Ask the Green Architect: Mirrors for Lighting; Radiant Heating for Floors; Efficient Exit Signs

Ask the Green Architect: Mirrors for Lighting; Radiant Heating for Floors; Efficient Exit Signs

Green architect Eric Corey Freed answers your questions on sustainable building performance, materials, and design.

  • Mirrors for Lighting
  • Radiant Heating for Floors
  • Efficient Exit Signs

    Why are mirrors not used more often in green buildings as a way to double the amount of light without using energy?

    The typical mirror is a sheet of flat plate glass coated with aluminum or silver on one side. The opposite site is a highly reflective surface. Since the coating is on the backside, it is protected from scratching and will typically last a very long time.

    First used in widespread production in Venice during the 16th century, early mirrors were backed with an amalgam of tin or mercury. Before that, people simply used convex metal tins.

    Justus von Liebig discovered the modern chemical process of coating a glass surface with metallic silver in 1835. Present-day mirrors are made by sputtering a thin layer of molten aluminum or silver onto the back of a plate of glass in a vacuum.

    Today, the modern mirror uses less than 3% lead content in the paint coating. In fact, lead free mirrors are now commonly available. Since glass itself it a natural material, mirrors are an inherently green materials.

    Mirrored glass cannot be recycled, but if protected, could last many years with minimal maintenance. They do not off-gas and glass is one of the most commonly recycled materials in the world.

    How important is the recycling of glass? For every ton of glass recycled, the equivalent energy of 10 gallons of oil is saved. Recycling a single glass soda bottle saves enough energy to light a 100-watt bulb for four hours. Since glass never wears out, it can be recycled in endless cycles forever.

    The simple use of mirrors in buildings is obvious. Since a mirror reflects light with virtually no loss, well-placed mirrors could be used to reflect the light, doubling the amount of ambient light in a room. Bulbs of half the normal wattage could be used, or half the number of fixtures could be used with no loss in light level. The potential energy savings would be enormous (50% in this simple example).

    While there are examples of the use of mirrors to reduce the electrical lighting, it is not a common practice. The reasons for this are not clear, but could include: unwanted reflections from sunlight, less controllability or even the cost of the mirrors themselves.

    Additionally, mirrors have numerous other applications in regard to green building practices.

    Light Shelves

    The principle behind daylighting is simple: by using natural light, you reduce the need for electricity. In practice, however, bringing daylight deep into a building can be challenging.

    Light shelves are horizontal fins located above eye-level at the windows to "bounce" the light back up onto the ceiling. This diffuses the light and brings it deeper into the building. Well-placed light shelves will also act as a shading device and reduce glare. Studies have found workplace productivity increases in workplaces that include natural lighting. In order to work effectively, both the light shelf and the ceiling would need to be white or at least a light hue.

    If mirrors reflect light so well, one could argue the light shelf should be topped with a mirror surface. Unfortunately, reflecting direct sunlight into a building could create enough heat to burn the ceiling.

    Solar Energy

    A new trend in solar electricity is the use of concentrator technology. In short, a concentrator uses a series of highly focused mirrors to concentrate an intense beam of light. In some current concentrator technologies, this light is focused onto solar panels; in others, the light is used to quickly boil water in a steam engine.

    Energy Innovations has developed an ingenious heliostat design using 25 mirrors to concentrate the sun onto a single solar cell. The Sunflower will be available later this year and stands only five feet square, but produces 25 times the amount of energy.


    EnviroGLAS produces a line of their popular recycled glass terrazzo (often referred to as "vetrazzo") made with recycled mirror glass. Their EnviroFLECT products use recycled ground mirrors to create a durable, reflective surface.

    More Information:

    A Sourcebook for Green and Sustainable Building: Lighting

    U.S. DOE Building Technologies Program: Lighting

    Light Shelves

    NREL: International Conference on Solar Concentrators

    "The Dotcom King & the Rooftop Solar Revolution"

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Based on your previous article, I am installing an earthen floor. I wish to use radiant heating in the floor, but does it use the same tubing and systems?

Eric: We human beings are such fragile creatures. Only comfortable between 68 to 75 degrees or so, the human comfort zone is incredibly narrow in range. Buildings go to great lengths to maintain this comfort zone, and most of these systems waste energy.

A radiant heating system uses an efficient combination of hot water pumped through tubes to warm the floor. Unlike traditional forced air heating, where hot air blows to heat the air, radiant heat uses the principle of radiation to heat the surface. Radiant heats the occupants, not the space, and this is an important distinction. The result is a wonderfully comfortable and cozy feeling of warmth.

Comfort is the most compelling reason to consider radiant heat. Since the heat source is under the floor, temperatures are warmer at floor level (where you are) and the heat rises to the ceiling. Since the floor is warm, walking barefoot and sitting are the floor are cozy possibilities.

Since no air is pushed around, radiant heat offers a dust-free alternative for those suffering from allergies or asthma. Unlike forced hot air, radiant heating also will not dry out the air. Children are safe from contact with hot radiators or dirty vent ducts. Finally, a radiant heat system is virtually noise and maintenance free.


Those innovative ancient Romans used radiant underfloor heating in 670 A.D. using terra cotta pipes. Already masters of plumbing and irrigation, the Romans heated their villas and baths by directing flue gases from wood fires in pipes beneath stone slab floors.

Rediscovered early in the twentieth century, modern radiant heat substituted copper pipes for terra cotta and hot water instead of hot gas. Frank Lloyd Wright would often employ this method of comfortable heating. The tract homes of Levittown featured radiant heat. In the 1970's, plastic tubing ( replaced the faulty pipes.


The hot water in the tubes can be produced through a gas or electric boiler or hot water heater. Obviously, if you are generating your own electricity with solar panels, the electric version is a better choice. The most energy efficient option would be the addition of solar hot water panels to pre-heat the water and reduce the operation of the boiler. The energy saved will pay for the $2500 price tag of the solar hot water in just a few years.

Floor Types

Although you can adapt a radiant system for any floor type, an earthen floor or concrete slab works best. The thermal mass of these floors holds in the heat from the tubes and will maintain a much more consistent temperature. So, in answer to your question, your earthen floor will work perfectly with a standard radiant heating system. No special tubes are necessary.

Consider using zones to better control where the heat is located. For instance, the living areas can be one zone, and the sleeping areas on another. You can have as many of these zones as desired, but each zone adds an additional manifold controller and thermostat.


While radiant heat can be installed in both floor joist systems or slab floors, you will find installing in a slab will be slightly less due to the easier installation.

As a Rule of Thumb, the system will cost about $1.50 - $1.75/sq ft, not including the heat source.


The advantages of radiant heat have brought it to the mainstream market in the U.S. (it is already commonplace in Europe). A slew of specialty products such as wood installation WarmBoard and thermal transfer plates are available. For areas where tubing is not an option, electrical radiant heat companies such as Nuheat provide a thin wire mesh one can install as a thinset under tile or carpet.


The comfort and health benefits of radiant heating make it an attractive and affordable option for any building. The energy efficiency also makes this one of the greenest ways to heat your building.

More Information:

Earthen Floors For Domes -- Often Called "Cob Floors"

WarmZone Radiant Heat Systems

EERE Consumer's Guide: Radiant Heating

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Do illuminated building exit signs really use much electricity? Are there energy-efficient options available and if so, how do they compare?

Although the specifics vary, all building codes require illuminated exit signs in some form in all commercial buildings. The signs are so commonplace; few even notice them anymore in daily use. Not only do they draw a constant stream of electricity, but they also contain backup batteries to continue illuminating the sign in the event of a power outage.

How Much Power Is Used?

The typical exit sign has two-20 watt incandescent bulbs. These bulbs last only 2000 hours, give off more heat than light, and need to be replaced four times a year. A simple fix to your existing exit signs would be to change the bulbs to compact fluorescent bulbs. Compact fluorescents will last four times longer and use a fraction of the energy. But what other options exist?

LED Bulbs

A light-emitting diode (LED) is a type of semiconductor bulb. Unlike incandescent bulbs, LED's do not have a filament that can burn out, use less than a tenth of the energy and do not give off any noticeable heat. With no filament to burn out, they last as long as a standard transistor. LED exit signs consume only 2 watts per exit sign, as compared to the 40 watts of a standard sign. Although each exit sign uses only a small amount of electricity, the total potential savings are large because about 40 million exit signs exist in the U.S.

Some manufacturers of super energy efficient LED exit signs:

Lumatech Corporation
2 Marlen Dr.
Hamilton, NJ 08691
Phone: 609-689-3122
Toll-free: 800-932-0637
Fax: 609-689-3091
Lumatech manufactures both red and green LED exit sign retrofit kits.

Astralite, Inc.
20 Pocono Rd.
Brookfield, CT 06804
Phone: 203-775-0172
Toll-free: 800-832-5483
Fax: 203-775-0797

Chloride Systems
272 W. Stag Park Service Rd.
Burgaw, NC 28425
Phone: 910-259-1000
Fax: 800-258-8803

Crescent / Stonco Division
2345 Vaux Hall Rd.
Union, NJ 07083
Phone: 908-964-7000
Fax: 908-810-4524

101 Corporate Dr.
Spartanburg, SC 29303
Phone: 864-599-6000
Fax: 864-699-1428


Photoluminescence is a chemical, non-electric reaction where light is emitted. Similar to the glow of fireflies in a summer field, the soft light of photoluminescence has numerous potential applications in buildings. Given the low energy requirements of an exit sign, it makes sense to use this natural technology for illumination. There are various types, but these will provide up to 20 years of free exit sign operation.

American Permalight Inc.
2531 W. 237th St. #113
Torrance, CA 90505
Phone: 310-891 0924
Toll-free: 888-737-6254
Fax: 310-891 0996
American Permalight Inc. manufactures non-electric, non-radioactive UL-924 compliant exit signs, plus glowing wall & floor paints, wall and anti-skid floor tapes, step markers, and other self-illuminating exit path marking products using patented strontium-oxide aluminate pigments. The EnergyStar approved exit signs are available in aluminum or PVC (but avoid the PVC version!).

Tritium Gas

Using tubes filled with tritium gas to create a chemical reaction, these radioactive fixtures glow without electricity. Although they comply with code requirements, they are extremely dim, and the hazardous ingredients may pose a serious liability to building occupants and owners. They are also the most expensive fixtures on the market today.

The Exit Store
[email protected]
Tritium Gas Exit Signs


The oft-overlooked exit sign represents only one small example of the many energy-efficiency opportunities surrounding us. By examining all of the systems within a building, other opportunities to reduce energy loads will appear. An LED or photoluminescent exit sign is a "no-brainer" alternative over standard incandescent bulbs.

More Information:

Exit Sign Guide

Electric Utilities and Energy Efficiency


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Eric Corey Freed is principal of organicARCHITECT and teaches sustainable design at the Academy of Art University in San Francisco and University of California Berkeley. He is on the boards of Architects, Designers & Planners for Social Responsibility (ADPSR), Green Home Guide, and West Coast Green.