Nonsense, I suggest that you familiarise yourself with the science of Psychrometrics which is the thermodynamics of air moisture mixes. An outdoor relative humidity of 90% at 5 degrees has a much lower water vapour content than an indoor relative humidity of 70% at 20 degrees and the vapour pressure indoors is higher as well. The second law of thermodynamics equates to heat moves towards cold. That's why we insulate our homes to slow the passage of warm air that's trying to leave.

I am familiar with Psychrometrics etc, I would suggest you read more carefully as the word could is the main word, if there was no issue with having windows open on damp, wet, or moist days then the general advice would be to have windows open at all times, but when your dealing with social housing tenants the majority are not familiar with relative humidity, dew points, etc so its easier to recommend not opening windows on wet, damp or moist days. Please be careful not to get a nosebleed while up on that high horse!

Then explain, using your knowledge of Psychrometrics how you arrive at your conclusion.. "You shouldn't open windows if the weather is raining, damp, etc because you could let in more moisture than your letting out, this is the simplest way to put it"@@phils749