When building or renovating, it's likely your windows will require an energy rating. Double-glazing is where 2 layers of glass come together separated by a 6-20mm layer of argon or gas, which significantly improves the thermal quality of the window. This allowing the transfer of heat to be reduced. Thermal performance is affected by the quality of the installation, thermal breaks in the frame, weather seals, gas & the type of glass and frame used & the frame to glass ratio.