Recently I completed a custom training workshop to a large company on how to use THERM and WINDOW to analyze windows to the National Fenestration Rating Council (NFRC) and PHIUS standards. Throughout the workshop, I kept being reminded just how complex a high performance window can be.
As most of us know, windows comprise the frame (and sash, if operable) and the insulated glazing unit (or IGU). The whole window U-value (or total product U-
value) is an area weighted average of three discrete component U-values: (1) Uframe,(2) Uedge of glass, and (3) Ucenter of glass. The PHIUS standard uses a similar, but different three components: (1) Uframe, (2) Ucenter, and (3) ?“edge” of glass. Both standards have standard test sizes for different window types (e.g. casement) in order for comparable total product U-values between manufacturer’s available products. Depending on the weakest link of the three components, growing or shrinking your total window area becomes extremely important when considering performance. For example, if your Ucentre of glass is the lowest of the three, a large window will have a lower total product U-value as the relative effect of the frame is decreased proportionately to the increase in overall glass area. This is simply because less energy enters the IGU, resulting in lower glass surface temperatures (i.e. the interior glass surface temperature may fall below the dew point of the interior house air!)
Further, U-value is not the only metric to consider. In fact, a decrease in U-value, may lead to increased risk of condensation on the interior surface of your glass. For example, if you opt to put a low-emissivity coating on the interior surface of your glass, you are effectively ‘trapping’ long-wave radiation within your building. The result is less heat loss to the exterior. This is pertinent in cold climates where long-wave losses through windows can represent a large percentage of overall building heat loss.
Couple these aspects with the important passive solar gains that are the backbone of the Passive House standard, and one starts to appreciate how important the design of windows becomes in the context of the overall building. At Passive Science, we have in house expertise in window technology. Two of us are members of the PHIUS Window Performance Technical Sub-committee and perform window performance simulations for PHIUS. If you would like more information on this, please do hesitate to reach out to us.