But we can make it even better: let’s renovate the Pantheon, so we have glazing, not an open aperture. The right glass can help. First, we’d need to increase the aperture area to compensate for the light transmission of the glass. So we increase the Pantheon aperture to 1,000 square feet to make up for a light transmission of 70%. Now we still have the original amount of visible light, but what about heat? Using a high-quality low-E insulating glass, the heat gain per square foot would be reduced to about 35% of what the same aperture would admit with no glass. So despite the increased aperture area we actually have only half the heat gain of the original open oculus, with the same amount of admitted daylight.
Let’s compare that to the electric lighting equivalent. All of the daylight is going to eventually end up as heat in the space, and so will the electric light and the electricity used to produce it. So with the new glazing our daylight is adding 5,000 watts of heat gain. The equivalent metal-halide system is adding 15,000 watts. So the daylighting is now creating only about one-third as much heat gain as the electric lighting that would achieve the same light level.
So that’s a big win for the daylight, right? Maybe. It’s a win only if we assume that the daylight light levels and the electric light levels are the same, and that’s tricky. Clearly, if the daylight levels are higher there will be more heat gain; if they’re three times higher the daylight will produce the same heat gain as the electric system. This is not as unlikely as it may sound. It’s easy to design an electric lighting system that maintains virtually the same desired light level all the time – it’s not at all easy to do that with a daylighting system.
For the Pantheon, what happens on a nice sunny May afternoon around 2:00? Now we have well over 5 million lumens coming in through the oculus, and even with the new glass the heat energy in the daylight will be about 22,000 watts, or about 75,000 BTU per hour. But the electric lighting system would still be plugging along at the same 15,000 watts. Sure, the daylight level is now higher than the electric light level, but we don’t need that extra light, and now the daylight is producing more heat gain than the electric lighting would. And that means we’re going to increase the amount of energy we have to use to air-condition that heat away (did I mention we’re adding air conditioning, too?). And by June the problem will be even worse.
This illustrates one of the important challenges of designing daylighting: maintaining reasonably consistent light levels at different times of day and different times of year (and also under different weather conditions). There is a related design challenge: maintaining consistent light levels at a given time but in different parts of the space. Because the Pantheon aperture is at the center and high above the floor, light levels at the floor will generally be very uniform (although occasionally on mid-days in summer direct sun will hit the floor, and all bets are off).
But in multistory buildings the daylight often has to enter from the sides, and we don’t usually have 142-foot-high ceilings, so daylight levels near the windows tend to be much higher than those farther away. The issue is the same for both challenges: if we design for an adequate daylight level for less-than-favorable conditions or locations, we can end up with much higher than needed daylight levels for the favorable conditions or locations.
There is also a tendency to simply overdesign daylight levels under all conditions. Our Pantheon example had a daylight aperture of only about 4.5% of floor area: a glass curtain-wall building could easily have a ratio of 40%. Without extensive shading and low-transmission glass, that is very likely to result in daylight levels much higher than needed. And those higher levels bring higher heat gain. They can also be visually uncomfortable, but that’s another subject.
Photo Credits: Irene (2), OliverN5 (3)