GBlist: Re: Rating Windows

[Stephen Thwaites <stephen@thermotechwindows.com>]

First a point (or more) of clarification. 

No rating system is perfect.  The Energy Rating system was designed to
help homeowners, Canadian homeowners, cut through the conflicting claims
of window makers. It considers Passive Solar Gain a good thing.  As such
its useful to anyone who pays more for heating than for cooling. It's
not applicable to commercial bdlgs, where, even in the great white
north, cooling bills are bigger than heating bills. Its of no use to the
building envelope intelligensia who have access to simulation tools that
give them exact answers to exact questions.  For the rest of us
northerners its a great tool. If that makes me provincial so be it.  


The Canadian Energy Rating (ER) system is based on the average sized
window facing an average direction in an average Canadian temperature
and solar radiation regime.  It is useful, but only to a point. For
comparisons of specific orientations in specific locations change the
inputs from the Canadian averages to the site specific and the standard
calls the result not an ER, but an ERS (Energy Rating Specific)  You can
see an example of how it can be used to select glazing options for
different orientations by going to;
	http://www.thermotechwindows.com/energy.htm


Technical chats like this are fun, but i make my living selling
windows.  If i show one of our typical university educated northern
dwelling customers the following two triple glazed casement windows and
they ask me which one will save me the most money?  How do i answer?

window A                                
visible transmittance  0.57
solar transmittance    0.24
window SHGC            0.23
Window U               0.17 (BTU/ft^2/F)

window B
visible transmittance  0.63
solar transmittance    0.43
window SHGC            0.42
window U               0.19 (BTU/ft^2/F)

Well lets see -  does the fact that A insulates 12% better than B
compensate for the fact that B allows 83% more solar gain?  

What i like about the NRFC approach is that it puts all the key
information in a consistent format.  What i don't like about it and the
Energy Star Program is that they don't take key information to its
logical conclusion - an single number Energuide label (how successful
would an refrigerator Energuide label be if it only listed compressor
efficiency and insulation U value?)


BTW i did look at www.efficientwindows.org  It's a great site. Much much
improved from my last look.
Looking at its selecting windows section I took a look at how they
thought a moderate solar gain triple glazed window compared to a low
solar gain triple.  If you chose between these based on ER (picked the
moderate gain window), efficientwindows.org said you'd make the right
choice such northern latitudes as California Mountains, Salt Lake City
and Boston.  To borrow an American ?Floridian? phrase, cities that were
"too close to call" included Albuquerque, Omaha and Dayton. So i think
saying the ER approach is 'useless south of North Dakota' is a bit of an
exaggeration

Stephen Thwaites
Thermotech Windows
Ottawa Ontario




"Nehemiah I. Stone" wrote:
> 
> Stephen,
> 
> It may not be ver "states"manlike of me to point your seemingly "provincial"
> point of view on rating organizations, but, hey, I'm from 'Murrica, as Rob
> Tom calls it.
> I'd like to give the list a slightly different take on the difference betwen
> the Canadian way of rating windows and the National Fenestration Rating
> Council's way.
> You said, "... the Canada's Energy Rating system. This approach  reconciles
> all of the losses (glass, frame, air leakage) with the  passive solar gains.
> A positive rating means a window gains more than  it loses."
> Translation: the Canadian system balls up all the individual performance
> characteristics into one relatively meaningless number.  I say relatively
> meaningless, for two reasins:
> (1) because it assumes away any energy costs for too much heat gain when you
> don't want it.  Now, this isn't much a problem in the frozen north where you
> seemingly always want heat gains, but it make the rating useless south of
> North Dakota.
> (2) because it assumes that all windows face the same direction and have the
> same amount of chading applied to them.  A window that looks great in the
> Canadian rating system facing west with or south with no overhang above it
> is supposed to be just as great facing north or under a three foot overhang?
> Sorry, it doesn't pass the laugh test.
> 
> So, how does 'Murrica's NFRC rating system do it?  Well, if you want to know
> the window's ability to keep the heat in, it gives you a U-factor
> (Btus/(sf*deltaT*hour*)).  If you want to know how good it is at keeping
> out, OR LETTING IN the sun's heat, NFRC gives you a Solar Heat Gain
> Coefficient.  The higher the SHGC, the more passive gain you can expect.
> Let's say you want to know how well the window lets the visible light in; it
> gives you a Visible Transmittance rating.  (Whereas the Canadian system
> basically says, "Oh don't worry your pretty little head about all that
> difficult technical stuff.  This here's a good window, and that one over
> there is not so good."  Just don't ask what they are good for, OR WHY!
> Nehemiah Stone
> Stone Ranch
> Penryn, CA
> 121 09' 17" west; 38 51' 39" north
> hm <nistone@cwo.com>
> ----- Original Message -----
> From: Stephen Thwaites
> > >  I have to admit that sometimes we are not good joiners.  I haven't
> > >  looked at the Energy
> > >  Star or Efficient Windows Collaborative Sites recently, but will when i
> > >  get back to my office next week.
> > >
> > >  Our inital relectance to join these groups stemmed from the fact that
> > >  most
> > >  industry groups are advocates for status quo energy
> > >  performance. We'd like to think that we are better that.
> > >
> > >  Most US window makers are focussed on air conditioning loads, as that
> is
> > >  the main concern in the majority of their market, so they don't offer
> > >  windows optimized for passive solar applications.  Industry pressure
> > >  seems to have stalled a more realistic rating system for passive solar
> > >  applications like the Canada's Energy Rating system. This approach
> > >  reconciles all of the losses (glass, frame, air leakage) with the
> > >  passive solar gains.  A positive rating means a window gains more than
> > >  it loses. US rating systems focus only on the losses.
> > >
> > >  Now if there was a Super Window Council....
> > >
> > >  Stephen Thwaites
> > >  Thermotech Windows
> > >  Ottawa Ontario
> 
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