Make the sun-facing glass about 7-12% of the building’s floor area in a moderate climate, up to 15% in a cold one, and keep east, west and north windows small. More south glass admits more winter sun but also loses more heat on winter nights and overheats the rooms on sunny days unless mass and shading balance it.
Part of Passive Solar Building Design
How much glass to put on the sun-facing wall, where to put it, and why the other walls get little or none.
Glass Is a Heat Gate That Opens in Both Directions
The carrier gives the sizing rule: sun-facing glass should be approximately 7-12% of the building’s total floor area for moderate climates, up to 15% for cold climates. It also states both failure modes: too little glass means insufficient heating, too much means overheating on sunny winter days and excessive heat loss on winter nights. This page uses Northern Hemisphere wording, with the sun-facing wall to the south; in the Southern Hemisphere that wall faces north.
The percentage is of glazed area, the clear glass that lets sunlight through, not the area of the frame and sash. A window is a hole in the insulation: at night it loses heat many times faster than the insulated wall around it, which is why glass beyond the rule makes a house colder through winter nights and hotter in sunny weather.
The Other Walls
- East and west windows — the carrier says to minimize them: they admit low-angle summer sun that is impossible to shade with overhangs. In summer the sun rises well to the north of east and sets well to the north of west, so it strikes east and west windows from a low angle in the morning and evening.
- North windows — small. They never receive direct sun and only lose heat.
What You Need
- The floor area — in square metres.
- True south — see Finding True South.
- Glass, if you can get it — see Glass Salvage & Handling; also Flat Glass.
Method: Size, Place, Check
Step 1: Compute the glass area Floor area x 0.07 to 0.12 for a moderate climate; up to x 0.15 for a cold one.
Step 2: Place the glass on the sun-facing wall Spread it across the wall so that the sunlit patch falls on mass: floor and wall, see Mass Placement & Sizing.
Step 3: Check how deep the sun reaches Direct sun reaches the floor a distance of (height of the glass above the floor) / tan(sun altitude) from the wall. This ignores overhangs, which shorten it.
Step 4: Plan the night and the summer Plan shutters or heavy curtains to close over the glass at night, and shade (see Overhang Design for Seasonal Control) and ventilation for summer.
Worked Example
A house with 80 m2 of floor area.
- 7%: 80 x 0.07 = 5.6 m2.
- 12%: 80 x 0.12 = 9.6 m2.
- 15% (cold climate): 80 x 0.15 = 12.0 m2.
So the glazed area should be 5.6 to 9.6 m2 in a moderate climate and up to 12 m2 in a cold one.
How far the sun reaches at 40 N at noon on the winter solstice (altitude 26.6 degrees, tan = 0.50), for glass with its sill 0.8 m and head 2.0 m above the floor:
- Reach from the sill: 0.8 / 0.50 = 1.6 m from the wall.
- Reach from the head: 2.0 / 0.50 = 4.0 m from the wall.
- The sun patch on the floor spans 1.6 m to 4.0 m from the south wall, and the patch is longer at higher latitude, where the winter sun is lower. At 48 N, altitude 18.6 degrees (tan = 0.337), the same window reaches 2.4 to 5.9 m.
A room deeper than the patch has a cold back. Overhangs shorten the patch, so the glass height and overhang have to be designed together.
Glass Substitutes
Salvaged glass is a priority resource. If no glass is available, the carrier lists oiled animal skin, oiled cloth or thin-scraped intestine; these transmit some light and give some insulation, though far less effectively than glass. Window Design puts oiled linen at roughly 40% of light passed and animal membrane at about 30%. Do not apply the glass percentages above to these: they admit less light and heat for the same area, and they are much weaker insulation. See Window Design.
Common Mistakes
- Counting window frames as glass — only the clear area admits sun.
- Glass without mass — the room swings hot by day and cold at night.
- Large west windows “for the sunset” — they overheat the house on summer afternoons.
Safety
Over-glazed rooms overheat
A room with a lot of glass, no shade and no night ventilation path can reach dangerous temperatures on a sunny day, and heat illness is a particular risk for infants, the old and the sick. Provide shading and a way to open windows on opposite sides or high and low for night cooling. Large panes also break and cut; handle and mount salvaged glass with care.
What’s Next
- Learn next: Overhang Design for Seasonal Control — shading the glass in summer
- Combine with: Thermal Mass Principles — storing what the glass admits