Building Regulations
A document, known as Part L of the Building Regulations, was introduced by the UK government in 2001 for England and Wales, in response to the growing mountain of scientific evidence showing global temperatures to be increasing at a rate beyond natural levels. This has been revised and amended over the years and the following documents provide all the information you will need:
Volume 2: Buildings other than Dwellings
The aim of the documents are to ensure that buildings are constructed or modified to provide greater energy efficiency and tackle the problem. The new and revised goals of the document also focus on reducing CO2 emissions.
The technical stuff
The best way to assess how well insulated, or efficient a building is at conserving energy, is to measure the U-value of its various building elements e.g. walls, roof, windows etc.
The U-value is a measure of the heat flow through a building element. The higher the U-value the more heat flows through the material, so a good U-value is a low one as you want to keep heat inside the building or outside, depending upon your local climate.
To explain in a little more depth, the U-value physically describes how much thermal energy in Watts (W) is transported through a building component of size 1m² at a temperature difference of 1 Kelvin (K) (= 1°C). So the unit for U-value is W/m²K.
What is a “good” U-value?
For a flat roof a U-value of less than 0.2W/m²K is described as good.
So it’s hardly surprising that Part L of Building Regulations stipulates U-values for various building components, and for flat roofs this is 0.25W/m²K.