U-values explained

In previous BWF blogs, Kevin Underwood BWF Technical Director, has addressed the Government’s changes to requirements for the thermal transmittance (U-values) of windows and doorsets.  In these blogs Kevin has described the target U-values and limiting U-values in terms of the scientific units W/(m².K). In this blog Kevin will explain what the units are and try to show what effects the changes in these values will have on the energy efficiency of a building.

The units for U-values are Watts per metre squared per degree Kelvin (W/(m².K).  A Watt is a joule per second and a U-value describes the amount of energy, measured in joules, that would be lost through a doorset or window every second for every square metre of the total area of the doorset or window and for each degree of temperature difference there is between the inside temperature and the outside temperature. A temperature difference in degrees Kelvin would be the same as a temperature difference in degrees Celsius or Centigrade but not degrees Fahrenheit which uses a different scale.

Consumers gas bills are charged on how many kilowatt hours of energy they have used. A kilowatt hour is a measure of energy and is 3.6 million joules. This energy can be utilised through a property’s heating system to maintain the inside temperature by balancing the heat lost through the fabric of the property, i.e. the roof, external walls, floors, doorsets and windows. To put this into context I will consider the heat lost over a year through a doorset with a U-value of 1.6 W/(m².K) which is the limiting U-value for timber doorsets that are to be installed into existing dwellings described in the building regulations guidance approved document L volume 1.

Heat will be lost only when the inside temperature is higher than the external temperature. The outside temperature will vary throughout the year and for the purposes of the calculation I have assumed that the inside temperature will be maintained at 21°C each morning and evening and at weekends, and 15°C during weekdays and at night.  This means that for half the time the inside temperature is at 15°C and half the time it is at 21°C unless, of course, the outside temperature is warmer than this. Using average monthly outside temperatures from the Met Office allows me to work out that, for a doorset with a U-value of 1.6 W/(m².K) and a boiler operating at 90% efficiency the energy needed to maintain the inside temperature at the levels shown above would cost £27.24 under the current Energy Price Guarantee. By comparison, the energy required to maintain internal temperature if a doorset with a lower U-value of 1.0 W/(m².K), the target U-value for new dwellings, was fitted would be £17.03, i.e. the lower U-value would save £10.22 in energy over a year.

The cost determined above assumes that the doorset is always closed which, of course, it is not. Every time the doorset is opened, and the inside temperature is warmer than the outside temperature, the inside air will be cooled as there is no insulation, or worse still, cold air could enter the property and would need to be heated to the inside temperature once the doorset is closed again. These conditions aren’t affected by the door’s U-value. It is most likely that the doorset will be opened in the morning or evening during the week and at weekends i.e. during the heating periods when the inside temperature is being maintained at 21°C. If the doorset was only open for a total of two minutes each day, the cooling effects would require £19 of heating to maintain the inside temperature. A cost comparable to the cost due to the heat lost throughout the year when the door is closed.

For further information on U-values please contact BWF Technical Director Kevin Underwood at bwf@bwf.org.uk