How to Insulate Your Walls

I read this interesting report about how lots of people who are most interested in saving energy have no idea what saves a lot of energy and what is not worth the hassle.
Anyway, Most of you know to insulate your loft well (fill the bit between the joists to the level and then go across the other way with the wider rolls to make a total of 300mm or more - nice and cheap) but what about windows and walls?
Well double-glazing saves a lot of energy over single-glazing but is expensive both in terms of installation price (I'm sure much more than it needs to be) but also in the energy required to construct the windows so for most people, this means waiting until the windows are broken or doing a few at a time. Leaving the curtains closed after sun-down also helps.
Now to walls. You have lots of construction types including wood, brick and cavity walls (two skins of brick or more commonly brick outside and concrete block inside) also solid concrete. As with most insulation, you should consider all of this to be unacceptably lossy without any insulation and regardless of the fact that 300mm concrete might be more efficient than 200mm brick, it still pays to use a very good insulator such as rigid phenolic insulation (often known by the trade name Kingspan but made by others also).
New houses generally have to use a cavity with 2 inches of rigid insulation in the gap to achieve an acceptable low heat loss but what if you do not have a new house?
If you have a cavity wall, you can opt to fill it with various loose fill materials. This is OK but I'm not convinced it is a good idea (I'm surprised it is even allowed) because it bridges the cavity which is designed primarily to avoid damp penetration (not heat insulation!).
If you want an alternative or have solid walls of any type, you can do one of two things. You can insulate the outside with heat resistant render mix (done by registered installers usually but shop around) or you can insulate the inside of the outside walls with 25 or 50mm rigid insulation either covered by plasterboard or integrated with plasterboard (which is easier but more expensive).
Both of these will make a big difference in heat loss through the walls so find out how much it is to get the work done. A builder or decent decorator will be able to fit the insulation and plasterboard but make sure you get someone who knows how to drywall to finish off the plasterboard.