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.
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