Older buildings were designed at a time when energy was inexpensive, and it may not have been economical to provide (what is now considered) an effective amount of insulation. A good portion of the envelope losses are through the ceiling space. This area is usually accessible, and it is easy to increase the amount of insulation there. Areas with colder winters and hotter summers should strive for at least R-50 insulation in the ceiling. Energy Star has a map with recommend R-values.
Attic insulation is especially important in cold climates: heat rises! In the summer, hot air forms a static thermal layer in the ceiling of top-floor rooms, acting to retard heat flow. That is, heat flows in direct proportion to the temperature difference, and a thermal layer of warmer air lowers the transport of heat from the room into the attic by reducing the temperature gradient.
Attic insulation is an easy DIY project. A typical modern home has R-25 insulation between the rafters. The wood rafters are exposed, however, and they form a “thermal bridge.” The R-value of the rafters is about R-15 (1.4 per inch of thickness), and so the net R-value of the ceiling assembly is less than that of the R-30 insulation; more like R-25. When you add a second layer of insulation, run it perpendicular to the rafters, covering them up, so that there are no thermal bridges through that layer.
Another option is to hire a contractor to put blown-in insulation into the attic. Be sure the product is fire-resistant.
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Excerpted from Going Green, available as an eBook at this list of eBook retailers online.
