The U-value of a material or construction element is the rate of heat loss through that material, taking account of both thermal conductivity and thickness. The lower the U-value of a material, the less heat can pass through it and the better it is at insulating. U-values are measured in watts per metre squared kelvin (W/m2K). Homes built to the passive house standard in Ireland or the UK typically includ a wall U-value of 0.15 W/m2K or better (More moderate U-values may be possible in buildings with more compact forms, which are inherently more efficient . A 2016 analysis by Passive House Plus of data from SEAI's National BER Research tool revealed that the average U-values for new Irish homes have been dramatically improved as a consequence of tightening building regulations - with respective average figures for walls, roofs and floors of 0.17, 0.13 and 0.14 W/m2K, though the backstops stipulated in the regulations are less ambitious. U-value requirements in building regulations in the UK lag some way behind at present.