At first glance, you might think that it's impossible to have too much or too little thermal mass. After all, if there's loads of it, your home will store more heat; if there's very little, you'll still get the benefits of what you do have.

But it's not as simple as that.

In an ideal passive solar world, homes would be built as a long string of single rooms, so that every room would be heated by the sun and there would be no need to transport warmth from warmer to cooler rooms. In practical terms, long houses are rarely appropriate: traffic flow inside such buildings is often difficult and housing lots in towns and cities tend to be square or rectangular, rather than extended oblongs.

One unusual feature of many passive solar homes is that they are partially buried. Given the natural human dislike of living underground, this often strikes people as a "caveman" approach to efficient living but, in reality, it is an efficient means of conserving potentially enormous amounts of energy. And no, living in a home that uses earth berming or earth sheltering is not like living in a cave!

Most, if not all, proponents of solar energy will exhort the long-term value of installing passive (and active) solar systems. The return on investment over the life of the installation is always impressive on paper and professional installers will happily spend hours explaining how the cost works out as a tiny percentage of your income. Then they'll just as happily bill you five figures!