In Tudor times, glass windows were so expensive that they were often removed and buried for safekeeping when the householders went away. Although modern windows can be left in place, people still pay large sums to replace windows during house refurbishment. So it is worth choosing the best possible design to match your budget and the style of the house.
Compared to single-glazed windows, modern double- or triple-glazed units will cut heat losses and energy bills, reduce draughts, insulate against outside noise, and lessen condensation. And if you are replacing double glazing that is 20 or more years old, new units should reflect advances in materials, design and performance.
Heat losses and gains
Iin the UK's climate we want a window that will minimize heat losses from the room to the exterior while allowing the sun's heat to enter and help heat the room for free. When assessing different products, it is worth knowing how windows lose and gain heat.
(Image courtesy of greenspec.co.uk)
The main routes of heat loss are via:
- radiation through the glazing (about two-thirds of the total)
- convection within the air or gas contained in the glazing cavity
- conduction through the window frame
- conduction through the spacer bars separating the panes
- leakage through gaps and openings
The amount of heat lost (thermal transmittance) is given by the unit's U-value, which is measured in watts per square metre per degree of temperature difference. Note that the overall U-value for window and frame is the most realistic reflection of the unit's insulating properties, rather than the 'centre-pane' U-value, which measures heat loss through the centre of the pane. For example, in England and Wales, building regulations now stipulate that replacement windows must have a U-value not exceeding 1.6 (centre-pane U-value 1.2).
The glazing unit should optimize the entry of solar radiation; this can happen in two ways:
- direct transmission of the sun's rays through the glass into the room
- secondary transmission of solar radiation absorbed by the glazing and transferred into the room by convection and radiation
The fraction of the sun's heat that is admitted by the unit is measured by its G-value, which is expressed as a number between 0 and 1. The higher the G-value, the more heat from sunlight enters the room.
The BFRC rating scheme: 'window energy rating' (WER)
The various performance criteria of window units are now combined in the rating system devised by the British Fenestration Rating Council (BFRC). This uses a simple colour-coded A to G rating label similar to those found on fridges and washing machines. The Energy Saving Trust endorses this 'window energy rating' (WER) scheme, which is also cited in building regulations. Apart from the band rating, the label also quotes the product's U-value and G-value, and other performance data. The higher the band rating, the greater the energy efficiency. But beware. there is currently no obligation for window manufacturers to carry this label.
More information about the scheme, and local suppliers of BFRC-rated products can be obtained from the BFRC website, by clicking here.
Energy-efficient window rating label under the BFRC registration scheme. Note the A++ rating for the most efficient designs.
This is an ultra-low-energy standard of construction developed by Wolgang Feist in Germany and now adopted in many countries including the UK. To achieve the high insulation values required, windows must have an overall U-value not exceeding 0.8 W/m2K, with triple glazing and insulating barriers embodied in the frame. For more information about PassivHaus construction see the PassivHaus UK pages hosted by the Buildings Research Establishment. See also Passivhaus: leading the way for sustainable buildings.