I don't know if you guys have seen this tiny package or not,
MANHR100W Heat Recovery Fan Heat Exchanger Ventilation Controls Condensation | eBay
It looks like it might be installable in a window, (Even a very small window)
to give at least one room some fresh air..
It would have to be inside an insulated window box, (and it uses 220vac).
Anyone know about the core used in the small Manrose products??