Windows are holes in the envelope
Even a good window loses heat several times faster than a well-insulated wall of the same size. Every window is a trade: light, view and fresh air against heat loss.
But windows can also gain heat. Sunlight passes through glass, warms the floor and walls, and that warmth stays inside.
Passive solar design uses the building itself, its shape, windows and materials, to collect, store and spread the sun’s heat, with few or no moving parts.
Which side the sun is on
In the US, the winter sun stays low in the southern sky. The summer sun climbs high and rises and sets farther north.
- South-facing windows get the most sun in winter, and are easy to shade in summer because the sun is high.
- East and west windows get low morning and afternoon sun in summer. It is hard to shade and can overheat the house, especially on the west.
- North windows get little direct sun. They give soft light but mostly lose heat.
DOE guidance says passive solar glass should face within about 30 degrees of true south. True south differs from compass south in most places, so check the local difference.
Text description
A plan view of a rectangular house, north at the top, with its long side facing south. Windows are drawn on each wall. South: most of the glass, because the low winter sun shines in and the high summer sun is easy to shade. East: some glass, for morning light, though low summer morning sun is hard to shade. West: limited glass, because hot summer afternoon sun is hard to shade. North: small windows, which give soft light but get little direct sun and mostly lose heat. A shaded wedge below the house shows that passive solar glass should face within about 30 degrees of true south.
Reading a window label
Most windows sold in the US carry a label with these numbers:
- U-factor: how fast heat passes through the whole window. Lower is better for keeping heat in. Cold zones call for low U-factors.
- Solar heat gain coefficient (SHGC): the share of the sun’s heat the window lets in, from 0 to 1. A higher SHGC lets in more sun heat. A lower one blocks it.
- Visible transmittance: how much daylight comes through, from 0 to 1.
For passive solar in a cold zone, people often choose a higher SHGC on south windows and a lower SHGC on east and west windows. In hot zones, low SHGC on most windows usually helps more. Ask your supplier for values suited to your zone and each side.
Storing the heat
Sun through glass arrives in a few hours around midday. The house needs heat all night. Thermal mass bridges that gap.
Thermal mass is heavy material, like concrete, stone, brick, tile or water, that soaks up heat slowly and gives it back slowly.
- Mass works best when the sun falls directly on it, such as a dark tile floor on a slab.
- Mass buried under carpet or far from the sun does little.
- Too much glass and too little mass makes a house hot by afternoon and cold by dawn.
Text description
A chart of one sunny winter day from midnight to midnight, shape only, not to scale. A tall, narrow hump shows heat from the sun coming through south glass, from about 8 am to 4 pm, peaking at noon. A lower, wider hump shows heat given back by thermal mass, such as a concrete or tile floor in the sun. It starts in the late morning, peaks in the evening and tapers off through the night toward dawn. Night hours are shaded at both ends of the chart.
Overhangs and shade
An overhang is the part of the roof that sticks out past the wall. Over a south window, a well-sized overhang:
- Lets in low winter sun, which passes under it.
- Blocks high summer sun, which hits the top of it.
The right depth depends on your latitude and the window height. Designers use sun-angle charts or software to size it.
Text description
A section through a south-facing wall with a window and a roof overhang. The high summer noon sun, about 73 degrees above the horizon at 40 degrees north latitude, is blocked by the overhang, so the window is in shade. The low winter noon sun, about 27 degrees above the horizon, passes under the overhang, through the window and deep into the room, where it lands on a concrete floor that stores the heat as thermal mass. North is to the left, south to the right.
Other shade helps too. Deciduous trees, which lose their leaves in winter, can shade east and west sides in summer. Awnings, shutters and exterior shades work well because they stop the heat before it reaches the glass.
Good move or common mistake?
For a house in a cold or mixed US climate, is each choice a good passive solar move or a common mistake?
- Size the south overhang to block high summer sun
- Plant a row of evergreens just south of the house
- Use a lower SHGC on east and west windows
- Put most of the window area on the south side
- Add many skylights for winter light
- Add large, unshaded windows facing west
- Let sunlight fall on a tile floor over a concrete slab
- Add lots of south glass with no thermal mass inside
How much glass is enough
More south glass is not always better. Past a point, the house overheats on sunny days and loses heat on cloudy nights.
Check
A new cabin in zone 5 overheats on sunny October afternoons and is cold before dawn. The south wall is almost all glass, with a wood floor on joists. What is the best fix to explore?
Summary
- Passive solar design uses the building to collect, store and spread the sun’s heat.
- In the US, put most glass on the south, within about 30 degrees of true south. Limit west glass.
- U-factor: lower keeps heat in. SHGC: higher lets sun heat in.
- Thermal mass stores daytime heat for the night.
- Overhangs let in low winter sun and block high summer sun.
- Too much glass overheats a house and loses heat at night.
Next, you will see why a tight house needs planned fresh air.
Go deeper: the five parts of a passive solar design
Designers often describe passive solar design in five parts. DOE’s current guide groups the absorber with the mass, but the ideas are the same. The parts are the aperture (the south glass), the absorber (a dark surface the sun hits), the thermal mass (which stores the heat), the distribution (how the heat moves around the house, by air movement or radiation), and the control (overhangs, shades and vents that stop overheating). Leave out any one and the design works less well. In the southern hemisphere, all of this flips: glass faces north.