science

My plan is to replace the bathroom extractor with a heat exchanger. It takes outside air, warms it using the exhaust air, then dumps it into the bedrooms.

The living areas are easier. Opening a window for 10 minutes isn't an issue when you're awake and moving about.

You can also get vent replacement versions. They flip flop between venting out, and pulling in, storing heat in a heatsink as appropriate.