U-values (measured in Watts per metre Kelvin - W/m2K) measure the rate of heat loss through building elements. As they impact on both the dwelling emission rate and fabric energy efficiency targets, they ultimately govern the result of the SAP (the Standard Assessment Procedure) test used to assess a house’s energy performance. Achieving ‘good’ (low) U-values gives you the baseline for achieving and exceeding Building Regulations. But what are the best ways to achieve a good U-value? There are so many options in terms of design and materials you can be spoilt for choice. So where should you start?