GBlist: Re: greenbuilding-digest V2 #1330

["Nehemiah I. Stone" <nistone@cwo.com>]

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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