Part V: Passive Solar Homes and SunspacesChapter 8. Designing a Passive Solar House

South Glass and Overhangs: Getting the Ratio Right

How much south glass a room can use, and how an overhang lets in January and blocks June.

Difficulty: Intermediate
Cost: Planning only

published

South glass is both the heater and the weakest piece of the wall. A common planning band for a cold-climate direct-gain room is about 7 to 12 percent of the floor area it serves, in glass that actually faces the winter sun. A mild climate wants less. A leaky room wants the leaks fixed, not a higher percentage. More glass than you have mass and night insulation for buys a wider temperature swing, not a lower fuel bill.

A 200 square foot room at 10 percent is 20 square feet of south glass. That is one large window, not a glass wall. It is easy to overshoot when a catalog shows a whole elevation in glazing.

The overhang is how that glass stays welcome in June. At solar noon the winter sun is low and slides under a properly sized projection. The summer sun is high and strikes the top of the overhang instead of the glass. The sketch below does that noon geometry. It does not size a beam, and it does not shade the morning or the afternoon.

Difficulty

Level: Intermediate
Time: An hour with a tape and the calculator, longer if you are designing a real header
Skills: Measuring glass and not treating noon as the whole day

Cost estimate

Planning only

The sketch is free. Building an overhang is structure, flashing, and a permit in many towns. Price that with a carpenter, not with this page.

Materials

Item	Qty	Notes
Floor area of the room the glass serves	Square feet	Only the room that receives the sun, not the whole house, unless the whole house is one sunlit space.
South glass area	The glazed opening, not the whole window unit	Frame does not collect. Count what you can see through.
Latitude, glass height, gap, and overhang depth	Inches and degrees	Gap is from the underside of the overhang down to the top of the glass.

How much glass

Divide the south glass by the floor area. If you are under about 7 percent in a cold climate, you are leaving winter sun on the table only if the wall actually sees the sun. If you are over about 12 percent, stop and account for mass and for night cover. A tight, well-insulated house should sit toward the low end of the band. The sun has less loss to overcome, and the same gain overheats the room sooner.

Glass is a poor wall at night. A decent double pane is roughly R-3. The wall beside it may be R-20 or more. Every extra square foot of glass is a hole you patch with an insulated shade or with a heating system. Put night insulation in the design when you choose the area, not after the first January bill.

Ask for two numbers when you buy the glass. Solar heat gain coefficient says how much sun gets in. U-factor says how fast heat leaves. South glass for heating wants a high heat-gain coefficient, often clear double glazing rather than a coating meant to reject the sun. You still want a low U-factor. A low-e coating sold for a west window in a hot climate can be the wrong pane for a south window in a cold one. "Low-e" by itself is not a specification.

East and west glass is not a place for this overhang trick. The sun is low there for hours. Use less of it, or shade it with vertical screens and trees from the shade guide.

Winter noon still reaches the glass. Summer noon hits the overhang. East and west need a different shade.

The noon sketch, and what it leaves out

Noon altitude is about 90 degrees minus your latitude, adjusted by how far the sun is above or below the equator. At the winter solstice that offset is about 23.45 degrees below. At the summer solstice it is about 23.45 degrees above. A horizontal overhang casts a shadow down the glass. The calculator reports how many inches of glass stay in sun at those two noons.

A useful result leaves most of the winter glass lit and shades the summer glass. If winter is already shaded, the overhang is too deep or the gap is too small. If summer is still fully lit, the overhang is too shallow or the gap is too large. Glass that runs all the way to a high ceiling is hard to shade with a short eave. That is an argument for shorter south windows, not for a deeper porch that darkens January.

The sketch is due south at solar noon. By mid-morning and mid-afternoon the sun is lower and farther around, and it walks in under an overhang that was perfect at noon. Vertical fins, a porch return, or plants help those hours. A roof overhang alone will not.

Compute the glass fraction
South glass divided by the floor area of the sunlit room. Compare it with a 7 to 12 percent band, and aim low if the house is tight.
Specify gain and loss separately
A high solar heat gain coefficient and a low U-factor. Do not accept a solar-control coating on the south heating glass.
Measure height, gap, and depth
Run the noon sketch at your latitude. Adjust depth or gap until winter stays lit and summer goes dark at noon.
Plan the hours that are not noon
East, west, morning, and afternoon. Trees and vertical shade, not a longer horizontal overhang.
Hand the depth to someone who can build it
Ledger, snow, and flashing are structural. A number in inches is not a permit.

South Glass and Overhangs: Getting the Ratio Right

Difficulty

Cost estimate

Materials

How much glass

The noon sketch, and what it leaves out

South overhang at solar noon

Compute the glass fraction

Specify gain and loss separately

Measure height, gap, and depth

Plan the hours that are not noon

Hand the depth to someone who can build it

Further reading