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  • Stefan Goebel

Window U-Value Spec. and it´s Impact to Comfort & Condensation

The minimum thermal performance comparison at a 5ft and 10ft window height for NYC, Boston, Chicago, Houston and San Francisco that will meet Phius Comfort Requirements and Condensation Risk. Utilize this simple and free Windows Comfort and Condensation Risk Calculator and ensure your projects will meet Window Comfort and Condensation Requirements!



Window Comfort: The maximum whole-window U-value that may be used in a building pursuing Phius certification is set based on the window height and ASHRAE 99% design temperature for that location.

The whole-window U-value must be sufficiently low to ensure that the interior window surface temperature is no more than a specified difference (∆T) lower than the indoor air temperature when the outside air temperature is at the ASHRAE 99% design temperature (Td). Choosing an appropriate window U-value is crucial for minimizing discomfort during the heating season. In contrast, shading devices, low-emissivity (low-e) coatings, and the solar heat gain coefficient (SHGC) are more effective for preventing discomfort during the cooling season.


Condensation Risk: When calculated according to ISO 13788, the interior surface temperature of the fenestration must be maintained to prevent condensation. This means the lowest temperature on the interior surface must result in a relative humidity of less than 100% at the surface when the interior air is cooled to that temperature.




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