You have to know certain things to be in our industry: How to read a tape measure; the difference between silicone and putty, how to cut metal, wood and glass; how to talk to a customer and run a credit card charge; and so much more.
We have to add to this list. If you want to survive in the future, you must understand the energy values and measurements of glass. Well, actually, you don't have to, but then you must understand the values of your 401(k) and reverse mortgages to carry you through your retirement. Yes, it is this black and white...learn, understand and use glass as an energy savings product rather than an a product which is clear and keeps rain out, or get the rocking chair ready.
Let's start with some basic terms and give some examples of how the term is used.
SHADING COEFFICIENT. The ratio of the solar heat gain through a specific glass product to the solar heat gain through a lite of 1/8" (3mm) clear glass. 1/8" glass is given a value of 1.0. The reason to know this is that glass lets in heat through solar radiation, convection and conduction.(well explain those in a while) If a glass type lets in less heat than the standard 1/8", it has a better shading coefficient. A lower number, such as .5, is better. A lower shading coefficient reduces air conditioning load, which reduces energy costs and saves on construction expenses throughout a project or home.
Sounds easy, make the glass dark and thick, (graylite 14, reflective) and you get a low shading coefficient. But make it too dark and you use more electricity to light the building. Too many electric lights means more heat and then more a/c. You need to strike a balance. A lot of light and a low shading coefficient are the goal.
So our next term is: VISIBLE LIGHT TRANSMITTANCE, which is simply the percentage of light that is transmitted through glass. The more light the better. People and plants respond to light. Everyone wants an office or a room with light. Put up a brick or spandrel wall and you don't have to worry about shading coefficient or visible light transmittance. You do have to worry that no one will want to live or work there. A higher VLT number is better. It is expressed as a percentage. A .6 or .7 number is really great.
If you are comparing two types of glass you can use the above two numbers to see which is more energy efficient.
Our last definition for today is the concept of how heat flows through glass. There are three ways: 1) RADIATION--the direct passage of solar light or heat through the glass. 2) CONDUCTION--the heat that enters the glass, or framing, and is then transferred to the room. U-value is the main measuring of this process, and we'll talk about that next week. 3) CONVECTION--the heat that enters your glass or IG unit, heats the airspace or the glass itself, and then radiates into the room. The newest cooking ovens have convection heating because they are the most efficient in cooking your turkey. So a window or store front unit with good convection that allows a lot of heat in and then sends that heat to the room is a no-no. Filling a unit with argon gas helps reduce the convection.
The new sputter coated low-e products have extremely good numbers in both of these categories. Talk with your fabricators, go on line to PPG or Guardian or Pilkington and you will learn more. Just don't think you are going to survive in our industry if you sell clear units.
The trend today is as much light as possible with the least amount of solar heat gain.
The next step is NOT to underestimate Joe and Molly Consumer. They know these numbers. They read about them in the Sunday newspaper, they watch shows on the Discovery Channel, their kids learn this in school, and they study on the Internet before they make a purchase. Joe wants to spend less on oil and Molly wants sunshine for her plants. You have to please both of them to make the sale.
If you can't speak the lingo, they will not have the confidence in you to do the install in their home when they replace all their windows.
Subscribe to:
Post Comments (Atom)
3 comments:
Paul, you forgot the cold climates. Now you need passive solar heat gain to allow heat in and offset this off with shading devices to keep excessive heat out. New softcoats are being produced with higher solar heat gain and good u-value performance. Retirement is seeming easier than keeping up with what will be required where!
Warren MacLean
PFG Glass
Great post-Thanks for your contribution to the industry!
Hello Paul, to echo Sara's comment, thank you for your continued efforts to support the industry!
With your latest post, you've hit on a topic/issue that I've questioned for a number of years. My friend Don McCann (truly an industry giant in educating/assisting architects in the use of glass) with Viracon can attest to the fact that I have long questioned the optimum level of visible light transmittance in commercial construction applications.
As the industry has discussed many times at the Glass Association of North America (GANA) technical committee meetings, you can't (or at least shouldn't) apply the same standards to residential glass and commercial applications. If we say the higher the visible light transmittance the better, it may be true for residential applications (in particular in cold weather environments needing solar heat gain) but is it really true for the commercial office environment? At what point does visible light transmittance yield the need for interior shading devices that dramatically limit daylighting further into the building in order to provide comfort for those working at the parameter of the building?
I recently "tweeted" (Twitter.com: glassandglazing -- sorry for the self promotion), about an office environment where the solar-control coated glass provided an 8 (0.08) percent visible light transmittance on a west elevation exposure yet the occupants had the shading devices closed due to glare and solar heat gain.
The issue is that we can not say higher light transmittance is ALWAYS better. The use of today's high performance architectural glass products need to truly be designed by application and by the elevation of the building!
Greg Carney
C. G. Carney & Associates, Inc.
Post a Comment