The third way energy is transferred is through radiation. This is evi­dent in the way the sun warms the surface of the earth, which involves the transfer of heat through electromagnetic waves and absorption of that energy by a surface. A person sitting in the sun by a window absorbs radiant heat. Inside a home, surfaces may exchange heat with other surfaces through radiation, which can have some impact on indoor ambient temperature. Heat energy from radiation is most rele­vant to home comfort in the summertime, however, when the roof and exterior walls absorb heat from the sun. (This heat subsequently enters the interior space through conduction and convection.) This process is more critical in hot climates.

Radiant heat transfer between objects operates independently of air currents and is controlled by the character of the surface (emissivity) and the temperature difference between warm objects emitting radia­tion and cooler objects absorbing radiation. Emittance (or emissivity) refers to the ability of a material’s surface to emit radiant energy. All materials have emissivities ranging from 0 to 1. The lower the emit – tance of a material, the lower is the heat radiated from its surface. Aluminum foil has a very low emittance, which explains its use in reflective insulation. This will be further explored in Chap. 12.

Reflectance (or reflectivity) refers to the fraction of incoming radiant energy that is reflected from a surface. Reflectivity and emissivity are related, and a low emittance is indicative of a highly reflective sur­face. For example, aluminum with an emissivity of 0.03 has a reflectance of 0.97.

The resistance of these modes of heat transfer may be retarded by the elements of a building wall section. Elements include

1. Outside surface films. The outside surface traps a thin film of air, which resists heat flow. This film varies with wind velocity and sur­face roughness.

2. Material layers. Each layer of material contributes to the resistance of heat flow, usually according to its density. A layer of suitable insu­lation is normally many times more effective in resisting heat trans­fer than the combination of all other materials in the section.

3. Airspace. Each measurable airspace, as well as its thickness, also adds to the overall resistance. Foil-faced surfaces of low emissivities that form the boundaries of the airspace can further reduce the rate of radiant transfer across the space (provided the airspace is at least 3/4" to Iм).

4. Inside surface film. The inside surface of a building section also traps a thin film of air. The air film thus formed is usually thicker because of much lower air velocities.1