GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Frost on the windows vs. bloody noses

Krohner | Posted in General Questions on

Our house is in northern Minnesota (Climate zone 7a I believe) where we can have pretty wide temperature swings. Last week it was +35 Deg F this morning it was -24 Deg F (without the wind chill).  Some times we can have that swing in a day. And we usually see -40 at some point during the winter. On the really cold mornings we can have some frost on the inside bottom edge of our windows (pictures didn’t want to post but is only about the bottom 1/2″ to 1″). The frost is all thawed 10-11 am usually. However with the drier winter air our kids are getting bloody noses at night so we are consistently trying to match the air exchanger to the temperature. (Ok truthfully I am a bit lazy on matching it and really only do it when I realize they don’t match life is busy).

Our house is about 3000 sq ft of finished space that is handled with Venmar AVS Solo air exchanger with a fully ducted setup. Our heat is all hydronic, except for 1 mini-split for AC in the summers. The humidity controls are a dial on wall were you set the outside temperature manually and it tries to control to the appropriate humidity.

So a couple questions
1) How much damage is that frost doing?

2) What is the appropriate temperature to set air exchanger at: daily average, daily worst case, or weekly worst case?

3) Any better options for having an outdoor reset on the controls so I am not having to be adjusting daily/weekly?

4) Is that air exchanger undersized for the swings?

Let me know if I missed any critical info I am still trying to learn this stuff. Thanks.

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. charlie_sullivan | | #1

    Frost won't hurt your windows. Liquid water running down onto the frame might--if they are wood. Ideal is if it evaporates before it runs down.

    There's another Q&A topic going in which a reader is reporting that a window shrink film worked great for avoiding this problem.

    The temperature swings aren't that big a deal, and I wouldn't worry about setting the control based on the outdoor temperature. Instead, I'd set it based on achieving comfortable humidity without condensation problems. I would get a humidity meter to be able to track that and understand it.

    The other discussion I referred to:

    1. Krohner | | #5

      Thanks I read that post. Any recommendations on a humidity meter?

      1. charlie_sullivan | | #6

        There was an extensive discussion of accuracy vs. cost of various humidity meters here a while back. The upshot is the cheap ones from acurite are good enough for general use. They are carried at big box home improvement stores and probably your local hardware store. The main limitation is that they won't read accurately below maybe 20% or about maybe 90%. I'm linking the full discussion for reference.

  2. onslow | | #2

    Mr Sullivan has made most of the relevant points, but I will add that window shades can also aggravate the problem. In a prior home with double pane windows in a somewhat less severe climate zone, I had to deal with "rain forest" level windows during the winter. The kids aquariums didn't help. The shrink film did on certain windows largely by making them into triple pane. My spouse didn't like the visual effect so much.

    The double cellular shades we put up to manage heat loss proved to be a win-lose for the coldest months. Yes we did lose less heat through the windows at night, but the windows also got so cold behind the shades that actual ice would form on the bottom edges. We had to keep the shades up about a foot to avoid the ice and still mop up the water with towels every morning.

    If you have shades and don't have close neighbors peeking at you, consider leaving them up at least partially so the interior glass surface doesn't get so cold. HRV's are outside my pay grade. There should be many helpful people on GBA well versed on them.

    Running a ceiling fan might also help if the offending windows are in a room is as tiny as our former living room. Do keep after the water collecting on the window frame, once the mold takes hold it will stain the paint and degrade the wood underneath. Repainting the windows was not in my plans after only three years from the installation.

  3. Jon_R | | #3

    The most important purpose of ventilation is indoor air quality and health. So I'd never go below the recommended amount even when excessive humidity isn't an issue.

    What is your indoor humidity when you get frost?

    1. Krohner | | #7

      Not really sure what the humidity is since I currently don't have a measurement with a read out. By turning the knob on the humidistat it clicks around 23 deg F.

      How I believe this control works is when the humidity is higher then the setpoint for the given temperature, it will kick the air exchanger into high, if it is running in either intermediate or low operating modes. We normally run it in low but during winter it is frequently kicking to high for humidity control or at least the light next to "Maximum Speed Humidity control". Am I misunderstanding how these units work?

      Thanks for the feedback

  4. natesc | | #4

    I let our windows sweat. I try to wipe them off once a week to keep mold from growing. I do try to make sure interior RH follows the rules from Joe Lstiburek's book.

    IMO the only proxy you need for indoor air quality in winter is the relative humidity and temperature.

    1. Krohner | | #8

      Which book is that? Just googling it looks like he might have a couple that might apply.

      1. natesc | | #10

        Geez, looks like it is out of print now. It's the "Builder's guide to cold climates"

    2. Expert Member
      MALCOLM TAYLOR | | #9

      Not CO2?

  5. Expert Member
    BILL WICHERS | | #11

    Just because you have condensation or even ice on your windows doesn’t mean there is anything wrong inside your walls, but it can be an early warning sign.

    There are smart thermostats like the ecobee that can automatically adjust humidity setpoints based on outdoor temperature info.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |