Not that kind of envelope

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When it comes to creating sustainable spaces, never underestimate the importance of the building envelope—the walls, windows, roof, floor and foundation. This envelope forms a thermal barrier between the inside and outside and has the greatest effect on a building’s heating load—the heat that escapes during the heating season—and cooling load—the heat that enters a home during the cooling season. To maintain a comfortable temperature inside the home, furnaces and air-conditioners must “satisfy” these respective loads.

Hot stuff
Understanding heating and cooling involves the study of thermodynamics and heat transfer. (Don’t worry, this will be painless!) Thermodynamics is the fundamental study of energy and gives us definitive laws that govern energy transfer and provides mathematical relationships between quantities such as heat, work, pressure, temperature and volume. Heat transfer provides laws that specifically address material properties and the three modes by which heat is transferred: conduction, convection and radiation.

Conduction is the diffusion of heat through a material due to atomic and molecular motions; it is most important in solid materials, and its magnitude is determined by a property known as thermal conductivity. Pure metals and insulating materials are on opposite ends of the thermal-conductivity spectrum.

Convection is the heat transferred between a solid surface and a moving fluid such as air or water. It’s what your body feels when you jump into Sycamore Pool on New Year’s Day. Brrrrr!

Thermal radiation is the heat transferred between objects by electromagnetic waves, often called infrared rays. Radiation readily passes through space and our atmosphere, but is quickly absorbed and “blocked” by most liquids and solids (except thin transparent materials such as glass). The warmth of solar radiation is very familiar to us, but you can also feel the effects of radiation cooling when you stand in front of a picture window on a cold day—because the window is much colder than your skin.

Bad transfer
During the summer and winter, there is always heat transfer between the conditioned space of our homes and the surroundings due to the temperature difference. While we can never eliminate heat transfer, we can reduce it by using insulating materials in the building envelope. Insulation has a large thermal resistance—more commonly known as a high R-value. An R-value is equal to the thickness of the material divided by its thermal conductivity. For example, a 31/2-inch layer of fiberglass has an R-value of approximately 13 (R-13)—the insulation most commonly used in 2-by-4 stud walls.

A superior but more expensive insulation is polyisocyanurate (closed-cell plastic foam)—a 1-inch-thick board with an R-value of R-8! This insulation is particularly effective when only a small amount space is available. Some foam boards have a reflective foil on one side to reduce radiation heat transfer, known as a radiant barrier. When the foil side is placed next to an air space (such as an attic or wall cavity), an additional thermal resistance equivalent to about R-2 can be gained. For a good resource on different types of home insulation, visit http://apps1.eere.energy.gov/consumer.

Next time:
Where and how much insulation? And the law of diminishing returns.