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Solar gain tradeoff?

user-1135248 | Posted in Energy Efficiency and Durability on

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
radiative losses.

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?


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  1. user-3549882 | | #1


    You've asked:
    1) " ... how worthwhile is it to open my reflective shades on a sunny winter day?"
    2) " ... am I thinking about this in anything like the correct fashion?"

    It's been many years since I've done calculations like you're attempting so please allow for this.

    Ans. to 2: I think your method is OK. Sketchy maybe but OK.

    Ans. to 1: You've not said much about these reflective shades of yours. Are they traditional curtains and blinds? Are they reflective like white cloth or like shinny aluminum? Are they the industrial grade with sealed edges and dead-air-pillows? I presume the shades are internal.
    For traditional curtains, I'll say it matters but not worth worrying about.

    Here's how I might think about your questions from a slightly different perspective:

    a) If your house is like mine, you'll notice that on bright winter days the furnace (or heating device) will have less work to do compared to a cloudy day at the same ambient temperature. This is evidence enough, in the context of all the real effects occurring, that the sunshine is beneficial and should confirm your gut feel about the matter.
    b) If you want to quantify HOW beneficial, you can crank the numbers as you've outlined. You can also look for two days like mentioned in (a) and document the furnace runtime or fuel usage on the two days. The difference is the (net) benefit the sun is providing. You can even try a variation with the shades closed.
    c) The determining factor in heating the home by the sun is mostly the infrared portion of the spectrum. When sunlight penetrates your windows, most of the infrared is absorbed by internal surfaces while most of the visible light is reflected about the room or out. That means most of the heating effect occurs once the sunlight has passed the window glass. That's true in summer too. Curtains will reflect only a small amount of IR (I'd estimate a few percent). So, if you forget and leave the shades closed, you'll still get most of the heating effect. In winter this is good. In summer, bad. To get the most of your sunlight in winter, open the curtains during sunny days and close them overnight. On cloudy days, open them if you need more visible light.
    d) As regards a 'break even' temperature: at any Earthly ambient temperature sunlight will heat the home. If a scientist were doing cryogenic experiments s/he would not allow sunlight to impinge on the subject material as it would needlessly heat whatever is absorbing the infrared fraction. It's conceivable that conductive losses at extreme low temperatures could exceed the radiant gains through a given window. I wouldn't worry about this scenario but if you like calculations, set the conductive/convective formula equal to the radiant gain you measured in (b) and solve for T. We can anticipate we'd long since have converted the window to a well insulated wall at the very least.
    e) Let's take it one step further: what might you do in summer? My answer is to try to intercept the infrared BEFORE it penetrates the window. That way, the house heating would be reduced, substantially. That's exactly what solar grates do. Sunlight impinges the grate. The visible fraction is mostly reflected and enters the building while the IR fraction is absorbed before it ever enters the building. Solar screens produce similar electrical savings for a/c but do it by blocking/absorbing 80% of the light. A device called Infiltrate does it by reflecting most light away from the window. The savings opportunities in summer using such window devices are BIG compared to what can be done in winter with traditional curtains.


  2. GBA Editor
    Martin Holladay | | #2

    Q. "On a cold winter day, should I open the reflective shades installed on my south-facing windows when the sun is shining?"

    A. Yes.

    Whenever the sun is hitting your glazing, the glazing is admitting more heat than it is losing.

  3. user-1135248 | | #3

    The shades are Reflectix radiant-barrier material, basically thin
    silvered bubble-wrap, adapted as shades as seen here near the
    bottom of the page. They have minor convective-loop issues but
    that small flow is mostly contained toward the exterior of the
    wall within the "outie" window recess so with them in place, radiant
    heating/cooling effects from outside have far less influence on
    the interior than with them removed.

    I'm thinking of tacking some XPS "rails" along the inside surface of
    these frames, to both narrow the edge gap to the shades and cut down
    some of the windows' conductive loss where it mostly occurs... the
    IGUs are fine; the frames get cold!


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