Retro-fitting insulation is an established practice in some parts of the world.

In Switzerland, the government periodically raises the insulation requirements for offices and apartmement blocks. Compliance with the new standard is optional for existing buildings, but after the date given in the legislation you are not allowed to use any form of central heating unless you do comply.

If you spend much time in Switzerland, you will realise that a number of buildings are having additional insulation added. The standard practice when I lived there was to attach a layer of insulating blocks (often up to 0.5m thick) to the outside of the building, cover this with a render and then paint it. Often the windows get upgraded at the same time.

Upgrading insulation in this way is relatively cheap and definitely effective.