We moved into this house over the (hot, dry) summer - and of course knew, that there would be no real way to tell how mildewy the house would get in the (cold, damp) winter.
What's really annoying is that our window frames (UPVC, double glazed, range of ages from early 2000s to mid 2010s) get really mildewy. Even if we leave a window shut for 48 hours, or don't fully draw the blinds.
We go around regularly with Dettol M&M remover, we leave windows open at least a crack most days, we try to remember to fully draw all blinds. But it's unrelenting, even if we miss a day of cracking open a window, a day later, little black spots on the UPVC frame, or in the corners where they're sealed in.
The newer windows have tickle vents, which are left open. The general humidity in the house is average, we run a dehumidifier if we dry washing indoors. There is no sign of mildew or dampness on any of the walls/ceilings, just around the windows.
I'd love to know WHY this happens, and if there is a better solution than the on-going cycle of Dettol M&M remover, opening and closing windows, etc.