Gnats fitting through screens in windws

Im not an expert at all, but I think the higher the number is the finer the mesh. And the highest Ive seen listed (therefore the finest mesh size to keep out tiny insects) would be 20x20. So if you can easily change them, try it. But you could also see if putting something like essential oils (designed to repel bugs) around the screens might help (and more cost efficient).

Well if the gnats are bothering you then I would say its worth it, but it is real easy to change screen yourself with just a single edge razor blade, a splining tool, rubber spline and screen. https://www.youtube.com/watch?v=UMDuYmHTabU

SPRAY SCREEN WITH RAID

Do you have a rep you can ask? I think most manufacturers have screens labeled for 'keeping insects out' type thing. I'm not sure how the 21x17 compares, sorry. But good look and maybe post back here once you contact Pella!

You might have an infestation already in the house. They lay eggs every week on fruits vegetables and in plants. Put a small jar with plastic wrap over it rubber banded. Poke a small hole in top slightly bigger then a gnat. They fly in can't fly out. In jar is a mixture of 3/4 water 1 teaspoon of sure 2 tablespoons of Apple cider vinegar and 4 drops of dish soap. They are very attracted to this. Empty and refresh when you see less bodies. Took a couple of days but if I see even 1 I put this out it works!