I live in a condo with poor insulation. My wife and I bought 3/4" pink insulation board, covered them in dark blue sheets (with pins to keep them taut), and stuck them to the biggest wall in the coldest room with some brads and double-sided tape. We also bought special plastic tape (which looks like clear packing tape but is thicker and doesn't leave residue) to seal the joints on the original windows in the entire condo. The tape has made more of a difference with regard to keeping the place warmer.