I'm currently living in a rented 1930's house in Bristol which has a cavity wall. In the bedrooms there is a sliding plastic vent that I can open or close. The one in my bedroom is broken and is permanently open. Its not been an issue in the summer but now the temperature is dropping i'm starting to notice it again.
During the winter the house is bloody freezing upstairs so i'm starting to think about what I can do to make it warmer. (or tell the landlord what I would like him to do).
Can I block these vents to make the rooms warmer during the winter months? What purpose do they serve? What has everyone else done with their cavity walls?