We have a partially half-timbered house -- in the old part of the house, not only is there nowhere to put insulation (no cavity, no roofspace) but there are also many unpluggable holes in the walls. I keep a duvet in the sitting room so that I can snuggle up even when the fire isn't alight, and find that 1 kW portable radiators work very well in warming the locality without wasting too much heat on warming the whole room.