VENTILATION is what you need alots of it Take a picture of the whole space please and post here in a comment .

@Patricia Colwell Consulting It's a 5x9 bathroom. There will be a ventilation fan installed with the remodel, but there isn't one existing.

Nothing that couldn't be "cleaned up", but the window being below the tile is problematic for long term moisture. Make sure your tile installer establishes what size window he needs with your window framer so that the water management details are continuous and in line.

Its not the window material, its the fact that the window is set low on the sill. Also, the weep holes on your windows looked to be clogged based on the pocket sill being filled with what looks to be dirt. That will also happen with fiberglass.

Any waterproof material will behave the same. A low window, that catches spray, with a track to fill with water, is THE big design mistake here. Inadequate cleaning and maintenance is the second mistake. Inadequate ventilation is the third.

The picture shows a lack of cleaning.

@Windows on Washington Ltd @William Rossman @Monique

If the window is too low on the sill, do we install a smaller or bigger window? Or do we keep the window the same size and change the tile height? Thanks!!

Here is a picture from the outside.

If you want to add on/repair stucco, make the window shorter and higher. If you don’t, then at least get rid of the slider aspect. Double hung would be better. And make sure your installer integrates the shower wall waterproofing with window. Windows arent waterproof from inside the house. I’m afraid you are going to need wall repair from rot when you do demo, and if that is the case, you will already be doing wall repair. So that is when you replace it with a shorter window.

Mildew, particularly mold and powdery mildew, is indeed caused by a combination of soiling (organic matter) and high humidity, which provides the necessary moisture and food source for their growth

@Monique When you say shorter, is that the height? So, would the window sill be higher and the top of the window be the same? About how much shorter? Thank you!

A proper detail eliminating the cause of the mildew is the solution, not window composition.

@Joseph Corlett, LLC If we were to install another vinyl window, how do we fix the detail?

Thanks!

Detail: daily maintenance., both inside and out.

You remove the window, the shower walls and siding around the exterior of the window. You pack the opening up to raise the window. Flashing tape the entire opening, treat the interior the same as you would the exterior. Vinyl or fiberglass window will not matter.

As others have posted fiberglass and vinyl are similar in their resistance to mildew.

It would be best to do some selective demo here, to assess the potential for rot inside the current construction. You can always tape up a 6 mil poly sheeting as an internal shower curtain over the window area after doing discovery. You may need to reconstruct the wall and exterior stucco already. By my bet is that you also will need a custom sized window, and those have lead times. Better to have that lead time on the front end, before a complete demo renders the bath useless. A partial demo lets you plan the end result much better, with less actual down time.

@millworkman Does packing the opening mean making the window smaller? Would that be to reduce some of the mildew from forming? If so, how much smaller does the window need to be? Thank you!

icenet333:

Something likethis.

Making the window smaller, raising it, would make the black less noticeable.

Spray and condensation inside, rain and condensation outside, coupled with: dust, will turn black.

Cleaning will eliminate the dust.

If you do not clean the window tracks, black stuff will grow.

I hate AI and this guy is making me nuts.

Just f l a g the AI b o t. It's an obnoxious regurgitation. Grab a cup of coffee, pull up the account, and go to town with the button. If they get no engagement, they will stop.

The bottom of the window should be above the splash zone, roughly the height of the tallest user's head. Take some blue painter tape and mark that height around the current shower including the window. With planning, you can order a window whose lock will be reachable while you stand in the shower and reach up to open the window without losing your balance (in other words, the lock should not be in the middle of the window or along the top). Any shampoo niche should be located where it won't get splashed or drenched by the shower to avoid mold.

You will also need a much more effective exhaust fan on a timer. Choose one with a light. Panasonic makes excellent exhaust fans.

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@apple_pie_order Great idea about the window size, which we considered. But there is too much fear of losing natural light and airflow in making a 35"35" window be 35"x 17". Nice photo!

Do you think the air flow in through the window may be improved with an exhaust fan running?

" But there is too much fear of losing natural light "

Worse than mildew and rot? Window cannot be a main source of ventilation. You must have a fan.

@apple_pie_order @millworkman We will be adding a fan as it is now code in our city, so with a new window, it should help reduce some of the mildew.