Building Construction Handbook, Eighth Edition

European Window Energy Rating Scheme (EWERS) ~ an alternative

to U-values for measuring the thermal efficiency of windows.

U-values form part of the assessment, in addition to factors for

solar heat gain and air leakage. In the UK, testing and labelling of

window manufacturer's products is promoted by the British

Fenestration Rating Council (BFRC). The scheme uses a computer

to simulate energy movement over a year through a standard

window of 1.480¾1.230 m containing a central mullion and opening

sash to one side.

Data is expressed on a scale from A†G in units of kWh/m^2 /year.

A > zero

B †10 to 0

C †20 to †10

D †30 to †20

E †50 to †30

F †70 to †50

G < †70

By formula, rating = (218.6¾

g value) † 68.5 (U-value¾L value)

Where: g value = factor measuring

effectiveness of solar heat block

expressed between 0 and 1. For

comparison:

0.48 (no curtains)

0.43 (curtains open)

0.17 (curtains closed)

U value = weighted average

transmittance coefficient

L value = air leakage factor

From the label shown opposite:

Rating = (218„6¾0„5)

† 68„5 (1„8+0„10)

= 109„3 † 130„15

= †20„85 i.e. †21

Typical format of a window

energy rating label ~

ABC Joinery Ltd.

Window ref. XYZ 123

Thermal Insulation, Window Energy Rating