Glazing for a loft conversion

Glazing (in windows, roof windows and glazed doors) fulfils several functions. It provides a view out, lets daylight in and assists ventilation.

However, the heat loss through one square metre of even modern, highperformance double-glazed windows is much greater than the heat loss through one square metre of insulated external wall or roof. Excessive glazing is therefore a cause of unwanted heat loss.

For loft conversions, building regulations in each part of the UK specify maximum allowable areas of glazed openings (windows, doors and roof windows).They also specify maximum U-values for new openings (windows, doors and rooflights). Adopting the higher, Best Practice standards for openings set out in Table 1 will reduce fuel use, fuel costs and carbon emissions.

There are many possible combinations of frame and glazing types for new openings. Examples of conventional window types (i.e. in vertical gable walls) that meet Best Practice standards are:

• timber-framed windows with double glazing, incorporating at least a 16mm glazing gap, argon gas fill and a 'soft' low emissivity coating;

• timber-framed windows with triple glazing, 12mm glazing gaps, and a 'hard' low emissivity coating;

• metal-framed windows (incorporating thermal breaks) with triple glazing, incorporating at least 16mm glazing gaps, argon gas fill and a 'soft' low emissivity coating.

Examples of roof window types (i.e. in pitched roofs) that meet Best Practice are:

• timber-framed roof windows with triple glazing, incorporating at least 16mm glazing gaps and a 'hard' low emissivity coating;

• timber-framed roof windows with triple glazing, incorporating 12mm glazing gaps and a 'soft' low emissivity coating;

• timber-framed roof windows with triple glazing, incorporating 12mm glazing gaps, argon gas fill and a 'hard' 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 is obtainable at a higher cost by filling with krypton or xenon.

All windows and external doors must be weather-stripped.They should also be equipped with good-quality locking mechanisms that ensure good compression in the seals when closed.

Window energy ratings

The British Fenestration Rating Council (BFRC) Window Energy Label provides an objective standard against which to judge different window types. Selecting an A or B rated product ensures that the windows achieve the manufacturer's claimed performance, and also 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).