Within the last year we moved house into a newish build (circa 96).

In the lounge there is a vent grill near the skirting, which
corresponds with a matching grill on the outside wall. In the summer we
didn't notice it, but now it's getting colder we can feel a massive
cold draught being generated by it.

Why do such vents exist? It is to prevent damp problems?

Can we cover it over in the winter? Surely it will send our heating
bill through the roof if we don't!

Many thanks