Thanks for the reminder and reinforcement of one of my own "pet peeves". We
must remember that WINDOWS HAVE EVOLVED A LOT! And those single, double and
triple paned windows with which the old "mass-to-glass" ratios were developed
have been technologically left in the proverbial dust.
While the "rules of thumb" are still pretty good around window U-values in
heating dominated climates (depending on whose thumb you use), the
spectrally-selective glazing that exist today are dramatically different that
their low-e predecessors of just a few years ago!
The ol' "visual inspection" is no longer sufficient to assess window
performance. Two windows that LOOK the same (say, vinyl, with two panes of
glass) can perform dramatically differently. One may be in fact vinyl-framed
with dual clear glass and have a whole product U-value of around 0.53 and
solar heat gain coefficient of around 0.60 (ASHRAE) while the other may be
vinyl or vinyl-covered wood or a vinyl composite with spectrally selective
glazing have a U-value of around 0.30 and an SHGC of 0.31.
What's the message regarding "visual inspection"? DON'T TRUST YOUR EYES!!!
Not only do we have to make sure that we actually USE the modeling tools
available, but we also have to ensure that we put the RIGHT DATA IN. (GIGO)
To that end I will repeat my mantra about window performance.... (1) demand
NFRC certified energy performance ratings (WHOLE PRODUCT U-values and SHGCs)
(2) make sure the glazing spectral data used in your "favorite" program has
been updated (call LBL or get the new spectral data files in WINDOW4) (3)
don't calculate heating and cooling loads with "center of glass" performance
data.
Lastly, those that actually USE these tools have a responsibility to TEACH
THE TEACHERS. Make sure that our architecture and building science graduates
know about this stuff when they graduate.
Regarding the next solar design book.... it IS about time for that, isn't
it....
Though not on "solar design" a great new book documenting some of the latest
in window technologies just hit the street. Check out "Residential Windows:
A Guide to New Technologies and Energy Performance" by John Carmody, Steve
Selkowitz and Lisa Heschong. Best book on the subject in a decade or so.
Available for $22 from Norton.
Best Regards,
Chris Mathis
MCSquared
In a message dated 97-03-18 01:10:17 EST, you write:
<<
A recent exchange on optimum glazing, mass etc. for a sunspace, reminds me
of
one of pet peeves. That is the "seat of the pants" solar designer. They've
usually been around for 20 years and are often very dismissive of anyone who
uses a computer to design a house. They're very convincing in explaining
that by now they've already made every mistake so the next house they build
is bound to be optimum.
I don't dispute they may be able to do a decent job with some classic
designs
(attached sunrooms etc.) I doubt they've come very close on optimizing the
cost/benefit.
Given the fact that we're using lots more insulation in the walls and that
it's getting hard to even buy a plain old double pane window, who knows what
that does to all the old axioms of glass to mass ratios, etc. And with
easy
to use software (like Solar 10 so cheap or like Solar 5.4 almost free) why
NOT use it?
And who's writing the next solar design textbook?
Dave Brook >>
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