A building can lower its environmental impact through a reduction of resource consumption and waste output. When it comes to energy, most property owners focus on energy-efficient equipment and renewable generation, but this approach is incomplete because it does not address the performance of the building envelope itself. You can have a property with the most efficient lighting and HVAC, and a rooftop blanketed with solar panels, but you will not achieve top performance if the structure itself suffers from air leaks and poor insulation.

According to the Urban Green Council, vertical construction in NYC is suffering from reduced performance due to a modern design trend: excessive use of glass. These buildings look modern, but glass conducts five times more heat than the average insulated wall. As a result, they end up spending much more energy than needed in space cooling and heating. One could argue that the ample window area leads to electricity savings by reducing dependence on lighting fixtures, but the extra HVAC load far outweighs any lighting savings.

Ample window areas is one of the main selling points in real estate, but building surveys have revealed that most window area ends up covered by blinds, curtains and furniture during actual operation. In addition, a poorly-insulated envelope is a major safety concern during an extended power supply interruption.

Why Low Performance Building Envelopes are a Problem

Building envelope performance depends on three key aspects: insulation, air-tightness, and glazing area. The first two can be improved with relative ease in existing properties, but window upgrades are costly, and replacing windows with walls is even more expensive. As a result, glass buildings are likely to stay that way for their entire service life. In the best-case scenario they will be upgraded to triple-pane glass, which performs better than conventional windows but is still outclassed by walls.

High-rise glass buildings are likely to get lighting, HVAC and domestic hot water upgrades many times during their service life, but the thermal performance issues brought by a glass envelope will remain there.

Energy Performance of Glass Buildings

If NYC has some of the most demanding building codes in the world, you may wonder how buildings with a low-performance envelope get approved? This is because the NYC Energy Conservation Code lets building designers choose between prescriptive measures and performance-based compliance.

  • Prescriptive measures are lists of technical requirements regarding the building envelope and its energy-consuming systems. They provide a simple approach for smaller buildings, but which can be very difficult to meet in a larger development.
  • Performance-based compliance uses the results of energy modeling: if a building’s energy consumption is below the established threshold according to its size, it gets approved regardless of how that performance is achieved. One property owner may focus on a high-performance envelope, while another may opt for the best lighting and HVAC available (e.g. LED lighting and a VRF system) at the expense of building envelope performance.

In this example, the property owner with an efficient envelope has a higher potential for improvement, even if overall performance was comparable when both buildings were new. The efficient envelope of the first building will achieve synergy with any future equipment upgrades, but in the second case the building envelope will always cause extra load on whatever HVAC system is installed.

According to the Urban Green Council, modern glass buildings have an envelope performance comparable with that of typical constructions from 1,000 years ago. Even Victorian-era brick buildings have a superior envelope performance than modern high-rise glass buildings.

Even though glass windows are poor insulators, their performance can be improved drastically with triple-pane glass and fiberglass frames. Another option if the property hasn’t been designed yet is to follow the prescriptive approach of the NYC Energy Code, where the resulting envelope conducts 2 to 3 times less heat.

Poor Building Envelopes Are a Risk Factor During Emergencies

Hurricane Sandy left a large portion of NYC without power in 2012, affecting around one million citizens, and revealing that most buildings in the city are not equipped to deal with extended blackouts. This led to the creation of the Building Resiliency Task Force, with the goal of reviewing NYC code and suggesting measures that would improve building performance under emergency situations.

A poor building envelope and an extended power outage are a dangerous combination. Since the envelope does not block heat transfer effectively, the building’s temperature will rapidly move towards the outdoor temperature. The Urban Green Council carried out building temperature modeling, assuming a blackout occurs during the coldest days of winter and during a heat wave. They compared typical building envelopes with high-performance envelopes, for six major building types in NYC:

  • Single-family house
  • Row house apartment
  • Brick low-rise apartment
  • Pre-2000 brick high-rise apartment
  • Post-2000 brick high-rise apartment
  • All-glass high-rise apartment

In the case of a winter blackout and with typical NYC insulation, the temperature of all six property types drops below 43°F after a week. Single-family houses face the highest risk, since they only take four days to reach freezing temperatures (below 32°F). On the other hand, all six property types remain above 54°F after a week if they have high-performance envelopes. In other words, a well-designed building envelope can save lives.

During a heat wave and with typical insulation, all six property types are above 85°F after a week. High-rise glass buildings suffer the most, going above 100°F around noon. With a high-performance envelope, only the single-family home rises above 90°F around noon, and all other property types stay below 88°F.

Getting a Pleasant View With Less Window Area

Building surveys have revealed that around 59% of window area is covered during normal operation, despite the fact that floor-to-ceiling windows are among the main selling points according to real estate companies. Tenants ask for large windows, but what they really want is a nice view, and this can be accomplished with moderate window area. For example, the bottom portion of large windows often ends up covered by furniture, and in many cases it just provides sight of neighboring building rooftops and their mechanical equipment or water towers. You can have a nice view of the NYC skyline without windows that reach all the way down to the floor.

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