Part L of 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. The aim of the document was to ensure buildings were constructed or modified to provide greater energy efficiency and tackle the problem. As climate change research continued, Part L was further reviewed for England and Wales, to take effect from April 2006. The new goals of the document were to reduce CO2 emissions by 20% on new build dwellings and 27% on buildings other than dwellings, these reductions were taken from the baseline of 1990 and are thought to be achievable by 2010.
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 as illustrated below:
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.
Technical Stuff
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 can be called 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.