English Translation
If your windows are not airtight, moisture will seep in.
気密性でない窓 is grammatically broken and sounds unnatural. 気密性がない窓 is the correct expression.
～性 is equal to “…ness” in English. It’s a noun. ～でない is not used for ～性 because 性 stands for 性質 (nature/characteristics/functions). You cannot say “this window is not airtightness” in English so as この窓は気密性でない in Japanese is grammatically incorrect.
気密性がない窓ならば means “if it is a non-airtight window” (lit. if it is a window without the ability of air ventilation). 気密性がない in a set phrase functions as an adjective for a window (noun).