How did we estimate the savings shown in this fact sheet?2
It's not possible to test radiant barriers in every city and in every type of house. To understand their performance in other locations, the attic simulator computer program, AtticSim, was developed to calculate the radiative, convective, and conductive energy exchanges in a specific attic geometry, with or without ducts.ix,x This model has been benchmarked against experimental data from controlled laboratory experiments, showing excellent accuracy for attics without ducts and moderate accuracy for attics with ducts.iii, xi, xiiThe attic model requires that the air temperature below the attic floor and the temperature and timing of air entering the ductwork be specified. To provide these values, a whole-building energy model, EnergyPlus, was used. This whole-building model includes leaking attic ducts and radiant energy exchange within the attic, but does not yet include radiant exchange between the attic surfaces and the duct surface.xiii These programs were coupled by using the same physical geometry and materials, the same weather data, and the same rate of duct leakage to calculate the energy lost and gained in the attic for each combination of duct condition, attic insulation level, and radiant barrier use. The building model shown here was used as a base building for this study.xiv The building is a 57 ft × 27 ft single-story house with one conditioned zone, an unconditioned attic, and a vented crawl space.
Figure 3 Schematic of house used in EnergyPlus and AtticSim
The analysis was performed for eight cities, representing the eight ASHRAE climate zones.xv For all climate zones, an interior 21°C (70°F) heating set point temperature and 24°C (75°F) cooling set point temperature were used. Two levels of building quality were evaluated, one with adequate ceiling insulation (new), and one with minimal insulation (old). The new homes were taken to have code-level insulation, corresponding to R-30 for climate zones 1 to 3, R-38 for climate zones 4 and 5, and R-50 for climate zones 6 to 8. An attic insulation level of R-19 was used for the older home in all climate zones. The study considered three cases for attic ducts, representing situations with no ducts (and therefore no duct losses), insulated and relatively tight ducts, and uninsulated leaky ducts.
To estimate the energy savings attributable to radiant barriers, four values of emittance (ε) for the downward-facing side of the interior attic space and the gable ends were considered; 0.05, 0.1, 0.2, and 0.9. The upward-facing floor of the attic (that is, the top surface of the attic floor insulation) was given an emittance of 0.9. The building thermal load with no radiant barrier (ε = 0.9) was compared with the thermal loads with ε = 0.05, 0.1, and 0.2 to calculate the radiant barrier energy savings.
To evaluate the potential economic savings due to radiant barriers, state average fuel prices and representative HVAC system efficiencies were applied to the calculated energy savings. For heat pumps and air conditioners, the seasonal efficiencies required in the 2006 Department of Energy standards were used, a Seasonal Energy Efficiency Ratio of 13 and a Heating Season Performance Factor of 7.7, to translate energy savings to electricity savings. For gas furnaces, an efficiency of 0.85 was assumed. This table shows the energy prices used for each analysis location. For climate zones 1 to 6, all-electric heat pumps were used. For climate zones 7 and 8, gas heat with electric air conditioning was used.
The results of the parametric evaluation showed that the savings estimates are most sensitive to the climate, then the presence and condition of the ductwork, and third – the effective emittance of the downward facing surface of the roof sheathing. Analysis shows that the effective emittance of the downward-facing roof surface is very similar for roof sheathing materials with a foil-covered interior surface, and liquid-applied low-emittance coatings. Furthermore, the savings for an emittance ranging from 0.05 to 0.2 were very similar, so consumers are advised that these two approaches, as well as the use of aluminum foil or metalized film-faced materials stapled to the bottom surface of rafters, should provide similar savings.
When looking at these results in Fig 4, keep in mind that they represent a single, simple, house geometry and that duct conditions in real houses can vary widely and are seldom well characterized. Factors that could make your savings larger than the ones calculated would be: a summer thermostat setting lower than 24°C (75°F), lower efficiency air-conditioner or heat pump, and higher fuel prices. Factors that could make your savings less than the ones calculated would be: a summer thermostat setting higher than 24°C (75°F), light colored roof shingles, shading of the roof by trees or nearby structures, higher efficiency furnace or air-conditioner, and lower fuel prices.
Table 2. Energy Prices Taken From EIA 2008 State Average Residential Retail Prices
Zone |
City |
Electricity |
Natural gas |
1 |
Miami, FL |
11.65 |
21.29 |
2 |
Austin, TX |
13.04 |
13.79 |
3 |
Atlanta, GA |
9.93 |
18.5 |
4 |
Baltimore, MD |
13.84 |
16.05 |
5 |
Chicago, IL |
11.07 |
12.09 |
6 |
Minneapolis, MN |
9.74 |
11.3 |
7 |
Fargo, ND |
7.51 |
10.34 |
8 |
Fairbanks, AK |
16.55 |
8.72 |
Table 3. Time-of-Day Prices
| City |
Off-Peak price |
On-peak period |
On-peak price |
Miami, FL (FPL, Rate code RS-1) |
$0.074/kWh |
Nov.1 to March 31: |
$0.13396/kWh |
Austin, TX (Experimental voluntary residential summer time-of-use rate, 02/17/2010) |
$0.0456/kWh |
May 1 to Oct. 31: |
$0.1825/kWh |
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2This material is adapted
from a paper to be presented at the Building Envelope Conference in 2010.(Stovall et al, 2010)