W2B-038 Natural light-0.txt
GETTING NATURAL LIGHT INTO BUILDINGS

ANN MCNICHOLL

It is difficult to overestimate the significance of daylight, and of sunlight, in the character of a building and in the lives of the people who use it. There are, of course, some building types, such as theatres or nightclubs, where being cut off from the world outside is an essential part of the experience. In others, shops or museums for example, daylight may be excluded from large areas of the building so as to give full play to display lighting. But most interiors which are to be occupied by people (as opposed to goods or machinery) need plenty of light and, with global warming and 'Sick Building Syndrome' on our minds, natural light and ventilation have once again become critical determinants in the form our buildings take.

Daylight is variable and unpredictable. It is these characteristics which account for peoples liking for it and for the sparkle that it brings to the interior of a building but which also make it challenging to work with. We want sunshine and daylight, but we don't like glare, downdrafts, loss of privacy, ultra-violet damage or severe temperature swings. Too much glazing, the wrong kind of glazing, or glazing in the wrong place may produce heat losses or gains which have to be countered by artificial heating or cooling. So, good daylight design must form part of a considered architectural strategy for the building as a whole, controlling and exploiting the available light, maximising its advantages and minimising its disadvantages. Most of the critical decisions are made during the early design stages.

The design possibilities of the 'window' are, of course, extremely rich. Whether treated as a simple hole in the wall or as a complete three-dimensional element it makes a fundamental contribution to the quality of interior spaces and the external appearance of the building. The way in which it frames a view, captures light, or channels warmth, sound and air, helps determine the character of any room - whether it is intended to be simply humane and comfortable, visually exciting, or solemn, with symbolic and spiritual impact.

The following paragraphs review some new and traditional daylighting devices and products which can be used to control daylight and sunlight, and to moderate the conflict between good daylighting and a good thermal environment which glazing so often entails. Lightwells, roof monitors and clerestory windows are well established devices for getting light deep into buildings. The atrium can be seen as an elaborated and inhabited lightwell, lightshelves and coated or prismatic glasses as sophisticated shades and blinds, while the concept of the lightpipe or transparent insulation is relatively new. Shading devices per se are not described, but these are as important, and selection of the appropriate type is particularly dependent on local climatic conditions.

Rooflights

Because the sky is generally brighter at its zenith than near the horizon, a horizontal rooflight is proportionally three times more effective as a source of daylight than a vertical window - and it casts its light over a space in a more uniform way. Direct sunlight can be diffused by translucent glazing and glare controlled by baffles. Very beautiful effects can be created by fitting angled reflectors below the rooflight or locating one beside a wall, so that ceilings or walls are washed with light.

A disadvantage of the horizontal rooflight, compared to a vertical window, is that it collects more light and heat in summer than in winter - usually the opposite of what is desired. For this reason vertical or near-vertical rooflights, clerestories, sawtooth or roof monitors are often preferred for lighting single-storey deep spaces. They can be oriented North, South, East or West as necessary and screened with conventional shades. The duration and quality of light can be enhanced by placing light-catching scoops on the roof outside the glazing, and the distribution of reflected light into the space below controlled by rooflight geometry.

Lightshelves and Louvres

The lightshelf, a flat or curved element placed at the window opening above eye level, redirects incoming light onto the ceiling and simultaneously provides shading for the area of the room close to the window. The underside of the shelf can also redirect light from a high-reflectance exterior ground surface onto the floor inside the room. Lightshelves are most efficient when they are external, cause minimal obstruction to the window area, have specular reflective surfaces, and are combined with a ceiling of high reflectance. Interior shelves have been found to be less useful - they obstruct daylight entering the room while providing little compensating benefit. The shading control functions of a fixed lightshelf are least effective for low-angle sunlight, and in northern Europe should generally be considered only if glare is a severe problem, or window size is restricted and internal surfaces (other than ceilings) must be of low reflectance.

Adjustable louvres with a specular finish on the upper surfaces of their blades are more responsive than lightshelves and, if completely retractable, need cause no obstruction of daylight on overcast days. Sophisticated fixed-louvre systems, incorporating lenses and mirrored faces, are now available. These are custom-made for the particular latitude and facade orientations and can combine shading with redirection of both direct and diffuse light deeper into the building.

Lightpipes and Lightducts

These are among the more mechanically complex daylighting devices. Sunlight is collected by heliostats (mirrors controlled by a tracking device), concentrated by means of mirrors or lenses, then directed to the core of the building through shafts or along acrylic rods or fibreoptic cables. Because they depend on direct sunlight, and are relatively expensive to install, they are cost-effective only in climates where blue skies and clean air can be guaranteed for much or the year. (Energy-efficient back-up lamps can be fixed at the head of the shaft to substitute for sunlight during infrequent overcase conditions.) The recent development of thermo-hydraulic tracking systems powered by solar cells should improve economic viability.

Atria

The daylight performance of an atrium is complex. Its proportions determine the amount of direct daylight reaching the floor - wide, shallow. square atria perform better in this respect than do deep, narrow, rectangular ones. The design of the atrium walls significantly affects the distribution of light once it has entered the atrium. Dark finishes reduce internal reflectance and the deeper the atrium the more important this becomes. Windows in the atrium wall reduce reflectance also - the upper walls are the most critical in reflecting incoming light down into the atrium, so it is best to limit windows in this area.

This arrangement corresponds with the requirements of rooms facing into the atrium - rooms at the upper levels get of light but need protection from glare, while those at the base need to maximise the amount of light they receive. Design strategies include making rooms near the base lower, increasing their floor to ceiling heights, or stepping up the upper floors in successive steps so that all rooms have some view of the sky. (The same principles will apply to a building facade on a narrow street.) The closer the room is to the bottom of the atrium the greater its dependence on light opened from atrium walls and floor. Reflectors may be fixed on the windows of lower rooms to redirect more of the zenithal onto their ceilings, but this is generally cost-effective only when other considerations determine that the reflectances of atrium walls and floors must be low. And while rooms further from the atrium roof may have lower light levels, they may have better light quality in terms of uniform distribution and absence of glare.

Putting any glass roof over an open court will reduce daylight levels in the court by at least 20%, and sometimes by 50% or more. So the structure of an atrium roof should minimise obstructions to the glazing area and its connections to the building should be such that light is allowed to wash the atrium walls.

Transparent Insulation

Transparent Insulation Materials (TIM),. which tend to be translucent rather than truly transparent, were initially developed primarily as insulation for Trombe wall assemblies. But because they transmit light, they can also function as a glazing material. There are several categories of TIM, using different materials and a variety of forms - foamed, capillary, honeycomb, fibre and gel. Most need protection on one or both sides by glass or plastic sheets. Light transmission of TIM ranges from 45% to 80%, with a reduction of approximately 8% for each sheet of protective glass used. Insulation values are very much better than for glass. For example, 98mm hexagonal honeycomb polyamid TIM has a light transmission factor of 61% combined with an insulation value five times that of a double glazed window. TIM can be incorporated into purpose-made window assemblies by most window manufacturers. Costs tend to be approximately three times those of conventional double-glazed windows.

Glazing Materials

A conventional window, single-glazed with clear float glass may transmit approximately 85% of the light that falls upon it. Double or triple glazing will reduce light transmission to 70% and 60% respectively. Where lighting requirements demand larger areas of glass than would be thermally satisfactory, specially treated glass can be used to control heat losses or gains.

The early tinted glasses reduced solar heat gain to some degree but also cut down daylight transmission and distorted the colour of the landscape outside. Heat absorbing glasses do not reduce daylight transmission to quite the same degree, but reduce heat gain by only 10% because a large percentage of the heat absorbed is re-radiated into the interior. Reflective glass blocks solar radiation effectively (reflectances up to 50% are available) but, like tinted glass, it blocks light as well as heat, and it continues to do so in winter when heat gain and daylight may be beneficial. Selective 'low-e' double glazing. with a resistance to heat loss equivalent to that of triple glazing, has a light transmission factor of approximately 80%.

Current developments include the responsive chromogenic glasses. Electrochromic glass changes its optical absorption properties and becomes dark or cloudy in response to an externally applied electric field. The opacity disappears when the field is reversed. It can be readily integrated into a responsive building climate control system. Thermochromic glass switches between a heat-transmitting and a heat-reflecting state at selected temperature thresholds and photochromic glass darkens and lightens in response to changes in light intensity. At presen,t material costs of all three are high and durability uncertain but improving.

Glass to which a holographic film has been applied does not block radiation but diffracts it. The film can be designed to direct incoming sunlight on to a reflective surface, such as the ceiling, or deep into a room. It can be designed also to reflect sunlight coming from pre-defined angles (high-angle sun on South facades, or low-angle sun on East and West facades, for example). Up to four images containing different 'instructions' can be combined in one layer. A view out through the window is retained but from some viewing angles there is a rainbow effect. Its performance for diffused light is poor, but research is continuing. Costs are not high but at the moment holographic film is not available in the sizes needed for the building industry.

Prismatic glass (or plastic) controls transmitted light by refraction and can be used to redirect or to exclude sunlight. The direction of incoming daylight is changed as it passes through an array of triangular wedges whose geometry can be designed for particular conditions and orientations. In several recent applications it has been used to reduce glare. Prismatic glass is translucent rather than transparent, so cannot be used where a view outdoors is required. Normally a prismatic refracting panel consists of two sheets with their prismatic faces facing each other to protect them form dust accumulation. They can also be used within double-glazed units. While the sheets themselves are inexpensive, the overall construction cost is higher than for conventional glazing. Prismatic assemblies, including sophisticated systems incorporating silvered wedge-faces and several panel types, are increasingly available.
