I live north of the equator in a humid climate. The 2 north-facing windows pictured below only get direct light (lots of it) in the evening. We get a lot of rain. There is green mold growing from the windows for some reason.

I want to

  1. determine why I'm getting this mold
  2. clean off / get rid of the mold
  3. prevent it from coming back without having to replace the windows.

The white on the stucco between the lip of the window and the stucco is unpainted white caulk that has been there for 2 years, and that I should have painted, and will once I clean off the mold.

scale scale

Click for full size image


Mold and/or mildew (yours looks like mildew, but no big difference in regards to abatement) are both fungi and will grow where dark and damp conditions exist.

1) The cause is moisture.

2) You can clean if off easily with a scrub brush and soap, but to help prevent regrowth you should chemically kill it. A 1/4 cup bleach/1 gal. water solution will work or there are numerous consumer cleaners with bleach that will do it.

3) Take steps to keep the area dry, like rain gutters, an awning over the window, etc. Thoroughly clean, caulk and paint the stucco with a good quality exterior paint (most have mildew growth inhibitors). Chemically killing it will help prevent regrowth but be prepared to repeat if moisture cannot be controlled.

  • I won't "not" be able to keep the area from getting wet. We get rain coming in at all angles. It appears as though (especially in the first picture) some of the growth is coming out from between the cracks of the aluminum. Should I consider sealing that after bleaching it down, and if so, with what? – JoshDM Aug 7 '14 at 17:52
  • 1
    If you can get your hands on it there are actually fungicide additives for paint. Also its hard to tell from the pictures, but is the window sill flat or does it slope down away from the window? – James Aug 7 '14 at 17:53
  • 1
    Paint will seal (given that the gaps are narrow, which they appear to be) those cracks in the window itself. Any crevice water can get into it will, and then it won't dry as well which promotes mold/fungus growth. – James Aug 7 '14 at 18:01
  • 1
  • HomeDepot also can mix fungicidal paint additive and sells it in single-use sizes. – Jeff-Inventor ChromeOS Aug 9 '14 at 22:05

Edit: Correction for warm climate.

Mold/mildew results from a combination of dirt/dust/pollen/pet dander, skin cells, bug crap combined with moisture. Inside, you can filter to reduce this. Outside, you're have to clean more often.


  1. Wash the window frames periodically with a disinfectant. Ordinary bleach works, but can be tough on clothing and nearby fabrics. Check the cleanup aisle. Often this sort of cleanup is easier in two passes. First pass just get the mold wet. Do the whole house. Next day, scrub. Giving it a day kills it and makes it easier to remove. You may have to try a couple of different disinfectants to find one that is really effective.

  2. Dusting the windows edges can remove dust before it gets wet. Outside, try using a shopvac with a brush tip. This material is very fine. Some is going to go right through your vacuum.

  3. There are paints that have a fungicide incorporated. Often used in repainting bathrooms and laundries. If you wanted to do this, you would need to clean the aluminum frames with TSP, then prime, then paint. This is time consuming and finicky. You may be able to get this as an additive, allowing you to use any latex paint. You may want to use a colour that matches the colour of your mold colony. This doesn't eliminate the problem but it makes it less visible.

  4. Upgrade your windows. Present prices of natural gas are low enough that this isn't a good investment in terms of changing your heating bill, but if you are in a cold climate it will make your house more pleasant. In a cooling climate, better windows may save enough on air conditioning costs to pay for themselves.

If your are in a cold climate: your problem is winter.

In winter the aluminum conducts heat away from the house. Moisture condenses on the metal. (We can get 1/8" of frost on our aluminum window frames on a cold night (-40))

The air in your house is also dusty, with a substantial amount of the dust being human skin and pet dander. The combination provides enough food for molds and mildew to grow.

Options. In addition to the ones above, these may help:

  1. Add dust filtration to your house. (Only if indoor problem) If you have forced air heating, it's easiest to do this at the furnace. In a nut shell, you are just putting in a better filter system. This does the most good if the circulation fan runs all the time, cleaning the air. But try both ways.

An easy dust test is to look at a beam of sunlight slanting through the window against a dark background.

  1. Dust the windows to remove accumulated organic matter.
  • Your answer is regarding rooms inside a house. This was outside. Also Florida; not a winter issue. – JoshDM Mar 29 '19 at 13:29
  • The section starting "options" is valid for any climate. The treatment is the same for outside. – Sherwood Botsford Mar 29 '19 at 20:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.