The term direct solar gain simply means window placement and design. In warm climates, the largest windows should be on the north side to minimize heat gain in summer months but still allow plenty of natural light. In cold climates, large and insulating windows should be on the south side of the house. They will draw heat during the winter, and are easily shaded with awnings, trees and bushes, or window coverings in the summer.

EnergyStar-rated windows and many other models come with a label that lists four numbers. The U.S. Department of Energy recommends fitting south-facing windows with double- or triple-pane glass that has a solar heat gain coefficient (SHGC) of at least 0.6, a U-factor of less than 0.35 and a high visible transmittance (VT) to allow in plenty of light and reduce the need for artificial lighting. Also keep windows clean inside and out to maximize the incoming sunlight.