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

Power outage and mold

mikeysp | Posted in General Questions on

I was discussing with a friend who is also engaged in planning his house build and a couple of questions came up as I mentioned the building science lessons I am learning.

We have very significant mold problems in our area (Mixed-Humid, Zone 4a, west of Nashville, TN).

Does building a tight house add to the risk of mold in an extended power outage? Weeks.

Separate question: if building an off grid home (think ammish) with no power to house would a tight house be a catastrophe for mold promotion? If yes, what would be the best building materials and approach?

Thank you.


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. Expert Member
    BILL WICHERS | | #1

    I’m not sure why power outages would affect mold growth. A tight house will resist airflow in both directions, in AND out. If you have an extended power outage, the conditions in a tight house are likely to change less than they would in a less-tight house due to better air sealing.

    If you build everything correctly to limit moisture buildup you should be ok regarding mold regardless of if you have electric service or not. Remember that there are plenty of unconditioned sheds that don’t have mold problems.


    1. mikeysp | | #2

      Thank you Bill.

      My thought was that people living in a house will have humidity coming from them, drinking water, taking bath, heating water, etc... Whereas a shed would not face these and are typically very leaky, at least in my limited experience.

      1. Expert Member
        BILL WICHERS | | #5

        I suspect that if you have a long enough power outage for moisture levels to rise to problem levels, you’d probably not have hot water, or even water at all. As others have said, cracking a window will provide plenty of ventilation to keep moisture levels under control.

        As a side note, my regular work is designing and consulting for critical facilities, primarily in the telecommunications industry. We maintain pretty complex environmental control systems due to all of the fancy electronic equipment. Anyway, backup power is taken very seriously since no power = no buisness, and we have regulatory reliability standards to meet. Our fuel supply standard is for 48 hours of runtime at full load. This is a pretty common standard in the industry.

        My point is that if you’re worried about humidity during a major disaster-level outage, don’t. There will be far, far bigger issues with a 48+ hour large-scale power issue (loss of refrigeration for food, loss of sanitation for water/sewer, etc). The off-grid people will really be feeling luxurious! You’ll lose pretty much all utilities, water included, so you’ll be in survival mode and humidity buildup will be less, and not something you’ll be worried about.

        For smaller-scale (neighborhood) but lengthy outages, get a good generator and keep it maintained. I’d stay away from the common brand the box stores carry due to reliability issues with those.


        1. mikeysp | | #6


          Good analysis and perspective on the real issues on one's plate with an extended power outage. I was just wondering if all these well built houses would become greenhouses in an extended situation; but, alas we will have bigger fish to fry than mold in such a situation.

          As for a tight building being the best idea for a permanently off grid house, it is not registering (greenhouse). I am not going this route (off grid); however, a few folks I know are into that and I wondered how it would work for them?

          Thank you gentlemen for your imput.


          1. Expert Member
            BILL WICHERS | | #7

            Tight houses are more energy efficient, other things being equal. Off grid houses tend to have limited energy resources available, so maximizing efficiency is good. On grid houses pay for energy they use, so efficiency is good. On grid houses with power outages running on backup generators are limited on fuel or generator capacity so efficiency is good.

            On grid houses without backup power during power outages want to maintain heat inside for as long as possible. Tight houses will help to do this.

            Any way you look at it, building a tight house helps you. I’d always recommend building as tight as you can, and insulating as well as you can (within reason :). You’ll save energy and money in the long run regardless of where your energy comes from. I always advocate for efficiency, and it’s one area where there really is no downside. If your house gets too hot, open a window or two. Easy.


  2. Expert Member


    Tight houses need ventilation. If they can't have it supplied mechanically, because there is no power to do so, they need to have it supplied passively by opening windows. One way or another there needs to be a mechanism to remove both humid air and contaminants.

  3. Jon_R | | #4

    The same moisture sources with less ventilation = higher interior humidity. But tight building is still a good idea. If you rely on leaks, then ventilation is often too much or too little - depending on wind. If you use a window, you can make it exactly what is needed (for humidity and indoor air quality).

  4. GBA Editor
    Martin Holladay | | #8

    People lived without electricity for hundreds of years, and most homes weren't overwhelmed by mold.

    In the winter, most people keep their windows closed. They don't usually have mold problems because winter air holds very little moisture. Winter air is dry air.

    Before the invention of air conditioning, during the summer in Tennessee, when the outdoor air was hot and humid, most people opened their windows. The humidity conditions in the house were identical to the humidity conditions outdoors. There weren't any cold surfaces where condensation could happen, except for the glass of lemonade with ice cubes that most people had in their hands.

    1. mikeysp | | #9

      Thank you Martin. I thought the tighter envelope of our day made some difference. So, it is the humidity touching cool surfaces which turns it back into a dew, water state, and grows mold. So an open window on a tight house is the same as an open window on a two hundred year old log cabin. I don't fully ggrasp the whole concept, but I am satisfied for now. Thank you.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |