My greenhouse is near some fruit trees and every spring when I come to start off the growing season I find a certain amount of grime has settled across the glass over the preceding year.
Most years I give it a thorough clean and it looks much better for it. This year I didn't bother but how much does it matter?
To be more accurate about the extent of the grime ... it's certainly more than any normal houseproud person would tolerate on their kitchen windows, but it's not enough to feel as if any serious amount of sunlight is being screened out. But, maybe I'm mistaken about that, hence my question.