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The text and illustrations from this page have been reprinted from Passive Solar Energy.

Every material and principle incorporated into passive solar design is common and in everyday use. The melting of an ice cube or the ability of a stone to stay warm long after sunset—these are the kind of considerations on which all passive design is based.

The trick is to learn the labels so that it is easier to understand and discuss. Then you can say "thermal mass" instead of having to say (each time you discuss the phenomenon) "the ability of a stone to stay warm long after sunset".

Air stratification
Air stratification
Then tendency of heated air to rise and to arrange itself in layers with the warmest air at the top.

British thermal unit (Btu)
A measure of energy. It is the amount of heat necessary to raise one pound of water one degree Fahrenheit. Heat loss and gain can be measured in Btus per hour and BTUs per year.


The transfer of heat between objects by direct contact.

Degree Day
A unit used to measure the intensity of winter. The more degree days there are in total for the season, the cooler the climate.

Evaporative Cooling
Evaporative Cooling
Natural cooling caused by water's ability to absorb heat as it vaporizes.

Layers of glass or plastic, used in windows and other solar devices for admitting light and trapping heat.

Materials that conduct heat poorly and thereby slow down heat loss from an object or space.

Mean Radiant Temperature
The average temperature you experience from the combination of all the various surface temperatures in a room: walls, floors,

Mean Radiant Temp.
ceilings, furniture, and people.

Movable insulation
Insulating curtains, shutters, and shades that cover windows and other glazing at night to reduce heat loss.

Natural convection
The movement of heat through the movement of air or water.

Phase Change
Phase-change storage materials
Meltable materials store heat when they change phase from solid to liquid form, and release that heat when they solidify. The heat released is called the "heat of fusion."
These materials require less mass (and volume) to store the same amount of thermal energy than more conventional heat-storage materials. Only small changes in temperature are necessary to induce the phase change.

Shiny surfaces for bouncing sunlight or heat to where it's needed.

A measure of the insulating ability of any material or group of materials such as make up a wall or ceiling. The higher the R-value the better the insulation and the slower the heat loss.

Measures for blocking out unwanted sunlight that can overheat the house.

Thermal mass
Thermal mass
Materials that store heat. Heavy dense materials—concrete, stone, and even water—store a lot of heat in a small volume (compared with most lightweight materials) and release it when needed.

Thermal radiation
The transfer of heat between objects by electromagnetic radiation.

Thermal Radiation
A measure of the rate of heat loss through a wall or other part of a building. It is the reciprocal of the total R-values present. The lower the U-Value, the slower the heat loss.

Windows let light (and heat) in (and out).

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