Keep a solar house cool in summer by blocking the sun with overhangs and shade, closing the house in the heat of the day, and opening it at night to flush the stored heat out with cooler air. Ten air changes per hour in a 52 cubic metre room with air 6 degrees cooler than the room carry away about 1 kilowatt, roughly 8 kilowatt-hours over an 8 hour night.
Part of Passive Solar Building Design
Passive ways of keeping a sun-heated building from overheating: shade, ventilation, night flushing and ground cooling.
A Winter House Becomes an Oven Unless It Is Designed for Summer
The carrier warns that a building designed only for winter heating becomes an oven in summer, and that cooling must be built in from the design phase. The strategies work in an order:
- Keep the sun out — overhangs, shutters, trees, vines. See Overhang Design for Seasonal Control.
- Do not let in hot air — close windows and shutters in the heat of the day if the outdoor air is warmer than indoors.
- Flush with night air — open the house when the outdoor air is cooler than the mass.
- Use the ground — earth tubes and earth coupling. See Earth Tubes & Ground Coupling.
The direction matters. Ventilation helps only when the outside air is cooler than the inside. In the afternoon, when the outdoor air is hot, opening the house makes it hotter. At night, opening it cools both the air and the mass, which then absorbs the next day’s heat.
What You Need
- Openings you can open and close — windows and vents on opposite sides, high and low. See Cross-Ventilation Design and Stack Effect & Thermal Chimneys.
- Insect screens or cloth — or you will keep windows closed at night.
- Thermal mass — to carry the coolness through the day. See Thermal Mass Principles.
Method: The Daily Cycle
Morning Close windows and shutters on the sun-facing side as the sun comes round. Keep the house closed.
Afternoon Stay closed while the outdoor air is hotter than indoors. Shade stays on.
Evening and night When the outdoor air falls below the indoor temperature, open openings on opposite sides and, if you can, low openings on the shaded side and high openings on the other.
Early morning Close up again before the outdoor air warms.
Worked Example
Night flush of the 5 m x 4 m x 2.6 m room (V = 52 m3), with the outdoor air 6 degrees cooler than the room, at ten air changes per hour (an assumed rate; see the cross-ventilation page for what openings give):
- Heat removed = 0.33 x n x V x dT = 0.33 x 10 x 52 x 6 = 1,030 W, about 1 kW.
- Over 8 hours: 1.03 x 8 = 8.2 kWh.
Compare the 2.9 kWh stored per cubic metre of concrete per 5 K swing (see Mass Materials: Stone, Earth, Water): one night’s flush at that rate is the heat of about 2.8 m3 of concrete cooled by 5 K. As the room and mass cool, the temperature difference falls and the flush slows, so this is the start-of-night rate, an upper bound.
Common Mistakes
- Opening windows in the afternoon heat — it brings heat in.
- Ventilating a house with no mass — the air cools, but there is nothing to hold the coolness.
- Forgetting security and insects — owners who cannot leave windows open do not flush.
- Shading only the glass — hot walls and roofs also pass heat; insulate and shade them.
Safety
Overheating is a medical risk
In heat waves, indoor air above body temperature can cause heat stroke, particularly in infants, the old, the sick and people who cannot move to cooler rooms. Know the coolest room, keep water available, and move vulnerable people to it. Open ground-level windows only where children cannot fall out.
What’s Next
- Learn next: Cross-Ventilation Design — how much wind can move
- Combine with: Stack Effect & Thermal Chimneys — cooling on calm nights