I live in a house which consists of a 17 year old extension on a 40 something year old cottage. The two main bedrooms are the only rooms in the house with air vents. They are also two of the coldest rooms in the house.
The vents are basically just holes in the wall covered by plastic vent covers. They have a switch to slide them 'closed' (I leave them closed permanently). Even on closed I can feel a cold breeze if I hold my hand to them. I can also hear the wind whistling through them at night.
As it comes into winter I feel like all the heat in these rooms is escaping through these vents. I keep thinking I should just stuff some insulation in there, even just for the winter months.
Is blocking them a bad idea? Long term, can I replace these with a more insulated kind that won't lose as much heat?