Hi-
You are asking a really good question - if you look at this image closely you can see that the sill of this window is tile - it is slightly sloped towards the tub to shed water. The window is fully aluminum window - so water hitting the window will not damage the window.
So, this is what you can see.
Under the tile, which is mortar set, is an interlacing of waterproof membranes to fully protect the opening behind the mortar, so that moisture that does migrate into the assembly through the mortar -- as it always will - eventually hits the waterproof membrane which prevents the water from reaching the backer board and or the framing - and over time the moisture simply dries out through the same mortar it wicked in through.
The key is a combination of good waterproofing - and I would recommend metal, not wood windows. I have used wood windows in showers - but they require more special care - and I would not use a wood sill - this just seems like one is asking for trouble.
I am sorry to took so long to get back to you- I hope this was helpful.
Best,
Cathy
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White kitchen
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