Beginning in 2021, Europe introduced new rules for Uw, or U factor. The Uw is the thermal transmittance of the complete window assembly. According to the new regulations, the Uw cannot be higher than 0.9 (W/m2K) or 0.1585 (BTU/hr.ft2.°F). What does this mean?
• (W/m2K) and (BTU/hr.ft2.°F) are measures of thermal conductance (U value) — the ability to ALLOW heat (energy) to flow through a material
• If U value measures conductance, then a lower value makes it harder for heat to transfer through a material (higher insulation = increase in comfort, and less “money out the window”)
• Most codes in the US only require a U value of 0.30 (BTU/hr.ft2.°F)
• This means US codes allow for twice the amount of heat to transfer through windows than the European minimum standard
It is important to remember that it’s not only heat leaving your home twice as fast in the winter, but it’s also heat entering your home and displacing cool air in the summer months. The U value is made up of all window components — the frame, glass, spacers, dimensions, and install method, which is why it’s an excellent metric to understand the true quality and performance of a window/door.
