Winter 2004

Volume 18, Issue 2 Winter 2004

GETTING WARMER
Understanding the Elements of Warm-Edge Technology

by Jim Plavecsky

Prior to the emergence of the National Fenestration Rating Council (NFRC), the thermal performance of various window designs was a tough animal to get your hands around. One could find non-thermally broken aluminum windows boasting U-Values or R-Values (the inverse of U) identical to vinyl or wood frame designs with the same choice of glass.

The reason for this was that U-Values were once reported as measured in the center of the glass. This can be very deceiving because it fails to take into consideration the effects of thermally-efficient frame designs or warm-edge spacer systems. Overall window thermal performance depends on many factors related to both window design and material selection. So, where does the architect begin?

Factors to Consider
There are five main factors that affect overall window thermal performance, and for the window designer wishing to get the greatest bang for his engineering time, it makes sense to look at them in order of greatest to least significant impact.

In this regard, choice of framing material and frame design has the greatest impact, followed by the choice of glass, warm-edge spacer options, gas-fill and the spacing distance between the lites of glass.

One can see this relationship by looking at heat flow plots. A window with low-E glass and a non-thermally broken aluminum frame shows heat loss (the red heat flow lines) primarily through the frame area (Figure 1). When a thermal break is added (Figure 2) one can see that the frame is no longer the major culprit–now the heat loss is concentrated around the metal spacer bar. Heat wants to travel from where it is hot to where it is not, and it looks for the path of least resistance. In this case it is the aluminum spacer bar.

In a nutshell, this is what warm-edge technology is all about. It is simply using a spacer system with the lowest possible thermal conductance to minimize this heat loss at the edge of the insulating glass (IG) unit. Use of a non-metal system will result in the lowest thermal conductance at the edge, and spacer systems using designs that are comprised of metals with lower conductance (steel as opposed to aluminum) will help, although to a lesser extent.

Warm-edge technology has taken the residential market by storm, but is a relatively unknown technology in the commercial window and glass market, simply because most architects are unaware that the technology exists. In 1990, warm-edge spacers accounted for only 15 percent of the North American residential market, but by 2000, this had taken nearly a 180-degree turn. Warm-edge spacers now account for at least 80 percent of the market share for residential windows. But the new frontier is indeed the commercial window and glass market.

Warm-Edge Options
So how warm are the various warm-edge options, and how do we compare them? Thermal conductance of edge-seal systems has been studied and measured by the Advanced Glazing Materials Lab (AGML) at the University of Waterloo in Ontario. The AGML came up with a method of making an IG unit entirely of edge-seal (Figure 3), negating the effects of air within the IG unit. This serves to isolate the thermal conductance occurring at the edge of the unit so an accurate measurement can be taken. The amount of heat that travels through the mid-section of this unit is then measured and the thermal conductance of the edge-seal is thereby determined, called “K-lin,” which stands for lineal thermal conductance. The thermal conductivity chart (Figure 4) shows the thermal conductance of various warm-edge spacer/sealant combinations.

So how does this translate to glass surface temperatures at the edge of the IG unit? Spacer/sealant combinations with low thermal conductivity values will keep the edge of the inside lite of glass warm when it is cold outside (Figure 5). Of course, at the middle of the glass, glass surface temperatures will be the same (assuming the air gap and type of glass is the same). But the lower the conductivity of the edge spacer used, the less of a difference will be seen as measurements are taken from the middle of the unit moving toward the edge of the window (Figure 6).

Maintaining uniform glass surface temperatures has significant benefits. For one, it reduces stress on the glass. When spacers with high K-lin values are used in cold weather, the thermal expansion of the glass is higher at the center versus the edge on the inside lite of glass, and it is lower at the center (versus the edge) of the glass on the outboard lite of glass. In very warm weather, these same differences exist only in reverse. The main point here is that higher stress factors are in effect, which can contribute to stress cracks.

Secondly, a warm glass edge translates to lower overall window U-values. This is especially important now that U-values are reported as overall window U-values as opposed to center of glass U-values. Window manufacturers can use this advantage to help meet Energy Star® regulations or simply to report lower U-values on their NFRC label as a means of setting themselves apart from their competition. When an architect is working on a commercial building project, warm-edge spacers can help to reduce annual energy consumption significantly for the building owner. Software is available to enable architects to simulate the thermal performance of an entire building project and to estimate the annual savings in energy usage with various technologies available to improve energy performance. In one such recent study a three-story high school was simulated with 150,000 square feet of occupant space, 57,500 square feet of wall area, 50 percent of which was windows. Windows were modeled with insulating glass built three ways: clear glass, low-E glass and low-E plus argon. Each of these three combinations was also modeled with aluminum spacer versus a non-metal type of warm-edge system. The results for annual energy consumption in MM BTUs for this building if built in Detroit are shown for each glass option. The architect can also see the impact that these same technologies have in this same building if situated in the south. If this building is situated in Orlando, Fla., we have a similar effect, but the total energy consumption is less in each case. Solar heat gain coefficient (SHGC) is going to be a bigger factor in the South, and, of course, the simulation will show that as well.

Indeed, information about new window technology as well as where it is most cost-effectively applied is deemed to be highly valuable among the architectural community. Feedback from recent seminars indicates that architects are highly interested in warm-edge technology as a method of further improving energy conservation in new projects. It is also surprising to see how few of them really had even heard of it prior to the seminar.
Last but not least, warmer glass surface temperatures at the edge on the inside lite of glass in the winter means higher degrees of condensation resistance. Condensation is also related to relative humidity inside the home. In general, the greater the relative humidity inside the home, the greater the propensity for condensation to occur on the inside lite of glass during the cold winter months. So, at what level of relative humidity should a home be kept? Can we simply control condensation on our windows by running a dehumidifier? Health experts suggest that a certain level of humidity inside the home is quite desirable. It seems that bacteria, fungus and viruses thrive in humidity levels that are either low or high, but do not do well in humidity levels between 40-60 percent. Therefore, this desirable humidity range is where the homeowner should want to condition his environment for health purposes (Figure 7). However, if glass-edge surfaces are cooler because of heat loss through highly conductive spacers, condensation can occur on window surfaces at this moderate level of humidity and this part of the window then becomes a breeding ground for mold formation.

Mold
Mold keeps popping up as a topic of conversation with window manufacturers. Sometimes, the window manufacturer brings it up as he is looking for ways to protect his company from liability concerns. Other times, talk of mold results in a frustrated or angry reaction. Window manufacturers respond, “I don’t even want mold-resistance mentioned in my literature. I do not want to bring it to the attention of my customers or give the lawyers another bandwagon to jump on!”

This is a somewhat defensive strategy, but ignoring the issue will not make it go away. In California, we even have Erin Brockovich talking about it since her newly built home in Agoura Hills was infested with mold. Perhaps in building design and construction, a proactive stance is smarter.

So, why is mold such an issue in our homes? Well, ask yourself one question: Where do we spend most of our time? The answer is–probably indoors. What does this mean for our lungs? Well, every 24 hours, we inhale about 10,000 liters of air, most of it within our homes. So, how does the average home stack up when it comes to air quality? Not good. The problem stems from the fact that our homes are so air tight, concentrations of pollutants can be many times higher in our homes than outdoors.

According to Jason Miller, author of “You are what You Breathe,“ upwards of 70,000 synthetic chemicals are in use in the United States, and many of them can be found inside our homes. Many of these chemicals contain organic volatile compounds (VOCs) that outgas or vaporize. Due to the fact that today’s homes are so airtight, the average American home contains between 50 to 300 VOCs in addition to molds, dust and dust mites, combustion by-products and pet allergens. Some scientists believe this is a particularly bad combination. “Mold can trigger asthma, ” says Terry Yost, a consultant who has worked with the American Lung Association. He cites studies showing the incidence of asthma in young children more than doubling in the past two decades. Indoor air is increasingly viewed as the suspect.

“Dealing with mold is a very real issue today for builders and remodeling contractors,” says Mary Lynn Pickel, environmental policy analyst with the NAHB. The legal actions are different from lawsuits that builders have had to deal with in the past, because, “the litigants don’t just want money and actions to fix the problem, they also want damages for health problems, and those are far higher,” she said.

However, many cite studies showing no conclusive evidence that mold leads to specific health problems. Is this information enough to protect the homebuilder or remodeling contractor? Well, in many cases, perception becomes reality.

“Media reports of homeowner distress, large insurance payouts and legislative hearings make consumers more aware that there could be problems and more skittish about how their builder or remodeling contractor handles the situation,” said Craig Shutt, author of “When Mold Attacks.” “If mold isn’t specifically a problem, customers’ reaction to mold is,” he said.

Smart window manufacturers will be proactive in offering builders and remodeling contractors advice in selecting window products and installation practices with two things in mind: water management and condensation resistance. This approach shows customers that everything possible is being done to improve air quality within the home, and this can help to alleviate consumer apprehension about mold in addition to hindering the likelihood of any possible legal action in the future.

Pro-activity in window design also has many benefits in terms of marketing and promotion. The fastest selling household products are those promoting energy efficiency and health benefits. Energy Star is featured on many household appliances in addition to windows. There are also many other household products that have gained a foothold in the marketplace by promoting their antibacterial qualities. The next time you are unpacking groceries, just set aside all of the products you see that have some reference to “anti-bacterial” or those that “kill germs on contact.” This is a testament to what a health-conscious society we have become.

Yes, health-consciousness and a greater concern for energy conservation are trends that are very likely to continue for years to come. As the architectural community learns more about the benefits of warm-edge technology, design in commercial construction will definitely take advantage of the new technology that is available, and the payoff will be commercial buildings with increased comfort, healthier occupancy and more energy-efficient construction features.

Jim Plavecsky is the owner of Windowtech Sales Inc., a sales and consulting firm specializing in the window and door industry based in Columbus, Ohio.

Announces Debut of Mold Magazine
Key Communications Inc., of Stafford, Va., publisher of the Architects Guide to Glass, will debut the first trade publication for the mold prevention and abatement industry, Mold & Moisture Management magazine, at beginning of the new year, January/February 2005.

Mold & Moisture Management magazine is a quarterly publication created to address industry needs concerning the rising influx in legal cases, building concerns and repairs required to solve the ever-increasing caseload within these fields.

“Preventative measures and remediation will be our main focus for those in the building industry, contractors and others. We are pleased to provide this much needed media regarding the issue of mold, which affects thousands everyday,” said Debra Levy, publisher of Mold & Moisture Management.

Free subscriptions to qualified industry members are currently available online at www.moldmag.com as well as the initial premiere supplement, which was included in Door & Window Maker (DWM) magazine.
Info+ www.moldmag.com or call 540-720-5584.