The walls of an old house are likely to be the single greatest place of heat loss, so any serious plan to save energy is going to have to deal with them. Cavity walls can usually be filled relatively easily, but solid walls are harder to treat. We have both types. The walls of the main house are solid brick; the extension has cavity walls.
Ideally we want to wrap the whole house in something snug and warm, like wrapping yourself in a duvet. Insulating on the outside doesn’t take space from inside, so can be quite thick. The walls can store heat, re-radiating comfortable heat into the rooms and helping to smooth temperature variations from day to night, and warm days to cold.
But it changes the appearance of the property of course, and as part of a terrace that would look odd. Plus we love the look of the red brick.
So we decided on Internal Wall Insulation (IWI) front and rear to preserve the appearance, and External Wall Insulation (EWI) for the large, flat end of terrace wall.
The Energy Saving Trust have also produced useful guides on this topic. There’s a selection here.