Insulation

Insulation is any material that reduces the rate of heat transfer by conduction. All building materials have some resistance to the movement of heat, but some, insulation, are more resistant than others. A common characteristic of insulation materials is low density (pounds/cubic foot, kilogram/cubic meter). Expanded polystyrene, which has a low density, has a higher thermal resistance than concrete, which has a very high density. Also, as the density of a material increases, for example, by compressing the material or adding water, the thermal resistance is reduced.

Some materials, such as wood, concrete, and certain insulation materials, are homogeneous; that is, they have a uniform consistency throughout. For these materials the insulating values are listed as per inch of thickness, Appendix IX. Other materials such as concrete blocks and insulation backed siding are not homogeneous. They may have holes in them, or they may be composed of more than one type of material. The insulating value of these materials is based on a thermal test of a sample, Appendix IX, and a different value must be used for each different thickness.

There are three common reasons for insulating houses and other structures, Figure 22.1 and Figure 22.2.

(1) Insulation reduces loss of heat inside any type of enclosure during the winter months, reducing the amount of supplemental heat required to maintain the temperature of the enclosure; (2) insulation helps reduce heat gain during the summer months, reducing the load on cooling equipment, Figure 22.1 and (3) insulation is used to manage the condensation point so that it does not occur within the

FIGURE 22.1. Advantages of insulation for reduction of the loss of heat in the winter or grain of heat in the summer.

: /

Cold

Warm

outside

moist

air

'y inside air

J Condensation

No insulation

Cold outside air

Adequate Insulation

Cold outside air

Warm surface

Warm moist inside air

Adequate Insulation

Warm surface

Condensation

Si

Cold

Warm

outside

moist

air

inside air

c c

Inadequate Insulation

FIGURE 22.2. Adequate insulation prevents moisture condensation during the winter.

wall or on the inside surface, Figure 22.2 Condensation is the transformation of water vapor to a liquid, which occurs when warm moist air is cooled or comes in contact with a cool surface. Condensation is explained in more detail in the next chapter on psychrometrics. Condensation may occur on the outside surface of a wall, Figure 22.2, or on any surface inside the wall if no vapor barrier is used and the insulation is inadequate for the temperature difference.

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