Volume 36, Issue 7, July 2001


Thermal Performanceenergystar
When Bigger Isn't Better


In the next decade, the top priority for window manufacturers will be thermal performance. This is driven by three factors: increased competition that makes cost-containment imperative; new ENERGY STAR® requirements; and the energy crisis. 

Improving thermal performance while also maintaining costs will be imperative for window manufacturers to stay competitive. Manufacturers must concentrate on the two most important factors for influencing thermal performance: air spaces and the emissivity of the glass.

Considering Airspaces

There are several cost advantages of optimizing airspace size: spacer inventory might be reduced by eliminating sizes larger than 9/16 inch, spacer cost can be reduced since a smaller spacer costs less and sash costs can also be reduced. The smaller spacer may help offset some of the added cost of low-E options. 

A small airspace line may mean a lower price point product, thus requiring a decreased amount of frame, spacer and desiccant materials. However, these may have a hard time meeting ENERGY STAR requirements. In some zones, a small airspace might still be an option since low-E glass is probably needed for solar heat gain requirements. If you are unable to meet the requirements, will you still be able to sell a small airspace window line if your competitors are meeting ENERGY STAR?

A large airspace window line has many disadvantages. This line uses an increased amount of frame and spacer materials, has a higher overall cost and offers no performance gains. Large airspace IG units also have the potential for higher stress cracks and IG failure. 

When there is more than 9/16 inch of airspace, convection of the air or gas mixture begins to detract from the thermal performance. Why spend more on wider frames and wider spacers when energy efficiency declines in every dual pane design over 9/16-inch airspace? The differences between thermal performance of air-filled units with 1/2- and 9/16-inch spacers is minimal. However, once low-E glass is incorporated into the glass package, optimum thermal efficiency is achieved with further reductions in airspace below 1/2 inch. 


Utilizing low-E Glass

The largest single influence on thermal performance is glass. If you are just missing the ENERGY STAR requirements, a change to a lower emissivity glass would help. If you are using hard-coat low-E, this change may mean additional costs and more difficult handling. If you are already using soft-coat low-E, a lower emissivity might not be an option. 

While the maximum thermal efficiency for a clear glass air filled unit is just slightly better at 9/16 inch, hard-coat low-E may be optimum at 12 mm (15/32 inch) while soft-coat low-E is more optimized at 7/16-inch airspace. The incremental advantages of these slight airspace changes may be sufficient to push your current system over the minimum requirements.

Suppose your current window design can’t accommodate these changes in overall thickness. A more beneficial strategy in the longer term may be to redesign the sash to accept several different IG thicknesses. In a redesign, consider a design that can accept several different glazing beads to take up the difference in IG thickness based on maximizing the thermal performance of each glass package. 

With increased competition and higher prices for raw materials, remaining competitive will be an ongoing challenge. However, as the energy crisis drives demand for better-performing windows at both the consumer and fabricator levels, thermal performance will be key. 

POSTAKLori Postak serves as product manager for TruSeal Technologies based in Beachwood, Ohio.



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