[Greenbuilding] Window R-Value vs. Energy Efficiency, (formerly Glass overhead doors for passive solarapplication?)

Tue Dec 5 11:48:42 CST 2006

A short posting to help clarify Rob Tom's comments.......

Rob,
sorry to sort of contradict you,
while we did build a single publicity stunt window years ago, that
included an R-20 claim, we (and many others) build windows each and every
day that are more energy efficient than a  R-20 wall.

A long posting to help clarify Rob Tom's comments.......

Window R-Value vs. Energy Efficiency

Warning:
What follows is from the Canadian perspective; or anyone else who's
heating bill is obviously larger than their cooling bill

Even the best insulated wall (say R-100) would over a heating season still
lose some very small amount of energy. A window on the other hand can be a
net gainer, especially if it faces south; even if it is R-3. So a R-100
house with no windows will cost more to heat than a R-100 house with a south
facing R-3 window.

Basic Passive Solar right?

Here's the tricky part, higher R-value windows don't necessarily result in
lower heating bills.
Hubba?
Low e coatings designed for the sunny south (where cooling bills are
obviously larger than heating bills) are designed to block free heat from
the sun (low solar heat gain). It turns out they have a slightly higher
R-value than low e coatings designed for the north.
Low e coatings designed for the north admit about 50% more solar gain
(higher solar heat gain) than low e coatings designed for the south. So
despite their lower R-value they produce lower energy bills -- thus making
them more energy efficient.

Canada being universally a heating climate, means that our techies could
develop a window rating that includes energy losses (from air leakage,
conduction through both frame and glass) as negatives and energy gains (from
the sun) as positives. It's called an Energy Rating or ER for short. It's an
average number. It's based on an average Canadian location and an an average
orientation.  So a window with a positive ER is on average a net gainer, and
a window with a negative ER.......

So.....
 it's frustrating for us to see people talking about R-value as if it's the
sole arbitrator of energy efficiency. A little more than 10 years ago
Canadian window companies were falling all over themselves, producing
prototypes with ever higher R-values. We were frustrated for two reasons:
- the 'R-wars' were based on center of glass R-values, not whole window
R-values
- the 'R-wars' fed the assumption that higher R-value meant greater
energy efficiency

To try and put an end to it, we built a casement window with what must have
been 6 panes of glass and filled the 1/8" spaces between the panes w/ xenon
. We were told  1/8" of xenon insulates as well as 1/4" of krypton,(which
insulates as well as 1/2" of argon). So we claimed an R-20 centre of glass.

So here's a reality check on our claim 10+ years later, now that we have
software (FRAMEplus) that includes properties for Xenon
FRAMEplus says:
  Visible transmissivity 29% (a 'regular' window would be 70%)
  Center of glass:  R-16.8 (so our claim was 16% high -- oops, but still
higher than the others)
  Whole window (2'x4' casment): R- 7.7 (no wonder no one liked to quote this
number)
  ER: -6 W/m^2K

by comparison our best passive solar casement window:
  Visible transmissivity 63% (a 'regular' window would be 70%)
  Center of glass:  R-6.3
  Whole window (2'x4' casment): R-5.3
  ER: +6 W/m^2K

So looking at the above you can see why we never actually sold a 6 pane
window. It was ridiculously dark, heavy and expensive.
Besides, a regular triple glazed window was more energy efficient (better
ER). But it did, thankfully, end the 'R-wars'.

Creative types can determine an ER for a wall. There is no solar gain, so a
wall must have a negative ER. Turns out a R-20 wall has an ER of -6. This 
means a
window having an ER of -6 or greater is more energy efficient than the R-20
wall around it.

When you look at our ER numbers, and those of other companies, you'll find
almost any triple glazed window and some double glazed windows have ER's
of -6 or greater.

So Rob,
sorry to sort of contradict you,
but while we did build a single publicity stunt window years ago, that
included a R-20 claim, we (and many others) build windows each and every day
that are more energy efficient than a  R-20 wall.

Stephen Thwaites P.Eng.
Thermotech Fiberglass Fenestration
Ottawa, Ontario, Canada

Technical PS
Recently the ER calculation was amended (fudged upward) so that bottom
dwellers would not have to report negative numbers. Adding 33 to the orignal
ER will get you the new ER. I still prefer the original ER for explanatory
clarity, but will resort to the new ER when necessary for comparisons to
other windows.

----- Original Message ----- 
From: "Rob Tom" <ArchiLogic at yahoo.ca>
To: "Keith Winston" <keith at earthsunenergy.com>; "Greenbuilder list"
<greenbuilding at listserv.repp.org>
Sent: Tuesday, December 05, 2006 1:14 AM
Subject: Re: [Greenbuilding] Glass overhead doors for passive
solarapplication?

> On Mon, 04 Dec 2006 23:36:20 -0500, Keith Winston
> <keith at earthsunenergy.com> wrote:
>
>> Uh, an R-17 window? I don't think you'll find that.
>>
>> The very best glass I know of is Superglass 88 or whatever it's called,
>> which has a center-of-glass value of about R-10.
> [snip]
>>
>> If you find an R-17 window, I'll use it in every single
>> project from now on, I promise you!
>
>
> Keith;
>
> Uh oh.
>
> I think that you may have just committed yourself to buying Thermotech
> windows
>                http://www.thermotechwindows.com
> for the rest of your life, assuming that you are a man who honours his
> promises.
>
> Years ago, rumour had it that those wascawy wabbits at Thermotech, just
> for the fun of it, made a window that outperformed an R-20 stud wall.
> (Yes, with glass in it and through which one could see through.)
>
> So, I hope that you are located in the Northeast portion of the USA rather
> than some bizarre place like California (so as to keep shipping costs &
> transportation energy at a minimum).
>
> But I agree.
> Overhead sectional glass doors as a window wall ?
> Nifty maybe, but energy-efficient ?
> [acquiring a Cockney accent a la Monty Python gang]  Nought blowdy loikly
> !
>
> ===* ===
> Rob Tom
> Kanata, Ontario, Canada
> <archilogic at chaffyahoo dot ca>
> winnow the chaff from my edress in your reply
>
>
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