U-Values

A U-value is a measure of the rate at which heat escapes through a fabric. It is expressed as W/m²/K which means Watts (of heat energy) per square metre (of wall, say) per degree Kelvin (same as Celsius). So a double-glazed window with a U-value of 1.8 will conduct 1.8 Watts of heat per m² for every degree difference in temperature between one side of the window and the other.

Building Regulations set out minimum U-values for each element of a building – currently walls 0.3, floor 0.22, roof 0.16, windows 1.8 and doors 2.2.

Air-tightness

Which is also called permeability. That is, the rate at which air enters a building under natural conditions.

Obviously we need fresh air entering a building, for health and comfort. But fresh air needs heating so Building Regulations now require that we limit the amount of air entering to provide only enough for health and comfort, thereby minimising the amount of heat energy required to keep the house warm.

The Regulations call for a maximum permeability of 10m³ per hour at a pressure of 50 Pascals. How much air that is is largely irrelevant. It is a level of ventilation that can be achieved with standard building practices and without the need for mechanical ventilation.

The standard is set to change in the near future; probably to 7m³ by 2011 and something below 5m³ by 2016. At that level a ventilation system will be needed, be it mechanical or passive.