Appendix D

D.6.3 Designing High Performance Features and Systems

A passive solar design will work in nearly any climate, and would work well at Fort Carson. A passive solar building is designed to maximize the use of natural systems to maintain thermal comfort for the occupants. A passive solar building successfully integrates the site, the local climate and microclimate, the Sun, and local materials in order to minimize dependence on external energy sources. The term “passive” implies a conceptually simple approach that uses few, if any, moving parts or input energy, requires little maintenance or user control, and results in no harmful pollution or waste by-products. Because of the inclusive nature of passive solar design, considerations for passive solar design permeate the entire design process and become critical architectural elements.
The warm summers experienced at Fort Carson coupled with the intense high-altitude sunshine makes solar control of all fenestration one of the important design considerations. Uncontrolled solar gain results in high cooling loads, excessive illumination, and glare. The first strategy in passive cooling is solar heat gain avoidance, which can be achieved primarily through shading and glazing selection.

Use solar angle charts for the Fort Carson (Colorado Springs) latitude (38.8 °N) to design shading devices that block unwanted solar gain at specific dates and times. Glazing selection is also an important consideration in window design as it determines the visual, thermal, and optical performance of the window.

D.6.3.1 Solar Shading

SHADED ANGLES FOR A SOUTH ELEVATION. DIAGRAMS LIKE THIS ARE USED TO DETERMINE THE OPTIMAL SIZE AND LOCATION OF SHADING DEVICES. DEPENDING ON THE BUILDING TYPE AND INTERIOR SPACE, THE GOAL MAY BE TO PREVENT SOLAR GAIN IN SUMMER WHILE ALLOWING IT IN THE WINTER, OR TO PREVENT DIRECT SOLAR GAIN YEAR ROUND.

HORIZONTAL AND VERTICAL SHADING RATIOS.
tHIS FIGURE LISTS THE APPORPRIATE X/Y RATIOS FOR COMPLETELY SHADING A SOUTH-FACING WINDOW FOR VARIOUS MONTHS AT TWO DIFFERENT TIME RANGES. uSE THE LOWER PORTION OF THIS FIGURE TO DETERMINE THE APPROPRIATE AZIMUTH ANGLE FOR SHANDING AN EAST - OR WEST - FACING WINDOW AT VARIOUS DATES AND TIMES.

SHADING ANGLE EXAMPLE. tHIS FIGURE DEMONSTRATES HOW TO APPLY THE HORIZONTAL AND VERTICAL SHADING RATIO IN SIZING THE HORIZONTAL OVERHANG THAT WILL SHADE A SOUTH-FACING WINDOW FROM 9 A.M. TO 3 P.M., mARCH 21 THROUGH sEPTEMEBER 21.

The most effective solar shading devices are exterior to the building envelope. Shades and blinds located inside the building may be effective at controlling glare, but are not effective in reducing the solar gain entering the space. Consider light-colored surfaces on shading devices such as overhangs, louvers, or light shelves. These light surfaces can help bounce diffuse sunlight into the building. Diffuse daylight is ideal for providing lighting without glare.
Consider a deep exterior wall section that can be used to self-shade the window surfaces with overhangs and vertical fins. Move the plane of the glass toward the interior plane of the wall to get free shading from the wall thickness.
Solar shading is most easily and effectively handled on south and north elevations. One method of describing the horizontal overhang is the ratio of the horizontal projection (x) to the height of the window (y) below the horizontal projection. Horizontal overhangs can adequately shade south-facing windows. North-facing windows receive predominately diffuse solar radiation and indirect daylight, and therefore do not need overhangs.

East- and west-facing windows are the most difficult to shade. Early morning and late afternoon sun rays are approaching perpendicular to these windows, causing excessive heat gains and visual glare. Minimize use of east- and west-facing windows. When these windows cannot be avoided, carefully size and place them for daylighting and view purposes only. Use a combination of horizontal louvers and vertical fins to shade these windows as much as possible.

D.6.3.2 Shading Strategies

Glare control is another function of shading. Limit or protect the views of extremely bright exterior surfaces, such as parked cars and large paving or sand areas. The reflected glare from these surfaces can be visually uncomfortable.
Interior shading devices have limited solar control potential and they often depend on user control to function properly. More likely an occupant will set the shading device once and leave it in position for the remainder of the day. This is often the case on east- and west-facing windows where louvers or shades are drawn to keep out direct solar gain. When using an interior shade, select a light-color shade to minimize heat gain.
To maintain an exterior view while shading the window, consider fine mesh roll screens that reduce illumination and glare while allowing contact with the view. Another option is to use screens or louvers that operate upward from the window sill. This provides shade near the bottom of the window where it is often first needed while allowing an effective clerestory for daylighting.

D.6.3.3 Glazing Selection

Select insulated low-e glazing units to reduce thermal loads and provide better comfort in perimeter zones. Low-emissivity (low-e) coatings and argon between the panes can dramatically increase thermal performance. Low-e coatings also can be specified to shade a higher fraction of the heat-carrying infrared radiation, while permitting more visible light to pass through.
In general, spaces dominated by cooling loads should have glass with a low solar heat gain coefficient (SHGC), possibly with a reflective outer surface. Use glass with a higher SHGC in spaces dominated by heating loads to take advantage of passive solar heating. Always protect occupants of daylit spaces from glare and direct beam. Glazing optical properties, shading devices, glass area, and orientation are all highly interactive in terms of their effects on heating, cooling, and lighting loads. Simulation-based sensitivity studies are the best way to balance these effects.
Be aware that spaces having good daylighting designs are likely to become heating-dominated spaces; whereas without daylighting, they would be cooling-dominated spaces. Also, it may be necessary to vary the glazing visual transmittance, depending on the window orientation, space lighting conditions, and occupant lighting needs.

D.6.3.4 Guidelines for Good Window Design

Size all windows to provide the best daylighting.
Add additional windows for view glass. Frame views without overglazing the space.
Specify glazing properties to minimize heating and cooling loads, and maximize visual comfort.
Place external overhangs on sough-facing windows to prevent glare and summer solar gains. Depending on simulation results, some south-facing windows may be unshaded to allow for good daylighting.
Use interior shade devices to provide user control of glare. Windows intended to provide daylighting should have been designed to prevent glare. Do not use shading devices on these windows.

D.6.3.5 Glazing Properties

The choice of glazing materials for the various window orientations and functions is critical for both thermal and visual comfort. Consider these glazing properties when specifying windows:

GLAZING EFFECTS ON TRANSMITTED SOLAR ENERGY.

Visible Transmittance (VT or Tvis): percent of the visible spectrum striking the glazing that passes through the glazing. This value changes with angle of incidence. While it may seem desirable to maximize the visible transmittance for daylighting, doing so often results in excessive window brightness. In the clear, intense sunshine of Fort Carson, a reduced visible transmittance will often be the better option to maintain visual comfort in the daylit space. Lower transmittance glazing will also typically result in better distribution of daylight at a more appropriate illumination level.
Solar Heat Gain Coefficient (SHGC): ratio of total transmitted solar heat to incident solar energy. A value of 1.0 indicates that 100% of the solar gain enters the building. A value of 0.0 indicates no solar gain is entering the space. For most buildings at Fort Carson, a low SHGC is desired on east and west facades (less than 0.35). Windows shaded by overhangs on the south facade should have high SHGC (0.70 or greater). North-facing windows can typically have high SHGC values. Use the results of computer simulations to verify SHGC values for each building project.
Shading Coefficient (SC): ratio of solar gain of a particular glazing compared to the solar gain of clear single and double pane glazing and many tinted single pane glazing windows (term found in some older documentation). The lower the number, the less solar gain is admitted. SC = SHGC x 1.15.
Visible and Solar Reflectance: percent visible light or solar energy that is reflected from the glazing.
UV Transmittance: percent transmittance of ultraviolet-wavelength solar energy (0.30 to 0.38 microns). High UV penetration will fade fabrics and can damage sensitive artwork.
U-Value: measure of the rate of conductive heat transfer through the glazing due to a temperature change between inside and outside surfaces. Often given in a winter night Uvalue and a summer day U-value format. The lower the U-value, the better the thermal resistance of the window. Current window U-values are a composite of three heat transfer components of a window: the center of glass, the edge of glass, and the window frame. The total window U-value should be less than 0.35 for Fort Carson buildings. U-value is the inverse of R-value (U = 1/R).