Windows

This week we’re looking at windows and two performance metrics in particular: 1) U value, or USI in metric (inverse of R value). Measures the insulation or thermal resistance of a window; driven by the number of panes + gas in between. 2) SHGC, or solar heat gain coefficient. Measures how well the window blocks heat from the sun; driven by low-e coatings on window’s surface that reflect heat.

When I was in grad school, there was a lot of talk of passive solar design for northern climates. Maximize solar heat gain, particularly in winter, with large windows on south facing aspects, and control the heat with overhangs in the summer.

Passive solar design however can easily lead to overheating. And in homes with highly insulated walls + small mechanical systems, it can be very hard to dump that excess heat! (many passive house examples of this)

We have a lot of south facing windows in our design and a reasonable but not excessive overhang. We plan to spec the same USI value for windows throughout (<1.0), but are trying to figure out how much we can play with SHGC and still stay on track for net zero. Can the systems we have set up cope with a 40 degree day, e.g. BC heat dome last summer? Will lower SHGC <0.25 mean more winter heating and nudge our overall energy use higher?

#netzero #netzerohome #renovation #greenbuilding #sustainability #energyefficiency #windows