Urban Green Council Warns Against Glass Envelopes
(07/23/2014)

NEW YORK —The nonprofit Urban Green Council (UGC) is warning builders that choosing subpar materials to meet energy performance standards may actually consume more energy than planned.

According to “High Cholesterol Buildings,” a report from the New York-based UGC, the envelope (walls, windows and roof) of today’s average building doesn’t insulate very well, and some are “outright terrible, creating a devastating tax on our energy resources.”

The biggest problem for curtain wall buildings, the UGC says, is insulation. Large, complex buildings often add more glass, which is a poor insulator. These buildings offset the energy performance by investing in more efficient mechanical systems.

“These tradeoffs ensure that in the first year a typical glass building is occupied, it won’t consume more energy than allowed by code,” the report states. “But as the rest of the building’s systems are upgraded over time, today’s glass envelopes will be responsible for a greater and greater share of energy consumption in the future. And we’ll be stuck with them.”

The UGC notes that building envelopes outlive other building elements. For example, office equipment is typically replaced every five years, lighting every 10 years and HVAC systems every 20 years on average. However, glass walls can last 50 years. More building designers are choosing the glass envelope design because it attracts more tenants, and that means higher revenue. UGC recommends that if glass must be the chosen envelope, it would be more energy efficient to choose a triple-pane window. Additionally, improvements to mullion design and material (such as those that limit the amount of heat lost, and use of fiberglass instead of highly conductive aluminum) can also help with energy efficiency.

Additionally, the UGC advises against unnecessary floor-to-ceiling glass, especially in typically private areas such as bedrooms. The council adds that glass near the floor is often blocked by furniture, and in places such as New York City, building occupants only get an unattractive view of rooftop mechanical equipment.

The health of the envelope depends on the construction crew, UGC says. Unfortunately, typical construction practices don’t stress air sealing and eliminating thermal breaks, especially for masonry and window walls.

“The shape of any big city— what we see when we walk down the street — is defined by the envelopes of its buildings,” the report states. “While the market favors all-glass construction and energy modeling makes this seem sustainable, we must remember to see beyond the surface and consider the long-term health of buildings.”

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