Glazed openings of a house extension

There are many combinations of frame type and glazing type that will meet the Best Practice standard shown in Table 1 (i.e. maximum U-value 1.8W/m2K). Glazing types include double- and triple-glazing with different spacing, low emissivity coatings, and argon filling between the panes.

Frame types include metal, PVCu and timber. Metal-frames should include thermal breaks to reduce heat loss through the frames. Examples of window types that meet the Best Practice standard are as follows.

• Timber-framed windows with double glazing incorporating at least a 16mm glazing gap, argon gas fill and one ‘soft’ low emissivity coating.

• Timber-framed windows with triple glazing, 12mm glazing gaps, and one ‘hard’ low emissivity coating.

• Metal-framed windows (incorporating thermal breaks) with triple glazing incorporating at least 16mm glazing gaps, argon gas fill and one ‘soft’ low emissivity coating.

Low emissivity coatings

Low emissivity (‘low-e’) coatings for glazing are of two main types, known as ‘hard’ and ‘soft’. The soft coatings provide better performance at little additional cost.

Gas filling

The most common form of gas filling for double and triple glazing is argon. Better performance can be obtained (at higher cost) by filing with krypton or xenon.

All windows and external doors must be weather-stripped, and should be equipped with good-quality locking mechanisms that ensure that the seals are compressed when they are closed.

Window energy ratings

The British Fenestration Rating Council (BFRC) Window Energy Label provides an objective standard against which the relative merits of different window types for a home extension can be judged. Selecting A or B rated windows also ensures that the windows achieve the manufacturer’s claimed performance, and that air leakage and draughts are kept to a minimum. Since February 2005 Band C and above windows are Energy Efficiency Recommended by Energy Saving Trust (EST).