Where summer heat comes from

In summer, the heat flow from lesson 30 runs the other way. Heat moves from the hot outdoors into your cooler house.

It gets in from four main places:

• Sun through windows. Often the biggest single source, especially on east and west windows.
• The roof. A dark roof in full sun gets far hotter than the air.
• Hot outdoor air leaking in through gaps.
• Heat made inside, called internal gains: cooking, people, lights, a fridge, a computer.

Cooling without much power means cutting each of these, then flushing out what is left.

Shade first

Stopping sun before it reaches the glass works far better than blinds inside. Once sunlight passes through a window, most of its heat is already in the room (DOE).

• Overhangs over south windows block high summer sun and let low winter sun in. That is the passive solar idea from lesson 10.
• East and west windows get low morning and afternoon sun that overhangs cannot stop. Use exterior shades, awnings, trellises, or fewer windows on those sides.
• Deciduous trees shade the house in summer and drop their leaves to let winter sun through. Keep them clear of solar panels.
• A light-colored roof and good attic insulation cut heat from above.

Thermal mass and night flushing

Thermal mass is heavy material, like concrete, stone, brick, tile or water, that soaks up heat slowly and gives it back slowly. It evens out temperature swings.

Night flushing pairs thermal mass with cool night air. You open windows or run a fan at night to pull cool air through. The mass cools down. In the morning you close up and shade the house. The cool mass then soaks up heat through the day.

It only works when nights are cool. Night flushing does best in dry places where the gap between day and night temperatures, the diurnal swing, is large. One design guide suggests about 20 °F (11 °C) or more. Hot, dry inland and high places tend to have large swings. Humid places often have warm, sticky nights.

Fans cool people, not rooms

A fan does not lower the air temperature. Moving air helps sweat evaporate, so you feel cooler.

DOE notes that a ceiling fan can let you set a thermostat about 4 °F higher with no loss of comfort. A ceiling fan uses a small fraction of the power of an air conditioner.

Because fans cool people and not rooms, turn them off when nobody is there. A whole-house fan is different: it pulls large amounts of air through the house at night, for night flushing.

Humidity changes everything

Humidity is the amount of water vapor in the air. Damp air makes the same temperature feel hotter, because sweat evaporates slowly.

• An evaporative cooler, sometimes called a swamp cooler, cools air by evaporating water into it. It uses little electricity and works well in hot, dry climates. In humid climates it adds moisture you do not want, and it barely cools.
• In humid climates, removing moisture matters as much as lowering temperature. Damp indoor air also feeds mold, as in lesson 11.

So the same trick can be smart in one climate and wrong in another.

What works where

When you need a machine

In some climates, shade and fans are not enough. Long heat waves, humid nights and older people or infants in the home can all make real cooling necessary.

A small, efficient ductless mini-split heat pump often cools a small home with modest power. It also removes moisture. Off the grid, the timing helps: cooling demand peaks on sunny afternoons, when solar output is high.

Do the passive steps first either way. They shrink the machine you need.

Check

Summary

• Keep heat out first: shade windows from outside, use a light roof, and air seal.
• Night flushing with thermal mass works where nights are cool and dry.
• Fans cool people, not rooms. Evaporative coolers suit dry climates only.
• Where heat and humidity are high, a small heat pump may be the honest answer.

Next, you will look at hot water, often one of the largest energy uses in a home.