Customizing NRCAN’s Energy Rating formula for south-facing windows with deciduous trees blocking summer sun
The formula is:
Since most expense in Canada is for heating not cooling, the formula already gives more weight to the SHGC, but still it assumes many/most users have nothing blocking summer sun. This one-size-fits-all Energy Rating is in-a-way simple, with the same window getting the same rating wherever you are located and whatever direction your window faces and whatever you have outside your window, but there ought to be an easy way to customize it.
I downloaded NRCAN’s spreadsheet of all sliding doors. I added a column for a customized rating.
What weighting should I use for SHGC? In other words, what number should I use in the formula to multiply SHGC? And/or how can I figure out what weighting/number to use? My glass gets no direct summer sun. I’m at 44 degrees latitude. (-80 longitude)
As Michael Maines and others pointed out in response to my question about why 0.63 is the highest available SHGC sliding door, high SHGC high u-value will result in big temperature swings between day and night / sun and cloud / room to room. And re-reading Martin Holladay’s “Windows that Perform Better than Walls”, I noted “Anyone who has ever lived in one of the experimental passive solar houses built during the 1970s knows that houses with too much south-facing glazing overheat on sunny afternoons in February and March, forcing the residents to open the windows to stay comfortable.” Still, I expect I should weigh SHGC higher than the standard weighting.
———
Ignorable? Thermal mass: In winter the sun will land half on wooden floor, half on drywall (semi-dark-blue). Insignificant? [edit: I’ll guess the NRCAN formula assumes thermal mass like mine, but if I had 4″ concrete floors and _” concrete walls I expect factoring in thermal mass would be important.]
L75 is the air leakage rate at a pressure difference of 75 Pa. I’m guessing it is the sum of exfiltration and infiltration, not the average. Anyone know? In a windy location it might make sense to increase the weighting for air leakage – not important in my case.
40 is an arbitrary number added so windows don’t look so bad (even good windows had negative numbers without it).
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
"Ignorable? Thermal mass: In winter the sun will land half on wooden floor, half on drywall (semi-dark-blue). Insignificant?"
One of the claims made by proponents of "thermal mass" is that it matters what material the sunlight falls on, although I've never seen any science to back that up. Standard models for insolation assume that all of the solar energy that makes it through the window stays in the room, regardless of what the room is made of. If that's not the case, the question I would have is where does it go?
If I had a 4" concrete floor and a _" concrete wall , it would take far more insolation to heat the air - and the floor. Conversely, my feet and the air would be warm longer after sun-down. I think it's all about timing. Yes?