Open the Window to an Efficiency Upgrade

Like all potential large investments, there are numerous considerations and calculations that go along with upgrading a building’s windows to a more energy-efficient variety.

"In considering the decision to upgrade a building’s windows to achieve energy savings, there is a central issue and several surrounding issues," says Craig Bloomfield, vice president of public relations for Jones Lang LaSalle. "The central issue is a comparison of the upfront cost to the energy savings that will occur over time – the simple payback period."

Determining the simple payback period is just the starting point – there are several other factors in the upgrade equation.

Calculate and Compare
There are several tools to calculate the payback period and help make the replacement decision. "You can calculate the payback period quite accurately by using energy modeling programs to project the amount of energy that will be saved by implementing higher performing windows," says Bloomfield. "Having an energy modeling program does not make the calculations easy – you still need to know what you’re doing – but it provides an accurate picture of energy performance based on the improvements you’re considering."

Energy Efficient Empire

Jones Lang LaSalle planned several capital projects at the Empire State Building, including the addition of a new chiller to cool corridors throughout the building.

"Through integrated energy modeling, we found that retrofitting the building’s 6,500-plus windows would reduce the load on the chillers by approximately 33%, eliminating a $24 million chiller project," explains Craig Bloomfield, vice president of public relations for the company. "It was the window project that made a significant part of our chiller strategy so effective. In this case, improving the windows was very economical."

It is important to note, however, that the windows were improved, not replaced. "We found out that it would be less expensive and would result in better efficiency if we reused and upgraded the existing windows than if we replaced them," he says. "The existing windows were double-pane windows. In order to more than double their performance, we removed each window from its sash, cleaned the two separated pieces of glass, and inserted a third pane of suspended coated film with glazing that cuts down on heat and glare from direct sunlight." A krypton/argon mixture was injected into the air gaps of the windows, which quadrupled the window’s thermal insulation value.

In this case, a window upgrade project made sense and ended up saving a lot of cents.

"The most useful tools are the performance ratings of the products themselves," adds Jeffrey Lowinski, vice president of technical services for the Window and Door Manufacturers Association (WDMA). "National Fenestration Ratings Council (NFRC) energy ratings, ENERGY STAR ratings, structural performance ratings, and other nationally recognized certifications are the best means for comparing products to one another, outside of aesthetics and price."

Building Balance
Energy modeling can help you reach a balance between cost and savings by determining the best combination of energy improvement measures. "It is an integral step of a process we call an integrated energy retrofit," explains Bloomfield. "One strategy may impact the effectiveness of other strategies. If you look at window replacement by itself, you may be missing the real cost-benefit opportunity."

Calculating the return on replacing existing windows with energy-efficient ones requires a whole building energy performance analysis, including the direct efficiency benefits from the windows and the impact of that benefit on the HVAC system, says Lowinski. "Replacing existing windows with high-performing products will change the building operating loads, especially those related to balancing air movement, perimeter heating/cooling, and daylighting control. It is even possible that highly efficient window replacement would need to include resizing of HVAC heating or cooling equipment."

"For example, in cooling-dominated climate zones, use of high-efficiency windows could mean that air conditioning equipment runs less frequently," he adds. "Since AC also removes moisture in addition to cooling, buildings might experience excessively high moisture content if AC equipment does not operate often enough. Resizing the equipment along the use of high-efficiency windows will reduce overall energy usage and ensure effective building operation."

Adding together the payback period and the impact on other building systems will give you the true ROI of upgrading your buildings windows and let you know if that project is a window that you want to open.

Kylie Wroblaski ([email protected]) is associate editor of BUILDINGS.