Simple question...hopefully

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I ran a very quick and dirty EEM to compare the effects of using a
different glass type when compared to the baseline model, which was
created in the SD Wizard. Contrary to my expectations, the higher
performance glass resulted in higher annual energy usage! The glazing
is the only component I changed in the model. If anyone can speak to
this result, I would appreciate it.

Thank you...DD

David Delasantos's picture
Joined: 2011-09-30
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Are you talking higher gas usage, higher electrical, or both?

I could see using higher gas in the winter if the glazing has a higher
absorptivity and thus doesn't allow as much sunlight in the structure in
the winter, thus requiring a larger amount of gas to heat.

I can't think of a reason why the electricity would increase off the top
of my head. If your energy increase involves electricity, I would be
scratching my head. That is, of course, unless you are dealing with an
all-electric building.

My two cents,

Gary.Schrader at's picture
Joined: 2011-09-30
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In a recent project, our architect decided to "upgrade" to nicer (more
expensive) glass. These fenestrations had marginally better thermal
properties, so I had the same surprise to find the annual energy figures
going UP...

After some investigation, it turned into an important design lesson
which may or may not apply to your case: This project had a
significant amount of daylighting, and the "improved" glass had a
significantly lower Tvis figure. Less light in means more energy spent
on the lighting system!

In the role of lighting system designer, I used to ignore things like
glass transmissivity, as I was taught to design around the worst case
(i.e. nighttime where window contributions have little/no effect)... now
I pay very close attention when daylight harvesting is a part of the

Hope this helps either your current situation, or otherwise in the


Nick-Caton's picture
Joined: 2011-09-30
Reputation: 605

One final warning, however...

It may be dangerous to conclude that the windows make a difference with
the increase in energy usage in the winter time. Most buildings have
interior shades that people use when the sun is shining in their window.
I can't ever remember a time when I changed eQuest's default interior
shading, and in previous modeling, I always assumed that there was no
interior shading. (For the purposes of doing loads this made sense as it
would get me the worst-case load on the zone in question)

If people are drawing their shades, then the heating effect by sunlight
would be reduced. This is the case for either the "better" glazing or
the "standard" glazing. So while your model will still show greater
energy usage due to the fact that sunlight is reduced, it probably isn't
going to be to the degree that it indicates, unless you did account for
interior shading in your model.

Good luck,

Gary Schrader

Gary.Schrader at's picture
Joined: 2011-09-30
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Increased glazing performance may result in less thermal transfer through the envelope. This may result in increased or deceased energy savings. Depending on climate, varying heat transfer through fenestration may result in more energy use in the winter and less in the summer or vice versa. There is usually a happen middle point which will give the best returns. The same applies to increasing and decreasing insulation.

Kevin Kyte2's picture
Joined: 2011-09-30
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Thank you all for responding. I understand the issue more clearly now.
Your comments are very much appreciated.

David Delasantos's picture
Joined: 2011-09-30
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There was a paper written by Victor Olgyay & Myself (published at the
ASES 2006 conference in Denver I think) where we looked at optimizing
glazing with two case studies in different climatic conditions - Atlanta
& Hawaii. We separated out the three properties - U-Value, SHGC & Tviz.
The Hawaii building actually performed better with the single glazed
units, whereas Atlanta with a significant winter season was more
intuitive - performing better with the lower U-Values. The other factor
is balance point temperature - if you have a really high internal heat
load, sometimes the higher U-Value might be beneficial even in cooler

SHGC & T-Viz need to be examined in concert with each other. There are
high performance systems that get you the best of both worlds - Solarban
70 on Starphire glass is one of them.

Vikram Sami, LEED AP

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Is there a way to determine a building's balance point directly from
eQuest, or is it easier to manually do this calculation? Is it possible
to use heating and cooling design day schedules iteratively until the
LS-C report shows a net-zero total peak load? I guess my question
depends in part on whether eQuest bases the peak loads on the design day
schedules or the weather file, which I don't know.


Luka Matutinovic, B.A.Sc., LEED(r) AP

Matutinovic, Luka's picture
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