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

Is humidity above 60% above grade as much a mold problem as basements?

user-4053553 | Posted in Energy Efficiency and Durability on

For houses without A/C in the summer in humid areas why is not mold a big problem?

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. GBA Editor
    Martin Holladay | | #1

    In the old days, before the invention of air conditioners, people in hot, humid climates left the windows open and kept cool with fans. The indoor relative humidity was the same as the outdoor relative humidity.

    These homes without air conditioning could still have mold, of course, especially in bathrooms. So it's not as if these homes were mold-free. If mold occurred, homeowners would scrub their walls and ceilings and re-paint.

    If you close up all of your windows and operate an air conditioner, the air conditioner should lower the indoor air temperature and lower the indoor relative humidity. That should reduce the chance of mold.

    However, a poorly designed air conditioning system can actually increase the chance of mold, mainly because a house with air conditioning has cold ducts and cold registers. Whenever you have cold surfaces, you have opportunities for condensation. If your duct is at 55°F, and if there is a pathway that allows humid outdoor air to get near your 55°F duct, then the duct will start to drip. This scenario can't happen in an older house without air conditioning -- because the older house had no surfaces that were cold enough to allow condensation to occur.

  2. user-4053553 | | #2

    I understand the washroom mold, 100% humidity many times a day, liquid water and so forth, but without A/C with the windows open and the indoor humidity matching the outdoor in areas of high humidity such as provinces and states that border the oceans, great lakes, rivers and so on (hence far over 60% for long periods of time) mold does not grow nearly as readily as 65% humidity in the basement.

  3. GBA Editor
    Martin Holladay | | #3

    Sunshine kills mold, and wind helps dry out surfaces where mold might get started. Basements don't usually have sunshine or wind.

    Outdoor mold exists. If you get a chance to visit the Pacific Northwest, look for mold on siding on the north side of homes, and in other outdoor areas that are shaded. You'll find outdoor mold.

  4. user-4053553 | | #4

    Very interesting points, i never considered the effect of sunshine and wind on indoors, i wonder if the mold preventing rays (UV, IR?) bounce off interior walls to cover the entire room because vented crawlspaces are very susceptible to mold
    With outdoor mold now that i think about it, it does seem to appear in dark and of course moist places.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |