Passive Solar Window Design

Clear glass transmits up to 90% of solar radiation

Did you know that clear glass transmits up to 90% of solar radiation. It either absorbs or reflects only 10% of solar radiation.You would think since clear glass can transmit solar radiation in, it can also transmit it back. The beauty is that, it doesn’t, after solar radiation is transmitted through the glass and absorbed by the home, it is radiated again from the interior surfaces as infrared radiation.

Although glass allows solar radiation to pass through, it absorbs the infrared radiation. The glass then radiates part of that heat back to the home’s interior. In this way, glass traps solar heat entering the home. So that’s why many energy efficient homes have almost all glass walls or large windows.

Source: http://www.energysavers.gov

Passive Solar Window Design in Heating-Dominated Climates

In heating-dominated climates, major glazing areas should generally face south to collect solar heat during the winter when the sun is low in the sky. In the summer, when the sun is high overhead, overhangs or other shading devices (e.g., awnings) prevent excessive heat gain.

To be effective, south-facing windows usually must have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer.

Windows on east-, west-, and north-facing walls are reduced in heating climates, while still allowing for adequate daylight. East- and west-facing windows are limited because it is difficult to effectively control the heat and penetrating rays of the sun when it is low in the sky. These windows should have a low SHGC and/or be shaded. North-facing windows collect little solar heat, so they are used just to provide useful lighting.

Low-emissivity window glazing can help control solar heat gain and loss in heating climates.

Source: EERE, U.S. Department of Energy

Windows are an important element in passive solar home designs, which can reduce heating, cooling, and lighting needs in a house.

Passive solar design strategies vary by building location and regional climate. The basic techniques involving windows remain the same—select, orient, and size glass to control solar heat gain along with different glazings usually selected for different sides of the house (exposures or orientations). For most U.S. climates, you want to maximize solar heat gain in winter and minimize it in summer.

Source: EERE, U.S. Department of Energy