Volume 9, Issue 8 - September 2008


An Energy Modeling Primer
How Do Windows Fit In?

As debate continues to rage over proposed changes to ENERGY STAR®, I recall a very telling meeting with a general manager (GM) of a glass shop on the subject of U-factors and solar heat gain coefficients. I arranged the visit because I knew there was a disconnect in the industry. Why was it so important to have U-factors available in the marketplace? There had to be a compelling reason beyond just marketing. When I arrived, the GM was quite puzzled when I told him I really wanted to peruse his paperwork. I proceeded to pull out my computer, asking him to provide information I could use to fill out the window portion of the modeling program for performance energy code compliance or ENERGY STAR Homes certification. After a few moments, he observed, “How do you enter my product into this calculation?” I smiled—I had found my disconnect.

The answer was he couldn’t. It turns out this particular facility spanned the gamut of fenestration products, producing assembled fenestration as well as glass and lineal components. The only inputs in the program were for whole window products. There was no place to enter a fenestration component Ufactor or R-value.

On the surface, this limitation seems discriminatory. Other building envelope materials were entered as R-values. Why not windows?

Houses and buildings are made up of assemblies (walls, ceilings and floors). Each assembly is made up of multiple materials in proportion to each other. Each material handles energy differently. Some reduce conduction very well, but aren’t very good at preventing convection. Others conduct a lot of energy, but stop radiation. You can characterize how a particular combination of materials will performance if you know how much material you have, the ratio of that material to the whole and how that material conducts, convects and/or radiates. From there, you can figure out how much external energy will be needed to maintain a desired temperature. The relationship of the building envelope (the assemblies) to the external energy (mechanical system) allows any particular modeling program to deduce projected energy usage and consumer cost.

There are, however, arguably hundreds of inputs for these calculations. If the user had to input every single one, such modeling would take an excessive amount of time. Programmers have realized that since the properties of many materials are always consistent, only variable components inputs are needed. The program then adds together consistent and variable R-values of, say, wall materials to determine the overall transmission of the wall system, the U-factor.

So if it’s possible to calculate a wall, why not a window? Well, sometimes the program does do this. If a whole window U-factor isn’t used, there are often default options based on the window description using bare-bones calculations.

However, this is problematic for the window industry because it perpetuates the myth that windows can’t be efficient products, so builders and consumers should put their money into other efficient technologies. Over the last ten years, in fact, a whole host of efficient fenestration technologies have become mainstream. One fundamental truth still remains— windows are meant to be transparent, which means you shouldn’t be able to see the technology that makes them efficient. Windows today are far more efficient than you can tell by just looking at them. It would be next to impossible for an average computer modeler to know how to enter reasonably accurate information based on components that could then be field-verified.

As LEED continues to gain prominence and as we inch toward the Department of Energy’s goal of zeronet energy homes, the need for reliable thermal transmission performance for the vast array of whole window products is paramount. The rest of the codes and building science world simply can’t evaluate how efficient our products are better than we can.


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