Volume 10, Issue 7 - September 2009

feature

Gas Enhanced Window Technology
Here to Stay
by Jim Plavecsky


Argon is a real bargain when you consider what it can do for thermal performance. But before looking at argon, make sure that you have made smart choices in terms of the other factors that affect thermal performance from the top down.

The Crucial Components
Have you heard of the Pareto Principle? In a nutshell, it means making changes first in the areas that mean the most. When it comes to window thermal performance, my experience has shown that choice of frame material and design matters most followed by choice of glass, choice of spacer, then choice of gas type. When you look at your window design in a quest to maximize thermal performance, the first place to look is the frame. If you are using a metal frame, some kind of thermal break will be necessary for you to get U-values down to decent levels. This is why wood and vinyl framing have taken over the window industry over the last 20 years. Back in the mid 1980s, aluminum windows accounted for at least half the market. Now, vinyl and wood dominate.

Next, after frame design, is choice of low-E glass. The emissivity and the solar heat gain coefficient of the glass employed will have the next largest impact on a consumer’s energy bill. The relative importance of these two factors depends upon how much solar heat gain your location is desirable based upon annual heating versus cooling costs. The next factor to look at is spacer type. Get the metal out! The less metal integrated into the edge seal design, the better (lower) U-values and the better (higher) condensation resistance values you will find. Yes, steel is better than aluminum, since it has lower conductivity. But metal is metal. A spacer system with no metal at all is best.

Now after we have arrived to this point in the design process, assuming that we have chosen the optimal pane gaps for dual- and triple-pane units, we are now ready to look at the choice of gas. A vacuum would be best, but this has proved difficult in past designs. This is where argon and krypton come into play.


“It’s in There”
New Standard Designed to Regulate Gas Retention

For the first time in North America, gas-filled insulating glass (IG) units will have to be certified as not only containing argon (or krypton) but also retaining the noble gases that are used to impart improved thermal performance. The intent is to protect the consumer against fraudulent claims and inaccurate thermal data that may result from units which contain less than an adequate amount of thermally insulating gas. This serves to assure the consumer that the thermal insulation values reported on the label are the “real McCoy.” It also provides some assurance that workmanship and fabrication methods, which correlate to long-term retention of the gas and durability of the insulating glass unit itself, are in place.

Keeping the gas inside the unit long-term is essential not only to help maintain long lasting thermal benefits but also to help prevent other problems that can develop as a result of excessive gas leakage from the unit. This is because gas permeation occurs at different rates for different gases. Argon, in particular, permeates out of the unit faster than nitrogen, oxygen or carbon dioxide permeates to the inside of the unit. The result can lead to negative pressure within the unit causing unit implosion and as a consequence visible distortion, increased heat transfer and increased stress at the edge of the unit, which can consequently shorten IG unit lifespan.

 

"Starting in July 2010,
gas-filling on the ‘honor system’ will be a thing of the past—at least
as far as NFRC labeling program participants are concerned."

 


There was a time when the prevailing attitude was that filling a unit with gas and ensuring that gas retention is maintained adequately was a nice goal but not necessarily a reality given the high production demands associated with the boom years in the window industry. Indeed, more than a few fabricators were overheard to admit that at the end of the day, if they had units to get out the door, gas filling might sometimes be skipped. And most had inadequate knowledge of how well the gas was being retained within their units. “No one can see the gas or smell the gas leaking out, so how would one ever know?” was a remark frequently heard.


NFRC Participants: Be Prepared for July 2010
Starting in July 2010, gas-filling on the “honor system” will be a thing of the past—at least as far as national Fenestration Rating Council (NFRC) labeling program participants are concerned. The NFRC will require that IG units used in NFRC labeled products must be listed under an accepted IG certification program.

Under the requirements for IG certification, if a manufacturer chooses to market a certified IG unit as containing argon (or any other insulating gas) then the unit must pass the gas content portion of the certification test. A set of units will be constructed by the manufacturer and these will be evaluated for both initial gas content as well as gas content remaining after completing the weather cycling exposure requirements of ASTM E2190. The units tested for initial gas content must contain a minimum of 90 percent initial fill and after weather cycling, units must average a minimum of 80 percent gas content remaining.

If the product cannot pass this requirement but does succeed at passing the requirements of E2190 for durability and volatile fog, then the units can still be certified but not as containing insulating gas.

Because the required certification testing takes about five months to complete, assuming no backlog at the lab, the time to act on getting involved in a certified program is soon. Prior to jumping on board with the program, however, a thorough review of insulating glass fabrication materials, methods and practices is highly recommended.


Value of Gas
The chart on page 46 shows the Thermal Conductivity of the Noble Gases. You will see that argon has a thermal conductivity that is 26 percent lower (better) than air, while krypton insulates at a rate 61 percent better than air. (Xenon is even better, but we won’t dare go there because of exorbitant costs.)

I recently was involved in discussions with a customer pondering the dilemma of “how to hit the.30.” After reviewing his design, which included a thermally efficient (vinyl) frame, optimum pane gap, a high-performance low-E (e=.035) glass, and a non-metal (foam) spacer, adding argon enabled him to bring his NFRC-rated window U-value from a .33 down to the required .30, thereby qualifying his customers to receive the 30 percent tax credit made available by the recent stimulus bill. As an added benefit, the addition of argon also brought his window’s condensation resistance (CR) value from a 55 up to a 58, thereby further enhancing the value perception among potential customers. Up to this point, this customer had vehemently debated the long-term value of argon gas filling. However, with the recent passage of the stimulus bill, argon filling, at least as far as he is now concerned, has now become a “must have.” Isn’t it funny how a little market stimulus can completely change an opinion, literally overnight?


Krypton—A Different Animal
Now krypton is an entirely different story. Yes, the resistance to thermal heat transfer is twice that of argon. However, the cost of krypton is now roughly 30 times greater than argon as we have seen it triple over the last several years. This is due to the skyrocketing demand in recent years for this noble gas coupled with recent global shortages. According to Pavel Perlov at Electronic Fluorocarbons, demand in the lighting and fenestration industries has skyrocketed in recent years. This is coupled with the fact that steel production has gone down, dropping in 2008 for the first time in ten years. The air separation units that are used at steel mills to isolate gases used in the production of steel are also used to supply feedstock to the krypton suppliers, which then take components of this gas into additional purification processes to isolate krypton. The increased demand, coupled with the squeeze on the supply side, is what has led to the recent round of price increases. “Prices have stabilized however,” notes Perlov.

Despite the rising cost of krypton, however, the usage has remained steady in high-end window designs combining triple pane construction with several surfaces of low-E glass. Since krypton is much denser and has a higher molecular weight than either air or argon, it really is at its best in terms of low conductivity when confined in a smaller air gap, as is used in triple-pane units. Crowding the molecules more closely together causes an increase in random collisions between molecules. This scatters molecular motion away from the gradient of heat flow so that heat transfer from “where it’s hot” to “where it’s not” is less intense.

This combination of triple-pane technology combining two low-E surfaces with krypton-gas-filled cavities can result in some very impressive numbers. When combined with vinyl framing and non-metal spacer technology, I have seen U-values in the neighborhood of .21 and with CR of 65. The identical window made with argon would have a U-value of .27 and a CR of 62, so the krypton is adding another 22 percent in energy savings over argon and another three points on condensation resistance.

I am often asked, “What is the added cost to put krypton in my window?” I have been asked this question so many times that I finally developed a “Gas Filling Cost Calculator” spreadsheet. Using the calculator, and assuming $1.50 per liter for the krypton, the cost of krypton-filling the double hung window just mentioned, for example, is $9.94 per IG , adding a total of $19.88 to the cost of the window (not including labor). In comparison, the cost of filling the same window with krypton at $.03 per liter would add only $.40 (less labor) to the cost of the window.


Will the Customer Pay?
How much are consumers willing to pay to have this extra level of thermal performance? Well, it is obviously not for everyone, but there are quite a few window manufacturers effectively marketing these high-end window systems and the market for such products is alive and well. Once again, these companies are bundling the krypton with multiple low-E coatings, high-performance spacer systems, the very best hardware and aesthetically pleasing frame designs.

The future Energy Star® requirements currently being outlined for 2013 are pointing to even higher- performance window systems as requirements. This, along with an increasing concern in Washington for a “greener” earth, as well as a national desire for reduced dependence upon foreign oil, will lead to an increased push for the highest thermal performance standards we have ever seen. The cost of gas has been rising, and gas filling itself has seen its share of controversy over the years. However, my guess is that it will continue to be a technology that fenestration suppliers call upon to help them meet these higher performance thermal insulation standards that loom on the horizon.

Jim Plavecsky is regional sales manager for Edgetech I.G. and is the author of an industry blog for DWM magazine, located at at www.dwmmag.com. Mr Plavecsky’s opinions are solely his own and not necessarily those of this magazine.

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