By Michael Nicklas, FAIA

Daylit cafeteria at the Heritage Middle School in Wake Forest, N.C. Photo courtesy of Innovative Design

Over the past 31 years, Innovative Design’s 4,750 buildings have routinely incorporated sustainable building practices, and all have incorporated at least one of the many ways of using solar energy. Our buildings have cut our clients’ energy bills by $96 million and eliminated 760,000 tons of carbon dioxide. Our peak energy savings now exceed 40 megawatts (111 megawatts of primary energy). The annual energy savings for our clients is $6.8 million.

Like many other designers pursuing passive design, our focus in the 1970s was on residential applications. Mass and glass was the game. Three decades later, if you’re designing a passive home, it is still the game. Following technical guidance provided by Doug Balcomb, practical suggestions from Ed Mazria, and many helpful refinements learned from speakers at American Solar Energy Society conferences, our hundreds of designs resulted in more than 4,000 successful passive solar homes. The key was balancing mass and glass amounts and protecting against overheating in the summer. Our passive designs typically reduced heating energy requirements by 40 percent to 70 percent. Because we minimized east-west glass and provided shading for the high summer sun, cooling loads were also reduced.

Today, we mostly design green educational facilities. Our firm has worked on the design of more than 100 K-12 schools that have incorporated solar solutions. Daylighting is the key, particularly in schools. It produces the largest savings and is the most important form giver. With daylit spaces representing a half or more of the total area of our schools, the savings attributed to the daylighting and roof assembly design are significant.

Our strategies for saving energy in schools are obviously different than those used in passive residences, but the importance of pointing our schools south remains the same. Good orientation to maximize southern exposure opens up several cost-effective opportunities for energy savings. Over the years, our daylighting designs have evolved, but we still favor south-facing roof monitors and light shelves wherever possible — from Maine to Northern Florida.

Like other designers who moved from passive solar residences to daylit nonresidential buildings, we have found that consideration for passive heating is still a key factor in the design, even in commercial and educational buildings driven more by lighting — and much more than many engineers appreciate.

Consider Human Factors
Through decades of experience we have learned, first of all, never to overlook human factors. The top priority in daylighting is to create a condition that is superior to that of electrical lighting for the majority of the time the space is occupied. Otherwise, occupants will never lose the habit of walking into a space and turning on the lights. Designers need to develop daylighting strategies that provide superior lighting for two-thirds of the daylit hours. If the norm is just 25 foot-candles of daylight supplemented with fluorescent lighting, the lights will burn.

The second key human factor is the elimination of direct-beam radiation entering critical occupant spaces. If you fail to block direct-beam radiation from getting into someone’s face, the occupant will soon find a way to block it for you. This conflicts with residential passive-heating strategies, where we often want south-facing windows to put direct-beam radiation onto a dark-colored tile floor. In well-daylit spaces, direct-beam sunlight coming from southern exposures needs to be bounced, redirected or filtered, creating the effect of natural light that comes through high, north glazing.

Why not just prefer north-glazing solutions for daylighting?

Use the Window of Opportunity
From an energy perspective, the worst thing you can do is to implement a daylighting strategy that is not quite good enough. If you create a situation where you typically have insufficient natural sunlight, resulting in the lights being on, you have created a negative energy situation. You have all the heat produced from the lights as well as the heat created by sunlight. Dimming controls help, but if the daylighting is not adequate, we have found that, regardless of the theoretical calculations, many occupants tend to just override the controls and turn on the lights.

The flip side is also true. If you put in too much glazing you can also get burned. It is easy to just add glazing and achieve a very high daylighting contribution but, as with passive solar-heating designs, you can get too much glass and negatively impact cooling. The key is to implement an optimum amount of glazing that provides superior daylighting and reduces cooling loads, particularly during peak.

While passive strategies primarily attack the heating load, good daylighting strategies result in the lights being out 60 to 70 percent of the time. This reduces the lighting load but also, just as important, the cooling load. With a 60 percent contribution, the habit of flipping on the lights is overcome. More than 70 percent and you are beginning to employ too much glass in achieving the higher levels of illumination. This results in overheating in the warmer months. Until detailed daylighting analysis is conducted, “glass-to-floor area” rules-of-thumb are useful in estimating daylighting glazing amounts for particular approaches in most of the United States, assuming that the daylighting apertures are clear, double glazing.

By simulating the varying glazing amounts and overhang lengths during peak cooling times (as well as annual simulations) the best design can be determined. The optimum energy- efficient design allows only enough radiation to provide the necessary footcandles during peak cooling.

If designed correctly, a daylighting strategy can reduce —
• electricity for lighting and peak electrical demand;
• cooling energy and peak cooling loads;
• maintenance costs associated with lamp replacement; and
• electrical service to the building.

If done wrong, it can increase your cooling energy and peaks.

South Glazing Is the Better Energy Solution
In the hot months, cooling loads can be significantly reduced by providing just the right amount of daylighting in your building. When the lights are out, the cooling load is less because daylighting can produce the same lumens as fluorescent fixtures but result in half the heat. To achieve these cooling reductions it is essential that automated dimming is employed and, during peak cooling times, no more radiation is allowed to enter than is required to meet your foot-candle objective.

However, the lights are out not just in the summer. They’re out in the winter as well. While this strategy results in a “cooler” lighting in the warmer months, it also means that in the winter, the heat typically produced by the lights is significantly less. By implementing south-facing, vertically placed daylighting strategies that naturally allow increasing amounts of radiation to enter into the space during the winter months, passive solar gains can be used to offset the heating that was being provided by the lights. This is a factor very often overlooked by even seasoned engineers, who tend to use assumptions (typically on a square-foot basis) on the amount of scheduled internal loads generated from lighting. When you take away the heat usually produced by the lights, the areas of the country where limited passive heating makes sense moves southward — all the way to Florida.

North-facing monitors, while similarly effective as south-facing monitors in providing natural light, are not as energy efficient because they typically require at least 25 percent more glazing to achieve the same annual daylighting contribution. North monitors are beneficial, but, because of the additional glazing and the lack of passive heat benefits in winter, they are not as cost-effective as the south monitor.

The same is true when comparing south-facing light shelves to high, north-facing transom apertures. From a daylighting perspective, high, north-transom glazing can provide good daylighting in spaces that are not too deep. However, like north-facing roof monitors, it is necessary to increase the glass area to achieve the same annual contribution as southfacing light shelves.

By employing south-facing apertures, it is possible to create a strategy that maximizes winter radiation and optimizes summer gain. As can be seen from the accompanying charts, indicating the amount of radiation falling on different flat surfaces, a south-facing aperture is the only orientation that maximizes winter gain and minimizes summer radiation.

The worst glazing placement, from an energy perspective, is typically horizontal — like most skylights. In the summer, just when you don’t want the extra radiation, is when you get the most. More than twice the radiation will enter your building through a flat skylight in the summer than in the winter — just the opposite of what you want.

The best way to design a daylighting aperture is to size your glazing and overhangs so that just the right amount of radiation is brought into the space during your summer peak cooling condition. If your glazing faces south, more radiation enters into the space during the coldest months — just what you want. However, with a flat skylight, if the size is optimized for summer peak, there will not be enough daylight to fulfill typical needs the rest of the year.

While skylights can be designed with internal tracking louvers to produce very nice daylighting, it is still difficult to justify their use when it comes to reducing cooling peak loads. Only in a very few areas of the country, where the climate is mild and sky conditions are optimal, should skylights be considered a better energy choice than roof monitors or light shelves.

Summary: Turn South Light into North Light
To reduce conductive losses and improve cost-effectiveness, design your daylighting around south-facing options first. Then utilize north-facing strategies. Bring the sunlight into your building shell through the smallest aperture with the highest visible light transmission you can get. Then, capture the energy benefits of south-facing glazing but filter and bounce the entering sunlight so that it creates the effect of north light. Create north light out of south light and keep the energy savings.

About the author: Mike Nicklas, FAIA, is principal of the architectural firm Innovative Design, Raleigh, N.C., and a past chair of the American Solar Energy Society (ASES). Nicklas has held leadership roles in many leading solar energy and sustainable building associations and was the recipient of ASES’ highest honor, the 1996 Charles Greeley Abbot Award. Contact him at 919.832.6303 or nicklas@innovativedesign.net.

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