 that it would not completely overcome the negative impact of the wall insulation unless ventilation was forced, as with a whole-house fan. Also, according to Parker, many people in humid regions air condition their homes around the clock and never ventilate. Ventilation is possible, but only if interior humidity of 80% or even higher is acceptable.
Even in the hotter Phoenix climate, adding wall insulation may not becost-effective. ORNL had modeled a proto-typical house in Phoenix with a central gas, forced-air furnace and air conditioner to estimate the combined heating and cooling savings attained through additional insulation. For an average retrofit cost of $3,900, the simple payback was calculated to be 32 years at 9.4e/kWh. Ternes explains, though, that the simple payback is reduced to 12 years if the homeowners were planning to restucco the house anyway and only the insulation cost of $1,500-$1,900 is considered.
Parker cautions that what is true of walls is not true of ceilings. Ceiling insulation in cooling climates is often exposed to very hot temperatures due to attic heat collection, often up to 130°F in the height of summer. Thus it is more universally desirable than wall insulation.
On the other hand, the retrofits all slowed daytime heat gain through walls, reducing peak cooling demand. This can be a major benefit, primarily to electric utilities but also to customers with time-of-day rates or those who have an opportunity to downsize their cooling equipment. The 15% demand reduction found by both studies is comparable to reductions achieved by replacing old air conditioners with high-efficiency units, which is often supported with utility subsidies.
Another benefit not captured in the analyses is the improved comfort resulting from lower interior wall temperatures after adding insulation. Lower radiant wall temperatures may allow residents to raise their setpoints comfortably, thereby saving energy. Ternes reports that several occupants observed marked improvement in the comfort of rooms on the south and west sides of the house that once overheated unbearably.
Optimally, says Parker, we would have dynamic walls where thermal resistance is adjustable throughout the daily cycle during the cooling season. Though some energy researchers have experimented with creative ideas, such as movable insulation and vacuum insulation, no practical system has as yet been developed.
In the absence of such a breakthrough, people who design and retrofit homes in hot climates will do well to examine the specifics of their situation in light of these studies.
Doug Johnson is a freelance writer based in San Francisco, California.