Numbers - what do they mean?

On my website I try to include information that will be helpful to people in making environmental changes to their homes, businesses or lives. Where possible I try to include numbers to show what difference any particular measure will make, e.g. how it will reduce heat loss through your walls, windows or roof.

But where do these numbers come from and what do they mean in reality?

To what extent does people's own behaviour or even poor workmanship undermine these theoretical benefits? There doesn't seem to be much real data out there.

For instance, I quote the Energy Saving Trust advice that topping up your loft insulation to 270 mm could save you £45/year if you already have 50 mm insulation in your loft, and £150 if you have no insulation in your loft. This seems to be saying that adding 220mm to your existing insulation is only saving you £45; whereas starting from scratch with 270mm saves you £150.

How does that work? Why does that first 50mm make such a difference?

Does it matter whether the old insulation is all dusty and looks disgusting?

Is the insulation that was already installed when I moved into this house in 1994 still saving me money on my bills each year?

Do I top it up or does there come a point when I should replace the old stuff and start again?

Don't get me wrong, I do think it's the most important thing to do, to draught proof and insulate your house properly. I just wonder what's behind the numbers we all so glibly accept.

Does anyone know? I would love to hear from you.