Humidity

I knew this in theory but here's a great example of practice. After I've inconsiderately breathed all night there is condensation on the cold glass of the middle sash and none on the warmer perspex secondary glazing (almost completely airtight) covering the two side sashes.

The humidity of my breath that has not been collected by the cold glass is presumably in the air of the room and to avoid a build-up of damp will need to be removed by ventilation. That's easy - the kitchen window on the other side of the room is loose and draughty but as well as letting out the humidity it also lets out such limited heat as I have.

I am glad that when I move back into my own house I will have heat-recovering ventilation.