Building Construction Handbook, Eighth Edition

Low emissivity or ``Low E'' glass is specially manufactured with a
surface coating to significantly improve its thermal performance.
The surface coating has a dual function:

1. 
   

   Allows solar short wave light radiation to penetrate a building.

   
   


2. 
   

   Reflects long wave heat radiation losses back into a building.

   
   

Manufacturing processes:

1. 
   

   Pyrolitic hard coat, applied on-line as the glass is made.
   Emissivity range, 0.15†0.20, e.g. Pilkington `K'.

   
   


2. 
   

   A sputtered soft coat applied after glass manufacture.
   Emissivity range, 0.05†0.10, e.g. Pilkington Kappafloat' and Suncool High Performance'.

   
   

Note: In relative terms, uncoated glass has a normal emissivity of about 0.90.
Indicative U-values for multi-glazed windows of 4 mm glass with a 16 mm void width:

Glazing type uPVC or wood frame metal frame

Double, air filled 2.7 3.3

Double, argon filled 2.6 3.2

Double, air filled Low E (0.20) 2.1 2.6

Double, argon filled Low E (0.20) 2.0 2.5

Double, air filled Low E (0.05) 2.0 2.3

Double, argon filled Low E (0.05) 1.7 2.1

Triple, air filled 2.0 2.5

Triple, argon filled 1.9 2.4

Triple, air filled Low E (0.20) 1.6 2.0

Triple, argon filled Low E (0.20) 1.5 1.9

Triple, air filled Low E (0.05) 1.4 1.8

Triple, argon filled Low E (0.05) 1.3 1.7

Notes:

1. A larger void and thicker glass will reduce the U-value, and vice-versa.


2. Data for metal frames assumes a thermal break of 4 mm (see next page).


3. Hollow metal framing units can be filled with a closed cell insulant foam to con-
   siderably reduce U-values.

Low Emissivity Glass