We have a lot of windows in this house, for some reason black mold grows along the bottom of each from basement to upper floor. Its and R2000 so we don't pump any moisture into the house and we run with a dehumidifier in the basement. We also have central air and a gas furnace. I'm thinking of wiping it away but concerned it will come right back or my wiping activity will spread it.

Windows are all double pane, likely 15 to 19 years old, and in good condition. They are casement - the type that swings open with a crank.

  • Is the mold inside or outside the house?
    – Tester101
    Commented Oct 7, 2014 at 12:52
  • To clarify, this issue is on the inside.
    – BMNS
    Commented Oct 9, 2014 at 1:12

5 Answers 5


First, kill the mold with bleach and wipe down the whole area to try to get rid of as many spores as possible.

Next, identify and treat the root cause. Mold at the bottom of the inside would suggest that interior condensation is pooling there. Window condensation is caused by two factors:

  1. Interior glass temperatures below the dew point

  2. Humid enough interior air that there's enough moisture to condense on your cold windows

This suggests two potential solutions: decrease the interior humidity level, or get better windows that aren't as cold inside. Getting another dehumidifier or a bigger one is likely to be much cheaper than replacing all your windows with expensive, high-performance triple-pane units which are what you really need to stop condensation in the cold Canadian climate. Window technology has come a long way in the last 15-19 years, but it doesn't come cheap. I'd go with the dehumidifier now if the windows are otherwise in good condition, and start saving up for those better triple-pane windows. You'll feel the difference, and it will stop your condensation problems, too.

  • Another thing to check is the weather stripping on the windows. If they are old and worn and letting a draft through it will drop the temp enough next to the windows to condensate.
    – diceless
    Commented Oct 7, 2014 at 16:19
  • That's a good point, although from the sound of it, these are casement windows, which seal much better than windows that slide and protect the weatherstripping better.
    – iLikeDirt
    Commented Oct 7, 2014 at 16:28
  • Great feedback! Much appreciated. With the amount of windows in this home we'd need to take out a mortgage for those triple pains. I will try the bleach and wipe tactic knowing this will have to be repeated at some point. I'll have a look at the weather stripping too.
    – BMNS
    Commented Oct 9, 2014 at 1:20
  • Heh, "triple pain." So true...
    – iLikeDirt
    Commented Oct 9, 2014 at 1:45

To wipe, wear gloves and make a bucket of diluted Bleach (1 part bleach, 9 parts water). Use a rag or sponge to wipe the mold and clean the rag often in the bleach solution. The bleach will kill the mold and the damp rag will keep the spores down.


Kill the mold first (Bleach, Borax, Vinegar, etc.) then you may use thermal imager (e.g. Flir One) which can help you isolate places of possible cold leaks.


I prefer to use hydrogen peroxide (3% mixed with water) to kill mold. It does a fantastic job and doesn't stink like bleach. I learned this when working in a hospital many years ago. Just remember to add the peroxide to the water, not vice-versa. This prevents a thermal reaction, ( AAA = Always Add Acid).


Stop using bleach! It's bad for people especially kids. It can actually make your mold problem worse encouraging mold growth. There are inexpensive mold killers at Home Depot that are Environmentally friendly, like Moldex.

  • In what ways does bleach encourage mold growth? Yes, bleach is bad for kids and adults if they drink it or get it on bare skin, but 1000s of people use bleach every day for cleaning (especially laundry) without any harm. Please provide some credible research to support your claims.
    – FreeMan
    Commented Dec 15, 2020 at 19:32

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