Solar gain tradeoff?
While the topic of solar gain via south-facing windows recently
came up in the “Martin’s PGH manifesto” thread, I’m not going
anywhere near that flamewar with this question even if it’s
peripherally relevant. The question concerns how worthwhile
it actually is to open my reflective shades on a sunny winter
day, and/or if there’s a critical-point outdoor temperature
where increased radiative/conductive loss going *out* would
be insufficiently balanced by solar energy coming *in* and
thus suggest keeping the shades closed to try and limit the
I had already done the basic math on this before the Manifesto
thread came along, but I wouldn’t mind a sanity check. Warning:
nerdly mental-masturbation ahead …
I have about 32 square feet of south-facing sunlight-reachable glass
across five modest windows. Not a whole lot, but for calculation
purposes I wanted to reduce what I know to ballpark energy flux per
square foot in each direction and compare them as well as get an
idea of what that much glass area can actually do for me on cold
but sunny days.
The windows [Serious 720] claim to be U 0.18, which I’m going to
derate to 0.20 because of frame-area losses I already know about.
By definition, that’s 0.2 btu / hr / sf / deg-F, right? Great,
loss figure easy, and let’s leave aside delta-T for the moment.
I gather that winter sun at my latitude [Boston area] might get
as high as 500 watts per square meter. Maybe that’s optimistic,
but since the solstice doesn’t last all winter maybe that’s an
okay average for the sunniest daytime hours. That’s about 47
watts per square foot, and then comes the kicker: my windows are
0.25 SHGC, so take a measly quarter of that influx and convert to
BTU/hr and I get about 40 btu/sf insolation. Which should not be
dependent on the outdoor temperature, that’s just what the sun
delivers as pretty much constant. The five or so good hours of
sun on such days might give me as much as 6500 free BTU. Which
saves less than ten cents spent on running the heat pump.
So whether the stated loss figure is conductive, radiative, or
whatever, it seems theoretically worthwhile to pop the shades off
and let as much sun in as possible on those days, because these
results put the “break-even” temperature down at about -150F
where losses would match gains. Heh. If we ever reached that
point the polyiso would probably just explode all over the
yard, and windows would be the least of anyone’s worries.
So the question remains, am I thinking about this in anything
like the correct fashion? Even if the real-life gain increase
is barely worth the time it takes me to run around opening
the shades in the first place?
GBA Detail Library
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