for baths and kitchen when appropriate. PK -

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