back - table of contents


Building Geometry

The geometries for the residential building models were based on the compact, one-story, "ranch style" house specified by Hastings (1977). The elevation and plan views of the house are illustrated in Figures 4a and 4b, respectively (crawl space not shown). These architectural drawings are intended mainly as illustrations. Some of the details depicted were not included in the computer models. Figure 5 shows a cross-section view of the house illustrating a pitched roof, ventilated attic, living space, and crawl space foundation. The conditioned floor area for the house was 109 m2 (1176 ft2) and the total glazing area was 11.8 m2 (127 ft2), or about 11 % of the conditioned floor space, Table 3. The roof overhang extended 0.4 m (16 in.) over the front and rear elevations.

Reflective Roof NIST Report 1998 Surface Area of Building Components

Reflective Roof NIST Report 1998 Elevations and Floor Plan

Reflective Roof NIST Report 1998 Cutaway of Building Construction

back - table of contents

While in practice, the type of foundation varies with geographic location, in the analysis a crawl space foundation was modeled for all locations to simplify the comparisons of results and to minimize the effect of ground-coupling with the structure. This simplification was considered acceptable since this study focused on the interaction between the attic and living space.

Building Envelope

Using the prescriptive requirements for the building envelope given in ASHRAE Standard 90.2¬1993, design values of thermal transmittance (U) were determined for each geographic location, Table 4. In this study, the duct system was assumed to be situated within the conditioned space. As noted in Table 4, values of U were generally comparable among locations, decreasing slightly for locations in colder climates. These requirements reflect a renewed interest by the authors of ASHRAE Standard 90.2-1993 in utilizing higher insulation levels for new residential construction in hot climates.

Reflective Roof NIST Report 1998 Prescriptive Criteria for Building Thermal Envelope Components

The required levels of thermal insulation for the exterior walls, floor, and ceiling were determined using the design requirements given in Table 4. Values of thermal resistance (R-values) were determined using guidelines in the 1993 ASHRAE Handbook of Fundamentals (ASHRAE 1993). The exterior wall cavities (88 mm, 3.5 in.) of the living space were insulated with R-2.6 m2-K/W (R­15 h.ft2.°F/Btu) high-performance glass-fiber blanket insulation. The exterior sheathing consisted of 25 mm (1 in.) rigid foam board, R-0.9 rri2-K/W (R-5 h.ft2-°F/13tu). The area between the floor joists (238 mm, 9.5 in.) was insulated with R-3.3 m2.KJW (R-19 h.ft2-°F/Btu) and between the ceiling joists with R-5.3 m2.K/W (R-30 h.ft2.°F/Btu) for Miami and R-6.7 m2.KIW (R-38 h.ft2.°F/Btu) elsewhere. The crawl space walls were not insulated. These levels of insulation comprise the "base case" of the model for each geographic location. Table 5 summarizes the building envelope construction details for the attic, conditioned living space, and crawl space, identified as Zone 1, 2, and 3 in the model.

Reflective Roof NIST Report 1998 Building Envelope Construction Materials


To satisfy the prescriptive criteria for glazing given in Table 4, the windows were simply assumed to be single glazing units for Miami, Phoenix, Birmingham, and Washington, D.C. and double glazing, low emittance units for Portland ME and Bismarck. The shading coefficients were selected from values given in Table 4. In practice, shading coefficients of 0.7 are achieved with window treatments (i.e., shades and/or draperies), coefficients of 0.5 with special coatings in addition to the standard window treatment. The total window area of 11.8 m2 (127 ft2) was slightly higher that the 11.6-m2 (125-ft2) prescriptive requirement of ASHRAE Standard 90.2-1993.

Thermal Properties

The thermal properties of the construction materials were taken from the  1993 ASHRAE Handbook of Fundamentals (ASHRAE 1993), Table 6. In accordance with ASHRAE 90.2-1993, the solar radiation absorptance of all exterior surfaces, except the asphalt roof shingles, was set to a fixed value of 0.5. The thermal radiation emittance of all exterior surfaces was set to 0.9.

Reflective Roof NIST Report 1998 Thermal Transmission Data for Building Materials

Site and Shading
The residential models were oriented with their elongated dimension parallel to an east-west axis and the pitched roof was modeled with an attached overhang, 0.4 m (16 in.) in length. In the analysis, only shading from attached surfaces such as the overhang was considered. Shading from the surroundings, such as other buildings, trees, etc., was not considered.

back - table of contents

Thermostat Settings

The house was modeled as three separate zones: a ventilated attic (Zone 1); a living space (Zone 2); and, a ventilated crawl space (Zone 3). The entire living space (Zone 2) was assumed to be conditioned as a single thermostatically controlled zone. The other zones were unconditioned and the air temperatures drifted in response to outdoor conditions. The temperature set points for the living space were specified in accordance with ASHRAE Standard 90.2-1993 (ASHRAE 1993). The heating thermostat setting was fixed at 20 °C (68 °F) from 6 AM to 11 PM (06:00 to 23:00) and 15.6 °C (60 °F) from 11 PM to 6 AM ( 23:00 to 06:00). The cooling thermostat setting was fixed at a constant value of 25.6 °C (78 °F). The conditioned space was maintained at the specified thermostat settings during the entire year. Natural ventilation, although beneficial during the cooling season, was not considered in the analysis.

Internal Loads

The living space was assumed to have a daily internal load (Q) of 47.7 MJ/day (or 5.0 W/m2) from lights, people, and equipment based on Equation (1) (ASHRAE 1993):

Reflective Roof NIST Report 1998 Internal Load Formula

The daily profile for the residences was determined based on the hourly fractional multipliers given in ASHRAE Standard 90.2-1993 (see Table 8-1, ASHRAE 1993), Figure 6. For the analysis, the day schedules of the lighting, people, and equipment were adjusted to provide an hourly internal gain heat profile, Figure 6. Other non-conditioning loads, such as water heating, were not considered.

Reflective Roof NIST Report 1998 Daily Internal Heat Gain Profile


Building Description Input

The building description data was prepared by the TARP building description processor (BDP). An example of BDP input files (for Miami) is provided in Appendix B. The temperature and loads convergence limits for the heat balance calculations were 0.05 °C (0.09 °F) and 0.01 W (0.01 Btu/h), respectively. The radiant interchange between room surfaces was computed using the mean radiant temperature network method (Carroll 1980) and the exterior and internal heat transfer coefficients were determined using the an algorithm that included forced convection, natural convection, and radiant interchange (Walton 1983). The ventilation rate for the attic (Zone 1) was varied at constant levels according following the experimental design. Infiltration rates for the living space (Zone 2) and crawl space (Zone 3) were fixed at constant infiltration rates of 0.5 114 (ASHRAE 90.2-1993) and 1.0 111 (Samuelson 1994), respectively.

back - table of contents