Re: [GBlist] Low E glass

[Renewable News Network <rnn@rnn.com>]

Hi Maureen/Richard,

Corwyn's formulations are causing me some consternation. I did a
google search and could not come up with a clear and straightforward
definition of "solar heat gain coefficient" that could explain the low
numbers (.30-.60).  I thought a double glazed lite allowed about 85
percent transmission of sunlight normal to the surface.  If so, then the
low SHGC must represent an averaging of the times of reduced gain due to
increased angles of incidence (tilt and orientation.)  And if that's the
case, then one wants to know the ranges for those variables.

Most of the research on windows appears to deal with the space cooling
issues in the more tropical climates.  Tinted glass and spectral coatings
may offer benefits, but not so much in the North, I would guess.  If one
needs to block the light, curtains work, and such extras only add costs
for features that only reduce your Winter time performance, but that's
neither here nor there.

The analysis, also, appears confounded by the U-values of the window,
which involves the peformance of the whole window, including what (?)
other factors (?) -- when the point of the original question was, I
think, about the impact of low-e coatings on solar gain.

I get easily confused.  These matters can quickly evolve from a mix of
apples and oranges to bananas and grapes in a veritable fruit salad of
cause and effect confusion.  And then there's all the design issues --
reflection, wind loading, space use, movable insulation, etc.

I don't know that one can attribute a greater comfort to low-e alone,
when a replacement window, with improved infiltration, and what all, can
make for an improved overall situation.

As a rule, its wise to use standard products.  And, in the North, as I
understand things, low-e insulated lites are pretty much the thing to
do -- warmer windows, less heat loss, little reduced solar gain, and no
appreciable visual impact.  If you're installing fixed glass, you can
more easily specify low iron glass, also, to maximize the solar gain.

Have a merry happy, everybody!

Yours truly,
Ross@rnn.com

On Mon, 31 Dec 2001, Richard Averett wrote:

> We are building a house in Upstate New York,(42 North latitude).
Does anyone have an opinion on whether Low-E glass on the South side
will interfer with solar gain in the winter?  Thanks.
>
> Happy New Year!
> Maureen
>

Date: Tue, 1 Jan 2002 11:45:34 -0500
From: Corwyn <corwyn@m...>
To: Richard Averett <averettr@n...>
Cc: greenbuilding@c...
Subject: Re: [GBlist] Low E glass

On Monday, December 31, 2001, at 10:58 AM, Richard Averett wrote:

I am struggling with the same question myself, and though I don't have
many answers, I do have some thoughts (And in addition I am sure my
thoughts (if wrong) will be quickly corrected (no way easier to get
good information than to post bad)).

Anyway, the simple answer is yes, it will interfere.  The question then
becomes, is it still worth it?  The two attributes of windows that are
relevant are U-value and SHGC (solar heat gain coefficient).  You can
find these out for some windows by looking in <http://www.nfrc.org/>

One tells you how much heat you lose, and the other how much you gain.

I think the simplified equations are as follows

Heat Loss (in BTU/Day) = U-value (whole unit not center of glass) *
square feet of glass * temperature difference (in degrees F) * 24
hours/day.

Heat Gain (in BTU/Day) = SHGC * square feet of glass * Available Solar
Energy * % sunshine.

temperature difference = inside design temperature (probably 68-72) -
the outside temperature (you might want to use average daily
temperature for the coldest month)

available solar energy  can be found using the PEC solar calculator for
your latitude, orientation (degrees off true south)
<http://arch.ced.berkeley.edu/vitalsigns/res/res_online.html>

To take my situation on an average day in February for two (random)
windows

1) Double Pane, No low-e, Air filled:  U-Value = .48, SHGC = 0.60
2) Double Pane Low-e #2, Argon filled: U-Value = ..31, SHGC = 0.34

My house will face 7 degrees east of True South (10 W of Magnetic South).
At my latitude a (almost) south vertical window receives 1842 BTU/ft^2
on a sunny day.

The average temperature in the nearest measured city (Portland ME) in
February is 24.5 degrees F, the average % sunshine is 59%.

Window 1 is therefore:
Heat Loss = .48 * 10 * (68 - 24.5) * 24 = 5011.2
Heat Gain = .60 * 10 * 1842 * 0.59 = 6520.68
Difference = 1509.48

Window 2 is
Heat Loss = .31 * 10 * (68 - 24.5) * 24 = 3236.4
Heat Gain = .34 * 10 * 842 * 0.59 = 3695.052
Difference = 458.652

So, in this example. the (probably cheaper) non low-e, air filled
windows are better (all else being equal) than the argon filled low-e
windows (on the south side).  Note that this assumes that you are able
to benefit from all the solar gain (if you have to open the window to
cool the house off...)  Thermal mass will help with that.

Hopefully this will either help, or prod some more knowledgeable into
helping,

Corwyn



<RNN>


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