Daylighting, Coatings, Installation Can All Affect Role Of Windows In Energy Consumption
Facility managers looking to reduce the amount of energy consumed by windows can explore daylighting, coatings, and installation options to cut energy use.
The use of daylighting can influence window selection. "It can be a double-edged sword," Carmichael says. Most facility managers (and occupants) prefer light, inviting environments. However, the goal should be to balance the heat gain with the amount of light let in.
One way is through a ceramic frit coating on the glass, Carmichael says. This diffuses the light, minimizing glare and reducing the heat gain, and can be valuable in southern, cooling-dominated climates. "You want the sunlight in, but want to minimize the amount you have to cool the heat that results."
Installation plays a key role in the effectiveness of a window, as well. "Where things break down is the integration of the window to the wall," says Cook. With energy codes now specifying tight buildings and little air penetration, "you need to have a total envelope for a unified system. If you don't integrate, you have big voids where air and water can come in."
For that reason, a facility manager frequently needs something more than caulk, which can degrade, when marrying the window system to the building envelope, Cook says. To account for this, the connection should include a mechanical device, such as a screw or fastener, to link the window with the air and water barriers that are part of the building envelope.
As their name suggests, air and water barriers are designed to prevent water infiltration and air leakage through cracks in walls, windows and doors. This is important, because if air is able to move through drafty windows, it can cause energy loss as well as damage from water. The Air Barrier Association of America reports that up to 40 percent of the energy used to heat and cool a building is due to uncontrolled air leakage.
Window frame systems are evolving, as well. Hughes notes one new type consists of frames on both the inside and outside that are separated by a thermal-break material. This both saves energy and opens up new design possibilities. "You can have different colors for the inside and outside frame," she says.
While energy-efficient windows generally can cut energy costs in a building, it's rare that the windows truly pay for themselves just through this reduction, Carmichael says. Where more significant savings may come into play is when the energy performance of the building envelope (including the windows) is strong enough that it's feasible to reduce the size — and thus the investment — in the heating and cooling equipment. "Then you can see a reduction in capital costs," he says.
The performance of a facility's windows is likely to become increasingly significant. At the moment, the commercial window market is not as well-regulated as it could be from an energy efficiency point of view, McGowan says. While every state has a building energy code that requires NFRC 100 (the standard that looks at a window's U-factor), and NFRC 200 (which looks at windows' solar heat gain coefficient and visible transmittance), they often aren't well understood or enforced, he says.
However, McGowan adds that the NFRC is "working with energy code officials across the U.S. to better enforce the energy code, which will lead to more accurate window energy ratings."
"The world of construction is becoming both more unforgiving and restrictive, which impacts window performance," Cook says. He notes that building codes are specifying higher performance from all elements of a building; windows and doors must meet increasing performance values to be in compliance. At the same time buildings are becoming more unforgiving in that there's not a lot of redundancy — say, compared to years ago when many were constructed of several layers of bricks — to absorb minor water leakage or air infiltration. "Therefore windows must perform consistently well or their defects will be more apparent sooner," Cook says.
Karen Kroll, a contributing editor for Building Operating Management, is a freelance writer who has written extensively about real estate and facility issues.