Glass Glossary

Relative Heat Gain (RHG)

Last Updated: November 23, 2023

The total amount of heat gain through a glazing system at NFRC (National Fenestration Rating Council) and ASHRAE (American Society of Heating, Refrigerating and Air-Conditioning Engineers) specified summer conditions, incorporating the U-Factor and the solar heat gain coefficient (SHGC).

The conditions are 230 BTU/hr/ft2 (726 W/m2) outdoor temperature of 89 °F (32 °C), indoor temperature of 75 °F (24 °C) and 6.2 mph (2.8 m/s) wind (RHG = U-Summer x (89-75) + SHGC x (230). RHG is expressed in terms of BTU/hr/ft2.