7 World Trade Center

7 World Trade Center


Why are so many LEED-certified buildings all-glass? It seemed to defy logic. After all, no matter how much you can reduce artificial lighting by using daylight, the insulation value of glass is negligible compared to solid insulated walls, and anyway there are many overcast days and dark winter afternoons. It is even  more puzzling when an all-glass building is shaded. First you wrap it in glass, and then you wrap the glass in something else. I always suspected that this was more about announcing “I am a green building” than about actually conserving energy. And now it’s official. A recent NYT article reported on the results of a New York City study of the actual energy performance of large office towers. (The full report is here.) In the rating system used, 75 is considered the minimum score for a building to be considered “high efficiency.” The all-glass 7 World Trade Center (LEED gold) scored 74, and the Condé Nast Building, highly touted as the city’s first green building when it opened in 1999 received an unspectacular 73. Close but no cigar. Solid buildings with windows, even old ones, do much better: the venerable Empire State scored 80, and Chrysler scored 84. As for the old glass towers, it’s downright embarrassing. The recently renovated Lever House reached only 20, and Seagram scored a miserable 3.

8 Responses to LEED Lies

  • Mark Hogan says:

    It’s not a matter of insulating, it is a matter of overheating. Glass buildings with shading can be incredibly efficient and still provide excellent daylighting if the shading is tied to the location and climate. There are many inefficient buildings with punched windows just as there are ones with glass curtain walls. Most high rises use very little energy for heating over the course of the year.

  • Witold says:

    Traditionally office buildings gained heat from their lighting (and to a lesser extent from office machines). But as lighting becomes more efficient, and is are turned off at night, this effect diminishes. A glass office tower at night is like someone standing outside naked. It’s always warmer when you put an i nsulated coat on.

  • Marcus Sheffer says:

    LEED does not lie.

    This highly simplistic critique leading to such a conclusion is the lie.

    I agree that all glass does not typically result in exceptional energy performance in high rises. The lack of actual performance relative to the predicted performance used in the design and construction version of LEED does not mean that LEED lies. Making that connection demonstrates a significant lack of understanding of the issues.

    Should LEED certification be based on actual performance and not a prediction regarding energy performance? In my opinion LEED certification over time should be tied to actual performance but right now it is not for the design/construction versions of LEED. If there was a greater degree of understanding in the market regarding the connections bethween prediction and performance this kind of simplistic comparison would not result in accusations of falsehood but rather engender the questions “why is the performance not equal to the prediction?” and “how can it be improved?” There are many reasons why a prediction and the actual performance do not match but LEED lying is not one of them.

    Often overlooked in critiques of LEED is that the operations version of LEED is based on actual performance. At a fundamental level all energy performance comparisons are relative and all baselines are not perfect, including Energy Star. Without a detailed understanding of the specific issues regarding a specific building you simply cannot look at the raw data and infer anything but a superficial, marginally informed hypothesis.

    I would suggest that before you accuse someone of lying you might consider first telling the truth.

    • Witold says:

      “All glass does not typically result in exceptional energy performance.” Well, yes. Glass has no R value to speak of, while even the most conventional insulated wall will have an R of 20 or more. Glass is good to see out of, in windows, but all-glass buildings are simply irresponsible.

      • Marcus Sheffer says:

        I agree that all glass (double pane) results in poor performance in most climates but not in all cases. Adding insulation to an strongly internal load dominant building actually increases its energy use. Most of the office buildings you mention are probably not strongly internal load dominant (except the portions of those buildings which may house data centers) so insulated walls and the window to wall area ratio is likely to be important. In our experience the optimum window-to-wall ratio is between 25% and 40% window.

        Even from a daylighting perspective all glass results in poor performance. Good daylighting is well designed to allow the light in while minimizing the direct solar gain in the cooling season. The criteria for good daylighting is similar to good artificial lighting. One key criteria is uniformity and an all glass facade results in a very high contrast ratio which adversely affects our vision. An all glass exterior wall is too bright relative to the interior portions of the space so to compensate we turn on the lights seeking uniformity. More daylight is not better.

        So the insulating value of glass relative to wall is just one of the criteria to evaluate in determining the effect on energy performance. In a typical office lighting accounts for 25% to 35% of the total energy use. Good daylighting design can cut this in half which is likely to be greater than the effect of insulating walls alone. A well designed (from an energy perspective) building will seek to determine the optimum combination of window performance, window-to-wall ratio, and insulating values to reduce the heating and cooling loads and to maximize total energy performance.

        Long winded way of saying I agree – all glass building are irresponsible in most cases and we certainly can do much better.

    • We can debate whether or not LEED “lies,” but there is no doubt it is deceptive. That’s because it uses an additive point system. Add enough bike racks, solar panels, etc. and you can get a LEED Platinum building for something that in reality is not sustainable.

      In the case of these glass buildings, there are two major problems. First, the glass is engineered to keep out ultraviolet light, keep heat gain down, have some insulation value, etc. The mutit-layer window that results is sophisticated but requires a large amount of embodied energy to produce it. And to add insult to injury, the chemicals wear out over time, so the glass will all have to be replaced after an unknown period (unknown because the current experiments are still early in their life cycle). That’s more wasted energy. The best walls have a high R value, low embedded energy and are like the Energizer Bunny – they last, and last, and last.

  • Bob Schulz says:

    But the glass buildings are so shiny!
    As a university architect it was interesting to see how thrilled our campus community was to learn that our new Student Union will be designed in the same California Mission Revival Architecture as our historic campus core, and how angry the local design community was to learn the same thing.

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