I'm in the middle of renovating our living room:
We're insulating the external walls and floor and making it as airtight as possible. Airtight construction is good for minimising heat losses but not so good for breathing. So we bought a mechanical ventilation with heat recovery unit which pulls warm, stale air out of the room and sucks in fresh air. It transfers 85% of the heat from the warm exhaust air to the incoming fresh air. So you get fresh air without wasting too much heat.
If you're interested, I've written up my MVHR research on EnviroWiki: envirowiki.org.uk/wiki/Mechanical_Ventilation_with_Heat_Recovery
The unit has just arrived. I'll install it over the next few days.