Energy Model Settings

The glazing area must balance heat gain, heat loss, and potential glare problems with the need for natural light and an outside view. Lighting is often the largest use in a non-residential building. The design goal should be a building that uses daylighting with electric lighting as a backup.

In a sunny climate, a 3 square-foot (0.3 square meter) perfectly clear opening (100% visible transmittance) will provide 40-50 footcandles of daylight in a space the size of a typical office. This is the ideal amount of light for an office. If the glass has a visible transmittance of 50%, you need twice as much glass to achieve the same amount of light.

When designing a sustainable building, consider the window area of the project carefully. Most heating and cooling energy is transmitted in and out of a building through its windows. With intelligent design and careful selection of components, windows can be used to provide a comfortable and energy-efficient indoor environment.

Natural light and heat flow through a window can be controlled to some extent through appropriate size, window characteristics (Solar Heat Gain Coefficient (SHGC), U-value, and visible light transmittance), and solar orientation. Larger windows have more potential to lose or gain heat than smaller windows. South-facing windows (in the Northern hemisphere, and north-facing windows in the Southern hemisphere) transmit more heat and light than windows on the other orientations.

Overhangs and light shelves on south-facing windows can be designed to allow sunlight in winter and provide shade in the summer. However, it is difficult to control direct sunlight on the east and west facades, due to the lower sun angles.

Residential projects can take advantage of passive solar heating, but commercial projects typically make little use of this method. For non-residential projects, you need to be more concerned with controlling unwanted solar gain through the windows and providing daylight for interior spaces.

As a general rule, the daylighting zone is considered to be a depth of about twice the window head height (the distance from the floor to the top of the window). For example, if a space has windows with a head height of 6 feet, it may be feasible to daylight up to 12 feet deep into the space, assuming no internal partitions block the light.

Daylighting strategies must carefully balance a design with sufficient window area against the potential for glare and unwanted solar heat gain through the glass. A light shelf is one method of controlling glare and direct heat gain.

A light shelf is generally a horizontal element positioned above eye level. It divides a window into a view area on the bottom and a clerestory area on the top. In a detailed model, a light shelf can be external, internal, or combined. The light shelf can be integral to the building or mounted on the building. Glazing should be divided into daylighting glass and vision glass. A light shelf of 2.0 to 2.5 feet shades the lower vision glass and distributes light from the daylighting glass above.

Critical design factors include the orientation, position in the facade (internal, external, or both), and depth of the light shelf. For instance, an internal light shelf redirects the light but may also reduce the amount of light received on the interior.

In the northern hemisphere, light shelves are most effective on south orientations. They can be effective on north orientations for controlling glare but will not bounce light further back into the space. Light shelves on east and west orientations may not bounce light that much further into the spaces, but they are an effective means of reducing direct heat gain and glare. For south-facing rooms, the depth of an internal light shelf should be approximately equal to the height of the clerestory window head above the shelf.

Exterior light shelves reduce daylight near the window but improve the light uniformity. The recommended depth of an external light shelf is roughly equal to its height above the work plane. To reduce cooling loads and solar gain, an exterior light shelf is the best compromise between requirements for shading and distribution of daylight.

The design requirements for a shading device depend on a building's use and local climatic conditions. In typical office buildings, for example, heating is rarely required because of the heat gains from people and equipment. In this case, it makes sense for the window shading to completely protect the windows year-round to avoid unwanted heat gain.

In residential projects, however, the shading may need to fully shade the windows during the summer months. In winter, the shading should expose windows as much as possible to direct sunlight to take advantage of the heat gain.

Use the Revit sun path tool or the Sun and Shadows Settings dialog to study the proposed shade depth for all orientations on Summer Solstice, Winter Solstice, Autumn Equinox, and Spring Equinox.

In summer, shades prevent direct sunlight from entering the windows at noon.	In winder, shades do not prevent sunlight from penetrating the living space.

Because of low sun angles, the east and west windows are difficult to shade with horizontal overhang devices. Consider vertical fins, louvers, and smaller window sizes for projects that need to reduce unwanted solar gain on these orientations.

For the purposes of Revit conceptual energy analysis, you must add these types of shading devices manually. Use mass surfaces attached to the mass model. In Revit, the shade material must be opaque to cast shadows.

For 100% daylighting from traditional skylights, approximately 5% of the roof area should be skylights. However, the benefits of daylighting must be weighed against the unwanted effects of heat gain/loss through the skylights.

The skylight specifications depend on climate. In all climates, use skylights with a high visible light transmittance (Tvis or VLT). Hot climates should have a low Solar Heat Gain Coefficient (SHGC). Cooler climates should have low U-value. Tubular skylights require a lower skylight-to-roof ratio (SRR) than traditional skylights, approximately 1-2%.

As a general rule, calculate the area of one skylight as follows, where SRR is the skylight-to-roof ratio:

(Floor to Ceiling Height x 1.5)² x SRR

Choose a size that is appropriate for the project. Starting with 5% SRR, modify this value depending on climate and building use.

For example, with a 12-foot ceiling and 5% SRR, the correct size for a skylight is approximately (12 x 1.5)² x 5% = 16.2 square feet. Therefore, the project should use 4' x 4' skylights.