3.11 Window Comfort Requirements – Above Green

The optimization used to set space conditioning targets allowed for code-minimum windows in the mid-rise and high-rise buildings. While the relaxed window performance may be most cost effective from an up-front cost and energy savings payback, it does not provide an adequate guardrail on ensuring comfort. Therefore, a window comfort requirement will be enforced.

The window comfort criterion applies to all projects, regardless of size. The U-value required scales by window height – the taller the window, the lower the required U-value.

PHIUS Window Comfort & Condensation Risk Assessment

The whole-window U-value must be low enough such that the window surface inside is no more than a certain temperature difference (∆𝑻) lower than the inside air temperature when the outside air temp 𝑻𝒅 is at the ASHRAE 99% design temperature.

Maximum temperature difference (∆𝑻) =

𝟔. 𝟎 ℉ for double-height spaces;

𝟏𝟒. 𝟕℉ − (𝟎. 𝟕𝟒𝟐 𝒙 𝑯𝑯𝑺(𝒇𝒕)) for window head-height-from-sill HHS of 10 feet or less,

or 𝟏𝟑. 𝟑℉, whichever is less;

where the whole-window U is calculated as:

𝑈 = ∆𝑇 (𝑅𝑠𝑖 ⁄ )𝑇𝑎𝑖𝑟,𝑖𝑛𝑠𝑖𝑑𝑒 − 𝑇𝑑

𝑹𝒔𝒊 = 0.74 ℎ𝑟𝑓𝑡2℉ 𝐵𝑇𝑈

𝑻𝒂𝒊𝒓,𝒊𝒏𝒔𝒊𝒅𝒆 = 68℉

𝑻𝒅 = 𝐴𝑆𝐻𝑅𝐴𝐸 99% 𝐷𝑒𝑠𝑖𝑔𝑛 𝑇𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒 ℉ (ASHRAE Fundamentals 2017)

Exceptions:

Windows in non 'regularly occupied' areas Example: a college dormitory with an entry lobby and no seating, or a hallway in a school with only transient occupants.
ADA doors