by Mark P. Ternes, Kenneth E. Wilkes, and Howard A. McLain

Mark P. Ternes, Kenneth E. Wilkes, and Howard A. McLain are researchers at Oak Ridge National Laboratory in Tennessee.

Oak Ridge National Laboratory conducted a field test on eight single-family houses in Phoenix, Arizona, to study a site-fabricated insulation system (see "Adding Exterior Wall Insulation,"). We performed the field test in cooperation with the Arizona Department of Commerce, the Salt River Project, Dow Chemical Company, Western Stucco Systems, and the City of Scottsdale. Our goals were to

• evaluate the potential for reducing air conditioning electricity consumption and demand in Phoenix by insulating the exterior masonry walls

• extrapolate results to other U.S. climates by performing a simulation study in parallel with the field test

• develop and refine the exterior insulation system.

Our sample consisted of single-story, single-family detached houses constructed between 1960 and 1970 and ranging between 1,120 ft² to1,585 ft² in size. Their uninsulated exterior walls were constructed of concrete block, and the uninsulated interior walls between the living spaces and garages were constructed of wood-framed drywall. All houses had wide front and rear porch roofs, which significantly shaded much of the window area, and were cooled by single central air conditioners. The annual cooling degree days for Phoenix are 3,508 (base 65deg.F) and 2,797 (base 70deg.F). For comparison, annual cooling degree days (base 65deg.F and base 70deg.F) are 4,038 and 2,613 for Miami, Florida, and 1,589 and 778 for Atlanta, Georgia.

Retrofit Costs

Total retrofit costs ranged from $3,610 to $4,550 per house, averaging $3.34 per ft² of exterior wall area to be covered with insulation (which, for aesthetic reasons, included garages, attic gables and other unconditioned spaces), including a cost of $160-$200 per house to insulate the interior framed walls. Approximately 40% of retrofit costs went for materials--with insulation the single most costly item--and the two coats of stucco accounted for most of the labor costs. As a matter of fact, stuccoing accounted for about half of the retrofit costs. The incremental cost of insulating a house--in addition to stuccoing costs--ran between $1,500-$1,950.

To reduce costs, we opted for a site-fabricated exterior insulation system using common building materials. Although commercial exterior insulation and finish systems are available, they cost about $5 per ft². These packaged systems typically include proprietary technology for attaching the insulation directly to the wall, use a fiberglass mesh rather than wire lath, and employ acrylic-based coatings rather than cement-based coatings.

Occupant Feedback

Although we had been concerned about how the houses would look after the retrofits, homeowners generally felt that the property value and appearance of their homes improved after the wall insulation was installed. Additionally, three occupants reported that overheating in rooms with south and west exposures vanished after the retrofits. One occupant had previously installed a second thermostat in the overheated room to try to alleviate the problem.

Savings Analysis

We estimated energy savings using a regression analysis and calibrated DOE-2 models. The regression model assumed that air conditioning energy consumption was linearly related to the daily average difference between indoor and outdoor temperature. The regression analysis estimated annual pre-retrofit air conditioning electricity consumption at an average of 5,499 kWh for the eight houses (see Table 1), which was reduced to 5,008 kWh following wall insulation installation, for annual savings of 491 kWh, or 9% of pre-retrofit consumption.

We estimated average pre- and post-retrofit produced peak demand at 3.61 and 4.26 kW respectively (see Figure 1), for a demand reduction of 0.65 kW (15% of pre-retrofit demand). We also calculated the effectiveness of adding external wall insulation for similar houses in a number of other southern cities, using the DOE-2 model. That analysis estimated the highest annual air conditioning energy savings--between 450 and 700 kWh (12-13%)--in Phoenix and Las Vegas, in contrast to estimated savings of less than 50 kWh in Miami and Southern California. Peak-hour demand reductions ranged from 0.25 to 0.7 kW, or 8-12%.

The modeling showed that the largest contributors to residential cooling loads in an extremely hot, dry climate were internal loads and heat gains through the exterior walls and glazing. Wall loads were much lower in other southern climates (especially coastal regions), so internal loads and solar heat gain through windows contributed more to total cooling load. In all cases, insulating the walls resulted in a much lower rate of heat transfer through the walls when the outdoor temperature exceeded the indoor temperature, but the added insulation also increased the retention of heat generated within the house when the outdoor temperature fell below the indoor temperature. In some locations--particularly in Miami--the addition of wall insulation actually increased the cooling load during the spring and fall.

Retrofit Economics

Using the DOE-2 model for a prototypical house in Phoenix, we estimated space heating fuel savings of 14 MBtu/year. Measured results indicated air conditioning savings of 491 kWh, for a total energy costs savings of $121 per year, or a simple payback of 32 years on an average $3,900 investment. If the home is being stuccoed anyway, the simple payback period drops to 12 years on a $1,500 investment (without those finishing costs). This simple analysis excludes a demand reduction of 0.65 kW, which directly benefits electric utilities and could benefit consumers if they use a non-traditional rate schedule or receive a utility rebate.

Although the measure is not cost-effective from the consumer's perspective, exterior wall insulation does produce air conditioning electricity savings and peak demand reductions in hot, dry climates. Improved house appearance, elimination of overheated rooms, and space heating energy savings can add to air conditioning electricity savings and demand reductions. Modeling performed by the Florida Solar Energy Center (FSEC) suggests that the performance of exterior wall insulation in hot, humid climates may be better than the results of this study indicate. Thus, we are considering repeating the experiment with FSEC in several Florida houses next summer to verify results.

The publication of this article in Home Energy and the primary research were supported by the U.S. Department of Energy's Office of Buildings Research, Existing Buildings Efficiency Research Program. The article is based on the draft report "Cooling Season Performance of Retrofitted Exterior Wall Insulation" by Mark P. Ternes, Kenneth E. Wilkes, and Howard A. McLain.

Figure 1. Predicted pooled air conditioning electricity demand with and without exterior wall insulation for the hottest day of an average weather year in Phoenix, Arizona.

We used inch-thick extruded polystyrene foam insulation boards for this site-fabricated exterior insulation system. As shown in Figure A, the insulation was installed by attaching 1.5-inch thick furring strips to the exterior walls, installing the insulation boards between the strips, with a second layer of insulation boards over the furring strips, attaching a wire lath, and finally, applying stucco. After the stucco had dried, it was painted a light color. The addition of the exterior insulation system increased the thermal resistance (R-value) of the walls from about 3 h-ft² deg.F/Btu to about 13 h-ft²deg. F/Btu. In addition to the primary exterior wall retrofit, the interior walls between the conditioned living spaces and the garages and utility rooms were insulated with blown-in cellulose.

Figure A. A schematic of the exterior wall insulation assembly.

                Annual pre-retrofit     Annual          Annual 
                  AC consumption      AC savings      AC savings 
House                 (kWh)             (kWh)            (%)
________________________________________________________________
  1                   8,225             1,319            16%     
  2                   6,955                81             1%     
  3                   4,379               539            12%     
  4                   3,124              -106            -3%     
  5                   4,950               306             6%     
  6                   7,073               516             7%      
  7                   4,387               413             9%     
  8                   4,902               861            18%     
Average               5,499               491             9%

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