Walls can't generally be insulated without removing the wall lining, but if you are renovating, install batts and foil in the walls. You should have the highest R-Value products that you can afford. Make sure that you request materials designed for walls, because if you use ceiling insulation it's likely to slump down into a heap under its own weight - making it ineffective.

 

Wall insulation may be coupled with accoustic insulation to reduce noise transmission too - especially important if you live on a busy road or if you have teenages that like their music loud. Internal walls may be insulated for noise.