I realised my calculation for the heat saving of installed insulation for the lesson on economics was wrong. Instead of calculating how much heat the insulation stopped, I was just working out how much it let through. Of course to calculate how much less heat is going through a wall after you add insulation, you need to know how much was going through the wall before you added the insulation. So, I wondered, what's the U value of an uninsulated wall?
Google will have the answer, I thought.
But Google didn't have a simple answer. It did have several links to U value calculators. For example this one from British Gypsum
It works very well, and allows you to put in various wall structures, showing a nice diagram of what you are doing. You need to have one layer of Gyproc products, though.
Similarly, the innocently titled uvalue-calculator.co.uk
The best I could find was from Changeplan.co.uk. You can add as many layers as you like from a drop down list, or even add your own materials and put in thermal conductivities and thicknesses. When you add them to the wall, you can show what percentage of the wall they are covering, which is useful if you have insulation between pillars and studs.
In the end I decided to just assume an original wall U value of 1 W/m2K, for the sake of easy calculation.
One important thing to remember is that the more insulation you have, the less effective any extra layers will be. So the first 50 mm is worth a lot more than the last 50mm, although the cost may be no different. In fact as you add more it can sometimes become more expensive if you need extra materials to support the cladding.
