When Wall Insulation Doesn't Save
Wall insulation clearly saves heating energy, but does it save cooling energy as well? Sometimes yes, sometimes no is the conclusion of a recent study in Florida, which clarifies the variables that determine wall insulation's usefulness in a warm climate.
The study, conducted by the Oak Ridge National Laboratory (ORNL) and the Florida Solar Energy Center (FSEC), focused on concrete block houses on slab foundations. Block construction is common in warm climates, especially where termites are a problem. As FSEC's Danny Parker points out, half of Florida's six million residences are built of concrete blocks and have little or no wall insulation.
Mark Ternes and his colleagues at ORNL had performed field tests on wall insulation for eight masonry homes in Phoenix, Arizona in 1993 (see Cooling Benefits from Exterior Masonry Wall Insulation, HE Mar/Apr '94, p. 33). They found some energy savings, but wanted to test the insulation effects in the more humid Florida climate.
In the latest study, ORNL and FSEC tested two homes in Cocoa, Florida. The researchers retrofitted occupied single-family homes with exterior wall insulation. They hired contractors to install commercial exterior insulation and finish systems (EIFS) like those typically used on commercial buildings. In Arizona, they had fabricated the insulation systems on-site, using polystyrene foam board, wire lath, and stucco. In both studies, the insulation typically raised the thermal resistance of the walls from R-3 to R-13.
In Phoenix, where temperatures climb over ll0°F in the summer and rarely drop below 80°F in July and August, the added insulation slowed the heat gain through the walls and reduced the measured air conditioning requirements. On average, energy consumption dropped 9%.
The houses studied in Florida had more complicated results. Summertime temperatures are less extreme there, especially in coastal areas. For a significant portion of the daily cycle, in the evening and overnight, the outside air can be cooler than the desired indoor temperature (see Figure 1).
These evening hours overlap with times that residents are typically home and active. Internal gains from people and appliances become a significant load for the cooling system. This heat generated inside the house can be passively transferred out into the environment, but only as fast as the walls and windows will allow. During this period, Parker explains, the most poorly insulated building possible will lower the required degree of air conditioning, because it will lose internally generated heat to the outside most quickly. Added insulation actually traps unwanted heat and impedes natural cooling.
The researchers determined that whether insulation saves cooling energy depends significantly on the interior thermostat setpoint. If residents set their thermostat at 73°F, rather than at the 79°F setpoint shown in Figure 1, the outside air would be warmer than the desired inside temperature virtually all the time. Heat would only flow inwardly through the wall under these conditions, and slowing it down with insulation would be helpful throughout the day.
Figure 1. This graph shows the avaeage daily June temperatures in Orlando, Florida. In this example, the residents kept their thermostat set to 79º F. Although wall insulation saved energy during the day (when outside temperatures were higher than inside temperatures) it prevented the heat from escaping during the evening and nighttime hours (when it was cooler outside than in).
The study found exactly this. The researchers monitored the two Florida houses, one with a setpoint of 73°F and another with a setpoint of 79°F. Although the cooler house used more air conditioning energy than the warmer house, the insulation saved 9%-14% of its preretrofit use. By contrast, the air conditioning energy use in the 79°F home actually increased by 5% after adding wall insulation!
Ventilation (by opening windows) could add to natural cooling at night. However, when the researchers modeled the effects of open windows, they found 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.