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Triple-Pane Energy Gains

February 16, 2010

Windows are a weak link for buildings when it comes to heat transfer. As much as 30 percent of a commercial building’s energy loss occurs through its windows, according to a DOE study. Conversely, windows allow in 10 times more solar heat per square foot than walls.

Due to the high initial costs, upgrading windows solely for the energy benefit is rare. A notable exception: 6,500 windows are being refurbished onsite in the Empire State Building.

Most owners of multitenant buildings have not been able to justify the cost of window upgrades based on lower energy costs. Existing tenants who ultimately pay for energy do not want their short-term costs to rise in return for energy gains extending beyond their lease term. And new tenants are willing to pay higher rent only to the extent that the premium is offset by lower energy costs. For retrofit strategies to work, they must have a short payback period.

One of the owners of the Empire State Building, Tony Malkin, set out to prove that a whole-building retrofit to greatly enhance energy efficiency could be financially justified in a multitenant office building. Malkin assembled a team that included Jones Lang LaSalle, the Rocky Mountain Institute and Johnson Controls. Their task: Identify which of several dozen potential strategies would yield the best balance of low upfront cost and long-term financial benefit.

After looking at the cost and environmental benefits of more than 60 strategies, the team recommended eight projects to reduce the building’s energy use by 38 percent, saving $4.4 million a year and placing the building among the top 10 percent of office buildings in terms of energy efficiency.

The window refurbishment, one of the eight projects, will reduce energy usage by about 5 percent, or about 10 million kBtus, over 15 years after all windows have been refurbished. The analytical process showed a financial advantage for window refurbishment over window replacement or keeping the existing windows.

For the analysis the team used several software tools, including ENERGY STAR’s Portfolio Manager for measuring baseline energy performance, and Green Globes for rating the building’s level of sustainability and identifying opportunities for improvement. A key tool was eQUEST, the free DOE program for modeling energy usage.

The team conducted a series of parametric runs to determine the window replacement options with a refurbishment strategy. In both scenarios, the double-paned windows would be upgraded to triple-pane glazed windows. Adding a 3/16-inch pane between the two existing panes in each window was made possible by a 5/8-inch gap between the existing panes.

Glazing and films with different amounts of tint and thermal protection have a significant impact on heat transfer and these are measured in three key ways:

  • U-Factor measures the extent to which a glazing product helps retain heat within the building on a scale typically between 0.20 and 1.20, with lower values representing greater effectiveness.
  • Solar Heat Gain Coefficient (SHGC) measures the efficiency of a glazing product in keeping out solar heat on a scale of zero to one, with lower values indicating less heat penetration.
  • Visible Light Transmittance (VT) measures the amount of sunlight to penetrate on a scale of zero to one, with higher values indicating that more light passes through the glazing.

The analysis demonstrated the benefits of the refurbishment option over replacement. Refurbishing the existing windows will save $477,000 annually, almost 2½ times the anticipated savings from the replacement option. Upgrading the existing windows will also reduce electrical use by twice as much as the replacement strategy, and will have a much greater impact on steam reduction as well.

 

Window Strategy Comparison

Existing windows

Scenario 1

Scenario 2

Panes

Double 3/16” 5/8” Gap

Triple Glazed
Replace

Triple Glazed
Reuse

Film

Clear

3M Prestige 70

Suspended SC75

U-Value

0.48

0.46

0.16

SHGC

0.72

0.56

0.36

Visible Light Transmitted

80%

69%

62%

Results

Cooling Load Reduction

 

278 Tons

618 Tons

kW Reduction

 

255

495

kWh Reduction

 

975,103

1,817,370

Mlbs Steam Reduction

 

224.9

3,669

Annual $$ Savings

 

$194,147

$477,404

 

Based on the analysis, the window project is budgeted to cost $3.76 million in hard and soft costs, and with $377,000 in contingency expenses figured in, the total cost could be nearly $4.1 million. Anticipated savings include $477,000 in energy costs and $125,000 in operating costs for a total annual savings of more than $565,000. With a substantial reduction of CO2 and a payback period of six or seven years, the project was approved.

The decision was made to do all the glazing and insert the third pane into each window on-site. This would reduce the time, cost and vehicle emissions associated with transporting the windows to an off-site location. Another environmental benefit to re-using the windows and frames is the reduction of building waste being sent to the landfill.

The project began in 2009. All 6,500 windows in the building will be replaced by the end of 2013.

As the team accesses areas to conduct window refurbishment, they will also install insulated sheet metal barriers behind each radiator, where the local temperature is over 100 degrees F. during the heating season. Placing a layer of insulation between the radiators and currently uninsulated walls will reduce the building’s energy use by 3 percent.

Peter Belisle is International Director at Jones Lang LaSalle and President of Energy and Sustainability Services in the Americas. He can be reached at peter.belisle@am.jll.com and 213/239-6033. Ray Quartararo is an International Director at Jones Lang LaSalle and head of Project and Development Services throughout the Northeast and Mid-Atlantic regions. He can be reached at ray.quartararo@am.jll.com and 212/812-5857.