Now that it is cold in Boston where I live, I was thinking about all
the heat loss through my sliding glass doors.  I have seen ads for
low-e window glass or films that "keep the house cool in the summer and
warm in the winter". How can you have it both ways?  Specifically, if
the glass is blocks some heat transfer in the winter, it could it stop
some sunlight from getting in and warming the house, at the same time
it is stopping some heat from getting out.  How can I calculate how
much energy the film will save me?  i.e. will I come out ahead in the
wintertime, balancing blocking the sun versus decreased the heat
leaking out of the house?  Will this coating hurt me in the summer?
Hopefully, if relevant, your answer will refer to certain
specifications of the glass/films that would be provided by the
manufacturer.  I think it has something to do with "spectrally
sensitive" films. Thanks.

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Clarification of Question by depth-ga on 06 Dec 2005 12:18 PST

Thanks for the comment.  Interesting about the gold astronaut helments.

Also I found some info below, but haven't gone through it enough to
figure out what the implications are for a sliding glass door in a
northern climate where heating is the major concern, and which kind of
film characteristics should I look for in this application.  I am
basically hoping someone will do that to earn the answer.

Thanks,
Jason

Some links to get us started:
http://www.consumerschoicewindowfilms.com/Definitions.pdf
http://www.consumerschoicewindowfilms.com/SpecsSinglePane.pdf

From http://www.bobvila.com/ArticleLibrary/Task/Weatherizing/SolarHeatGainControl.html

Calculating Energy Savings
Energy savings from solar control glazing are difficult to accurately
predict. Predictions of savings are based on many variables such as:
size and orientation of the windows, solar heat gain coefficient
(SHGC), and the cooling load factor (CLF; the ratio of actual total
cooling compared with total steady-state cooling during the same
period at constant ambient conditions.) To make this somewhat simpler,
some references combine these variables into one figure: the Heat
Transfer Multiplier (HTM). The HTM will vary with location, seasonal
changes, time of day, shading, orientation, temperature, and building
color.

There are also computer programs for sizing of heating/cooling
systems. These can also be used to estimate solar heat gain from
different types of windows (given the SHGC and climate). Typically,
you run the same program for each choice in window type and find the
dollar value of the difference in energy saved between the choices.
You can then divide the purchase price by the estimated savings to
determine simple payback.

Some solar control films are very costly and may have very long
payback periods. In such cases it may make better sense to consider
other shading devices such as awnings, overhangs, solar screens,
shutters, roller shades, blinds, and draperies.

It would take me too long to figure out exact specifications for this
situation, but here's a random site that might give you some numbers
to throw at sales-people:

http://www.energystar.gov/index.cfm?c=windows_doors.pr_anat_window

In the mean time, here's some explanation of the physics.
The heat control provided by windows has a few different aspects, as
you probably know.

One is the thermal resistance to conducted heat, which is mostly
related to how many gas spaces fill the window stack, e.g.
double-glazed is a better insulator.
A better insulator reduces conducted heat-flow, i.e. if it's cold
outside and warm inside, OR hot outside and cool outside, then the
inside temperature is more easily kept where it is.  Hence heating is
more efficient in winter, and so is aircon in summer (assuming you
keep radiant heating under control).

Radiant heating is the fun part, as I'm sure you've realized.  Some
light from the sun passes through the window and is absorbed by
objects in the house, which heat up.  Light emitted from an object is
spread out into a broad-spectrum, with a maximum intensity at a
particular wavelength that depends on temperature of the object.  The
sun is very hot and has large output in the visible and near-infrared
(say 1 micron), whereas objects in the house never get as hot as the
sun so they re-emit most at longer wavelengths (around 10 microns). 
The trick is that glass can be engineered to pass only particular
wavelengths, and in fact ordinary window glass tends to block a lot of
infrared due to the presence of water, trapping the heat (which is the
reason that glass-houses get so hot).

The low-e glasses work by coating with a thin metallic film, which
acts as a reflector.  Different metals have different spectral
behaviour, e.g. gold reflects infrared very nicely but passes visible,
hence the coating on astronaut visors, but is way too expensive for
windows.  Overall the energy allowed in is reduced but then so is the
energy getting out, as you've pointed out.  I can't think of a fast
way to answer your question about energy balance, but click on the
link for some guidelines.

Incidentally, the best solution is active control of incoming light,
as I see you have alluded to in another question.  If we were all
megarich, photochromic glass would be great, since it darkens in
strong sunlight and so passes a similar amount of energy regardless of
outside conditions.

sorry for rambling, just my two cents.