This course is part of the Custom Home Academy

Building Elevation Drawings
Building elevation drawings are needed to determine the overall height of the building and the zone location of the assembly itself.

All of this information will be used to determine design pressure (DP)—the maximum pressure that a building or structure is designed to withstand from external loads, such as wind or seismic forces (see the DP sidebar for a more in-depth discussion).

Window Assembly Size
Finally, architects need to submit window assembly size, including individual window size. The size and shape of the window units are important factors in wind-load calculations. The larger the window, the greater the wind load it will experience. The shape of the window can also affect the wind load, as curved or irregular shapes can create turbulence and increase wind pressure.

The total wind load on the mulled windows is calculated by adding up the wind pressure on each individual window unit and multiplying it by the area of each unit.

Once the wind load is calculated, architects can use this information to make adjustments to the design of the mulled window units so that they are strong enough to withstand the forces of wind. This may involve considering windows with more appropriate wind-load ratings, making sure they have the proper framing and mullion design, and ensuring that the installation meets local building codes and standards.

Photo courtesy of Marvin

By selecting energy-efficient windows that reduce both heat loss and solar heat gain, architects can help reduce energy costs for occupants and improve the comfort of indoor spaces.

GLAZING COATINGS TO IMPROVE ENERGY PERFORMANCE

According to a recent National Association of Homebuilders study, of all the efficient features one could have, 83 percent of homeowners found it essential or highly desirable to have energy-efficient windows, making it number one on the list of priorities.

When selecting window assemblies for big-glass designs, there are multiple factors to consider. For instance, while increasing the amount of glazing can increase heating and cooling demand, the daylighting achieved through more glazing can significantly reduce electric lighting demand. Reducing the amount of heat transfer between the interior and exterior of a building is one way to reduce heating and cooling costs and improve occupant comfort. The ability to control solar heat gain is also important. While some solar heat gain may be desirable in colder climates, where it can help reduce heating costs, excessive solar heat gain can be a problem in warmer climates, where it can increase cooling costs and make indoor spaces uncomfortable. High-performance glass can also be used to help boost energy efficiency and control solar heat gain.

Energy-efficient windows are typically designed to reduce both heat loss and solar heat gain, and may include features such as low-emissivity coatings, reflective coatings, and insulating gas fills between panes of glass. By selecting windows that address both issues, architects can help reduce energy costs for occupants and improve the comfort of indoor spaces.

Low-emissivity (low-E) coatings are designed to reduce the amount of heat that escapes through the glass in cold weather. Emissivity is a measure of how well the surface of an object absorbs and re-radiates (or gives off) thermal energy. Very reflective surfaces have a low emissivity, and duller objects that absorb heat have a high emissivity. Emissivity is measured on a scale ranging 0.0 to 1.0, with 0.0 being a perfect reflector and 1.0 being a perfect emitter, aka a “blackbody.” (A blackbody is a theoretical object in physics that does not really exist, but is a useful concept when talking about emissivity.)

Low-E coatings work by reflecting heat back into the room, rather than allowing it to escape through the glass. In addition to reducing heat loss in winter, low-E coatings can also help control solar heat gain in the summer by reflecting some of the sun's heat back outside. The microscopically thin, transparent metal or metallic oxide layers deposited on a glass surface can be selected from a variety of options with different levels of heat gain control to match different climate needs.

Here are a few low-E coatings architects can consider:

Low E1: Low E1 coating is a good choice for maximum solar heat gain (or maximum heat transferring into the house from the sun) and radiant heating properties (keeping heat on the side of the glass where it originated). This type of coating is generally used in Northern climates where heating is prioritized over cooling. This type of coating reaps maximum benefits when windows are positioned to receive direct sun exposure.

Low E2: Low E2 is the most common low-E coating since it works well across most geographic regions and climates. Low E2 with two metallic coatings balances less solar heat gain and improved radiant heating properties.

Low E3: Low E3 is used in applications where solar heat gain may be a concern. The coating uses multiple metallic layers for radiant properties similar to Low E2. It is most commonly used in Southern, sunny climates where cooling is prioritized over heating.

Low ERS: Low ERS is a higher-performing low-E glass with significantly improved U Factors. This coating is placed on the interior surface of the glass and can be touched. It is used in conjunction with Low E2 or Low E3 and helps reflect radiant heat back into the room. This means it will have a slightly cooler interior glass surface since the heat is not absorbed by the glass. The cooler surface may see a bit more condensation in cold climates versus traditional low-E glass make-ups.

Keeping building orientation, climate, and daylighting goals in mind, design professionals can choose architectural glass that will help their specific project obtain the most daylighting benefits while controlling glare and solar heat gain. The key terms to remember when specifying these systems include U-factor, solar heat gain coefficient (SHGC), and visual transmittance (VT).

U-factor: A measure of the heat flow through the window due to convection, conduction, and radiation. The higher the U-factor, the more heat is transferred or lost through the window. Low U-factors can help reduce heating and cooling loads. The typical range to look for is between 0.20-1.20.

Solar heat gain coefficient (SHGC): The ratio of the solar heat gain entering the space through the glazing to the incident solar radiation. SHGC is measured on a scale between 0 and 1; the lower the SHGC, the less heat is transmitted. Choose low SHGC when cooling loads are high and a higher SHGC for passive solar heating.

Visible light transmittance (VT): The percentage of visible light entering the space through the glazing system. VT is measured on a scale of 0 to 1; the higher the VT, the more light that is transmitted. Clear glazing has a VT of about 0.90. A high VT is desirable for effective daylighting.

Tints and low-emissivity, or low-e, coatings can impact both VT and U-factors. Typically applied to the glass during manufacture, these coatings limit the transmission of infrared and near-infrared wavelengths and so reduce the emittance of radiant heat.