Volume 41, Issue 9 - September 2006


The Numbers Game
What Performance Numbers Matter Most?
by Dez Farnady

You are reading this column in a trade publication, not in a scientific journal. I am not a scientist or engineer and, more than likely, neither are you. If you are, then you should probably be reading something else. I am just a glass peddler writing for glass peddlers trying to make sense out of what the scientists and engineers are doing for us. 

What prompted this column were the numbers I discovered while searching for glass documentation on the Internet. The documentation of glass facts by manufacturers in their annual publications or on their websites is a great source of information but it seldom provides glass performance comparison between similar products from different manufacturers. In order to have the opportunity to evaluate comparative performances you need to have a catalogue from everybody or your own version of the Lawrence Livermore National Laboratoryís Windows 5.2 software with the Spectral Data Program. Of course, we all have that one right on our desktop, donít we? 

Performance Matters 
It turns out that somebody does care because, finally, Cardinal did us a favor by making its technical bulletin #IG05 Ė 07/04 available on its website. This is a low-E performance comparison table that lists most of the available products, along with more performance comparison numbers than you may have ever wanted to know. The apparent date on the document is 07/04 so it may be too early to include PPG Solarban 70XL, but most other products seem to be there. You can find the Solarban numbers on the PPG website; it is not too bad if you only have to go to one more place to have nearly all of the information. 

I generally concentrate on only a part of it to get the relevant comparison information. Insulation or prevention of heat loss is pretty easy to understand. U-value measures conduction and, because glass is a conductor, its resistance numbers are not going to be great no matter what you do. So just figure the reciprocal of conductance to figure resistance. With glass, forget the R-19 or R-30 thatís in your walls. With air space you get R-2, with most low-Es you get R-3 and you need argon or triple glazing to max out at around R-4. Thatís about all you are going to get, at least practically, off of this table. I am sure someone will point out that they can get R-15 on a spaceship to Mars, but most of us arenít going there.

Let in the Light 
The other two items that concern most of us non-engineers are light transmission and heat gain. It is pretty easy to measure light transmission when you figure that 100 percent is what comes through a hole in the wall. Anything you put in the hole is going to reduce the amount of light coming in. So even I can figure out that 50 percent light transmission is only half the light. Since windows and skylights are designed to bring in the light, I figure the higher the number the better.

For the final number in the puzzle I only look at one more numberóthe shading coefficient (SC). SC is an old-reliable comparison number using 8-inch window glass as a base 1.0 to measure the heat coming in. Conducted or transmitted, I donít care how the heat comes in as long as I know that an SC number of .50 represents about half the heat gain and a .35 is even less. Unlike the light transmission or insulation here, the lower the number the better the performance. 

There are other numbers providing other comparisons, but I am not smart enough to deal with more without blowing a fuse. Staying with two good numbers is just fine for me. The best combination of high light transmission and low SC should provide the most efficient product, if that is what you are after. Adding colored glass to reduce light or glare or to match other glass or architectural features opens another can of worms and provides another set of numbers with nearly unlimited options. But even with all those options the three key numbers remain the same.

So what the numbers really tell you is that you can choose (based on price and availability from your regular suppliers), from a nice range of products that can provide outstanding performance. A temperable product with a light transmission in the 70-percent range with SC numbers in the .40s with an R-3 is a readily available commodity from your friendly neighborhood fabricator, temperer or insulator. If you donít believe me, just check some of the numbers on the ďold productsĒ and you will find that you have never had it so good.

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