Daylighting

Natural Daylighting in New Buildings

Daylight is the best light a building can have. It connects occupants to the outdoors, supports health, alertness and sleep, and can replace electric lighting for much of the working day. But it is not only the amount of daylight that matters to the people inside; it is also its quality: color, contrast, variability, direction, and the absence of glare.

Good daylighting is a balance. The same windows that bring light in also bring in solar heat and, in winter, lose heat. Daylighting and the thermal performance of the envelope have to be designed together.

A checklist for designers

– Don’t over-complicate it. Use intuition and common sense alongside today’s analysis tools. Annual climate-based daylight simulation is now routine, and it should always be paired with thermal and energy analysis. Think about solar heat gain, heat loss and daylight together.

– Control the electric lights. Daylight only saves energy if the electric lights respond to it. Current energy codes, including the IECC and ASHRAE 90.1, require daylight-responsive controls that dim or switch off lighting in daylit zones. With LED lighting now standard, lighting is about 10% of total commercial building energy use (U.S. EIA, 2018), so the larger daylighting benefits are occupant comfort, well-being and productivity, as well as reduced cooling from lower lighting loads.

– Zone the lighting with the windows. Group fixtures in rows parallel to the windows so each row can dim as daylight falls off with distance from the glass. Tunable-white LEDs can follow the color of daylight through the day.

– Use tall windows to push light deep. Daylight reaches farther from the top of a window than from the bottom. Clerestory and transom windows raise the effective head height without increasing the window-to-wall ratio (WWR). Even a modest WWR provides ample daylight when it is well oriented and placed high.

– Mind the depth. As a rule of thumb, a conventional window daylights a room to a depth of about 1.5 times the window head height. Light shelves and light-redirecting glazing can extend that toward 2 times head height and even out the light.

– Use effective aperture as a starting point. Effective aperture (EA) is visible transmittance multiplied by window-to-wall ratio. An EA of about 0.20 to 0.30 on north and south elevations is a common starting point, with glazing on east and west elevations kept to what the views require.

– Skip glass below the work plane unless the view needs it. Glass below desk height adds solar gain and heat loss but little useful daylight. Floor-to-ceiling glass is a choice for the view and the architecture, and it calls for high-performance glass to stay comfortable.

– Choose glass by elevation, not by darkness. Older practice used tinted glass to control sun. Spectrally selective low-E coatings now do it far better, cutting solar heat while keeping visible transmittance high, so the glass stays clear and neutral. Use a low solar heat gain coefficient (SHGC) on west, east and, in hot climates, south elevations, and a higher SHGC on south elevations in cold climates, where winter sun is useful heat.

– Plan the building around daylight. Make daylight access a factor in the floor plan and in assigning perimeter space. Use building geometry, overhangs, fins and orientation to control glare and sun.

– Scale the strategy to the space. Tall conventional windows work well for shallow perimeter rooms. Deep spaces need more: clerestories, light shelves, skylights, atriums or light wells.

– Favor horizontal spans for even light. Continuous ribbons of glass spread daylight more evenly across open-plan perimeter space than punched windows do in small rooms.

– Treat glare separately. Light shelves and redirecting louvers work best under clear, direct sun. Glare control (blinds, shades, dynamic glazing or exterior shading) needs its own design, especially on east and west elevations.

– Size light shelves to the window. On south-facing rooms, an interior light shelf should be about as deep as the height of the clerestory or transom glass above it, with a white or bright, matte top surface.

– Measure it. Daylight performance is now measured with climate-based metrics, spatial daylight autonomy (sDA) for how much of a space gets enough daylight and annual sunlight exposure (ASE) for how much gets too much direct sun, defined in IES LM-83 and used by LEED and WELL.

Where windows make or break daylighting

The more daylight a design asks of its windows, the more those windows have to do. Large glass areas bring in light, but in an ordinary window they also bring cold glass surfaces in winter, drafts along the glass, condensation, and solar heat in summer. That is often what limits how much glass a design can afford.

GlasFëns windows are built to remove those limits:

– Warm glass, even in winter. Our ZERO hybrid vacuum glass reaches R-23 at the center of the glass. The room-side surface stays close to room temperature, so people can sit and work right beside large windows without cold drafts or radiant chill, and the glass stays free of condensation.

– Solar control tuned by climate. The coating on surface 2 of every ZERO package is chosen for a high, neutral or low solar heat gain, without changing the insulation, so glass can be matched to each elevation and climate.

– Large openings. Our fiberglass frames are strong and stiff enough to carry large, heavy glass, from tall fixed windows to the GR145 lift-slide glass wall, in a frame material that insulates better than vinyl or wood.

– Tall operable windows. Dual action windows tilt in at the top for secure ventilation high in the room, where warm air collects, and turn fully open for cleaning and egress.

See the glass, the performance data and U-Value & SHGC for the numbers behind each choice.

When is a new building a candidate for an innovative daylighting design?

An innovative daylighting system is an especially good fit when:

– The design is early enough for an integrated design process

– The site is unobstructed, ideally in mid-latitudes with frequent clear skies

– Occupancy is primarily daytime, for example an office or school

– Whole-building energy modeling and daylight simulation are planned

– The building has long north- and south-facing facades

– The program has open plans or glazed interior partitions

– Atriums, courtyards or light wells are part of the design

– Ceilings are high

– Dimmable, daylight-responsive lighting controls are planned

– Task lighting is planned for detailed work

– Blinds, shades or exterior shading are planned for glare control