building envelope

A window into better building envelopes

The case for finding ways to reduce load demand before completely replacing HVAC systems
Monday, January 11, 2016
By Steve DeBusk

The building envelope can truly make or break energy-efficiency efforts. Major improvements to heating, cooling, and ventilation systems may help reduce energy, but those savings could fly out the window if the building envelope lacks appropriate insulation.

Poorly performing building envelopes can allow heated or conditioned air to escape; HVAC loads increase as a result, leading to higher utility bills. When this happens, it may seem as if HVAC systems aren’t performing at their best.

To improve heating and cooling equipment performance, the only option is to replace the system, right? It depends on the situation. Uncovering the reasons why an HVAC system isn’t performing as expected will likely save more money in the long run.

Reduced heating and cooling loads

Most energy-efficiency consultants recommend reducing load as a first step toward improving energy efficiency. Before purchasing a chiller system to accommodate a building’s existing cooling demands, for example, it makes sense to try decreasing those cooling demands. This can be accomplished by turning off equipment when it’s not in use, establishing appropriate temperature set points, reducing electric lighting levels, etc.

The next step is to make sure the building envelope provides a good thermal barrier. There are several ways to improve building envelope efficiency, from installing exterior louvers to replacing drafty doors. Another option is to install high-performance, low-e window film. It improves the insulating power of existing windows and helps reduce heating and cooling demands. If loads are reduced enough, this step may also preempt unnecessary HVAC equipment replacement.

The Hyatt Regency Houston was contemplating replacing its HVAC system due to temperature complaints from guests. The existing system was unable to keep southwest- and southeast-facing rooms cool enough. But after installing low-e window film in those rooms, heating energy use decreased by 25 per cent and cooling energy use decreased by 23 per cent compared to the southwest- and southeast-facing rooms without window film. The HVAC system that once couldn’t keep guestrooms cool enough now provides the hotel enough cooling without retrofits or replacement.

Improving window-insulating power

The majority of energy transfer in or out of the building envelope occurs through windows. Thanks to solar heat gain entering through unprotected exterior glass, nearly one-third of a commercial building’s cooling costs can be attributed to windows. Any measures to better insulate exterior glass will help reduce the amount of heating and cooling lost through the building envelope.

Known for bringing cooling costs down in warm climates during summer months, most non-low-e window film provides savings in cooling season only. But newer window film technology allows year-round HVAC energy savings in almost any climate. This high-performance, low-e window film reduces heat loss in winter and solar heat gain in summer, maintaining comfortable temperatures through all seasons.

Window-insulating power can be increased by up to 92 per cent, based calculations made using the Lawrence Berkeley National Labs (LBNL) Window Program — even if windows are decades old. High-performance, low-e window film can provide single-pane windows with the same insulating performance as double-pane windows, and double-pane windows with the same insulating performance as triple-pane windows — without window replacement.

Window film’s impact on lighting

Solar heat gain, glare, and hot temperatures near windows can force occupants to close blinds in an attempt to control these issues. When occupants shut blinds are shut, however, they sacrifice access to daylight, which can raise demand for lighting.

Newer low-e films are spectrally selective, reducing a greater portion of infrared heat than visible light. In other words, this technology allows daylight in while minimizing uncomfortable heat and glare.

A research team at the University of Padua in Italy studied window film’s impact on lighting at the 21-storey MG Tower. As a modern Italian office tower built in 2011, the building had up-to-date HVAC systems and new windows. Despite this, facilities management struggled to relieve occupants who were dealing with solar heat gain and glare from exterior glass.

By installing window film, the facilities team was able to reduce these problems. It also noticed a considerable increase in useful daylight because window blinds were open more often. The University of Padua research team concluded that there wasn’t a measurable increase in interior lighting energy at MG Tower after window film installation, contrary to the commonly held belief that window film increases lighting energy use.

Maximize energy-efficiency investments

To make the most of energy-efficiency improvements, it’s imperative that the building envelope provides a thermal barrier to help keep conditioned and heated air inside — where it belongs. High-performance, low-e window film offers an immediate increase in window insulation levels to improve building envelope performance for years to come. (See sidebar “Install window film or replace the windows” for guidance on when to act and which measure to take.)

If a building’s heating and cooling demands are high, and the HVAC system struggles to keep up, consider reducing heat and cooling loads before investing in equipment replacement. By lessening the amount of hot and cold air required from the system, it’s possible to reduce equipment run time. This not only lowers energy bills, but can also lengthen the life cycle of the HVAC system.

Install window film or replace the windows?

What are signs the windows in a commercial building need attention? Occupant complaints are often a good indicator, whether they revolve around uncomfortable temperatures, glare, drafts, lack of natural light, or condensation on windows.

To fix some of these issues, a facility manager may consider window replacement. If the windows suffer from any of the following, or aren’t structurally sound, then replacement is likely necessary — not only to improve energy efficiency, but also to ensure safety and window integrity:

  • Air leakage
  • Moisture leakage
  • Failed seals
  • Damaged frames

New windows don’t always address complaints about heat or glare, and window film can’t fix leaks or moisture problems, so identifying the real issue is key to selecting the right solution. If a building’s windows are structurally sound and in good condition, and the goal is to increase efficiency and reduce energy use, then low-e window film may help.

Steve DeBusk is global energy solutions manager for the window film division at Eastman Chemical Company. DeBusk has 30 years of experience in energy efficiency. He is a Certified Energy Manager, a Certified Measurement and Verification Professional, and a Certified Sustainable Development Professional. You can find his blog at or follow him on Twitter @greenbldgs.

4 thoughts on “A window into better building envelopes

  1. Steve, what has been the downside regarding thermal stresses on the existing glazing systems you reference?

    Was it single glazed monolithic or IGU’s?

    Examples of BOTH?

    The low e film is a solid alternative to reglazing for sure. Thanks,


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