South Glazing

Sun-facing glazing is window area on the side a building takes sun from - south in the northern hemisphere, north in the southern - sized so low winter sun reaches far into the rooms behind it. It is the one part of a passive solar design that adds energy; the mass inside, the overhang above and earth banked outside only store, delay or exclude what it lets in. Earth banked on a wall admits nothing, so this elevation carries the whole aperture.

Glass is a poor wall: a single pane loses heat about three times as fast as the brick it replaces and nine times as fast as insulated timber, around the clock, while collecting a few hours a day. So it is a hole in the envelope first and a collector second, and whether it pays turns on glass quality, area and shading. Guidance answers two questions and a design needs both: where the glass goes - 40 to 50 percent of the window area on the sun-facing side, as little as possible east or west - and how much, about 8 percent of floor area in a tight envelope, where the glass is the main way heat leaves.

The aperture is fixed and the seasons are not: the glass that pays in midwinter overheats the room in midsummer, so it is inseparable from a roof overhang that shades it while the sun is high, and from mass inside to even the gain out rather than leaving a hot afternoon and a cold night. Loss runs the other way after dark, worst on the clear cold nights following the best collecting days. East and west glass is harder: low morning and afternoon sun comes in under any overhang and must be shaded outside the pane - trees, vines, shutters or screens. Windows are a weak point in high wind too, met with shutters or impact-rated glass. Upkeep: keep the glass clean, since dirt on a collector costs energy; misting between panes means a failed seal and a replacement; condensation at the edges says the assembly runs colder than the room.