A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Evaluation of Affordable 

Prototype Houses at Two Levels 

of Energy Efficiency 

Preprint 

R. Hendron 

National Renewable Energy Laboratory 

E. Hancock and G. Barker 

Mountain Energy Partnership 

P. Reeves 

The Partnership for Resource Conservation 

Presented at the 2006 ASHRAE Annual Meeting 

Quebec City, Canada 

June 24–28, 2006 

Conference Paper 

NREL/CP-550-38774 

October 2006 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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An Evaluation of Affordable Prototype Houses at 

Two Levels of Energy Efficiency 

Robert Hendron, Associate Member, ASHRAE; Ed Hancock; Greg Barker; Paul Reeves 

KEY WORDS 

Building envelope 

Heat recovery units 

Radiant heating equipment 

Residential buildings 

Solar equipment 

ABSTRACT 

Two high-performance prototype houses were built in Carbondale, Colorado, as part of the U.S. 

Department of Energy’s Building America (BA) Program. Each prototype was a 1256 ft2 (117 m2

), one-story, 

three-bedroom house and met the local requirements for affordable housing. The authors, representing the 

National Renewable Energy Laboratory, performed short-term field testing and DOE-2.2 simulations in support 

of this project at the end of December 2004. We also installed long-term monitoring equipment in one of the 

houses and are currently tracking the performance of key building systems under occupied conditions. One of 

the houses (designated H1) included a package of cost-effective energy-efficiency features that placed it well 

above the Energy Star level, targeting a Home Energy Rating System (HERS) score of 88-89. The other 

(designated H2) was a BA research house, targeting a HERS score of 94-95 and 45% whole-house energy 

savings compared to the BA Benchmark. The floor plans and other basic characteristics of the two houses were 

nearly identical except for the extended package of energy efficiency measures in the H2, including a 1.6 kW 

(5500 Btu/hr) photovoltaic system, a combination solar hot water and radiant space heating system, heat 

recovery ventilation, and orientation specific glazing. Preliminary results from the field evaluation indicate 

that the energy savings for both houses will exceed the design targets established for the project, although the 

performance of certain building systems, including the ventilation and foundation systems, leave some room for 

improvement. 

INTRODUCTION 

Two high-performance prototype houses were constructed in Carbondale, Colorado, in December 2004 as 

part of the U.S. Department of Energy’s Building America (BA) Program. Both houses were 1256 ft2 (117 m2

), 

one-story, three-bedroom designs, and each one met the local requirements for affordable housing, which 

limited the sales price to about $220,000, an amount affordable to a family earning less than 80% of the median 

income in Garfield County. 

One of the prototype houses (designated H1) was treated as the “base case” for the purpose of side-by-side 

testing, even though its energy efficiency was well above the Energy Star level, with an estimated Home Energy 

Rating System (HERS) score of 88-89. The other (designated H2) was the BA prototype house, targeting a 

HERS score of 94-95, and 45% whole-house energy savings compared to the BA benchmark (Hendron 2005). 

The floor plans and other basic characteristics of the two houses were nearly identical except for the expanded 

package of energy efficiency measures in the H2. A list of key specifications for both houses is provided in 

Table 1. Figure 1 shows both prototypes as viewed from the southwest. More detailed design specifications 

and trade-off analysis can be found in the BSC deliverable report about this project (BSC 2004). 

The authors, representing the National Renewable Energy Laboratory (NREL), visited the site in late 

December 2004 and performed a series of short-term field tests. The test results were used as inputs to hourly 

simulation models of each house for the purpose of calculating annual energy savings. The authors also 

installed long-term monitoring equipment in one of the houses in order to track the performance of certain key 

building systems over the course of one year. 

 1
Table 1. Specifications for H1 and H2 Prototype Houses* 

H1 (88-89 HERS Rating) H2 (94-95 HERS Rating) 

Building Envelope 

Attic R-53 hr·ft2·oF/Btu (R-9.3 m2·K/W) blown cellulose, 

14 in. (0.36 m) minimum, some cathedral ceilings 

R-53 hr·ft2·oF/Btu (R-9.3 m2·K/W) blown cellulose, 

14 in. (0.36 m) minimum, some cathedral ceilings 

Walls R-20.8 hr·ft2

·oF/Btu (R-3.7 m2·K/W), 2 x 6, 24 in. 

on-center (o.c.) optimum value engineered (OVE) 

with damp-spray cellulose; R-5 hr·ft2

·oF/Btu (R-0.9 

m2

·K/W) extruded polystyrene (XPS) foam 

sheathing 

R-20.8 hr·ft2

·oF/Btu (R-3.7 m2·K/W); 2 x 6 24-in. 

o.c. OVE with damp-spray cellulose; R-5 

hr·ft2

·oF/Btu (R-0.9 m2·K/W) XPS foam sheathing 

Rim joist Spray foam cavity insulation, R-10 hr·ft2

·oF/Btu 

(R-1.8 m2

·K/W) 2-in. XPS on outside 

Spray foam cavity insulation, R-10 hr·ft2

·oF/Btu 

(R-1.8 m2

·K/W) 2-in. XPS on outside 

Foundation Sealed conditioned crawl space, R-10 hr·ft2

·oF/Btu 

(R-1.8 m2

·K/W) 2-in. XPS on interior walls, 

concrete floor, supply registers to crawlspace with 

transfer grilles for return path 

Sealed conditioned crawl space, R-10 hr·ft2

·oF/Btu 

(R-1.8 m2

·K/W) 2-in. XPS on interior walls,

lightweight gypcrete slab on framed floor, transfer 

grilles from crawlspace to first floor 

Windows Low-E with high solar gain on south side, U = 0.36 

Btu/hr·ft2

·oF (2.0 W/m2·K), solar heat gain 

coefficient (SHGC) = 0.48; low-e spectrally 

selective on north and east sides, U = 0.33 

Btu/hr·ft2

·oF (1.9 W/m2·K), SHGC = 0.28; triple 

glazed on west side, U = 0.26 Btu/hr·ft2

·oF (1.5 

W/m2

·K), SHGC = 0.24 

Low-e with high solar gain on south side, U = 0.36 

Btu/hr·ft2

·oF (2.0 W/m2·K), SHGC = 0.48; triple 

glazed on north, east, and west sides, U = 0.26 

Btu/hr·ft2

·oF (1.5 W/m2·K), SHGC = 0.24 

Infiltration 2.5 in2 (0.0016 m2) leakage area per 100 ft2 (9.3 m2) 

envelope area 

2.5 in2 (0.0016 m2) leakage area per 100 ft2 (9.3 

m2

) envelope area 

Mechanical Systems 

Heat 92.1% annual fuel utilization efficiency (AFUE) 

sealed-combustion furnace, ducts in conditioned 

crawlspace, single control zone 

92% AFUE gas boiler, 80,000 Btu/hr (23 kW) 

capacity, radiant floor in lightweight slab, three 

control zones, solar assisted 

Cooling Alternate cooling strategies: ceiling fans, natural 

ventilation, one motorized window 

Alternate cooling strategies: ceiling fans, natural 

ventilation, one motorized window 

Ventilation Intermittent central fan integrated supply ventilation

with motorized damper, 33% duty cycle 

Continuous heat recovery ventilator (HRV) 

Domestic hot 

water (DHW) 

Direct vent water heater, 0.62 energy factor (EF) Gas boiler (with storage tank connection), EF 

estimated at 0.75-0.80 (not rated by the Gas 

Appliance Manufacturers Association, GAMA), 

solar assisted 

Solar hot water None Two 26.8 ft2

 (2.49 m2) panels on south awning, 

glycol loop, 105 gal (397 L) tank 

Photovoltaics None 1.625 kW (5540 Btu/hr) array on roof, 13 

modules,1.8 kW (6140 Btu/hr) inverter 

Lighting and 

appliances 

Compact fluorescent lighting (CFL) package, 

Energy Star appliances 

CFL package, Energy Star appliances 

* Items changed between H1 and H2 are shown in italics 

 2
SHORT-TERM TESTING 

Air Infiltration and Ventilation 

A local HERS rater performed blower door tests on both prototype houses on December 17, 2004. A 

motorized window was partially open during the H1 test and could not be easily closed because the location 

was high and the motor was not yet operational. The measured envelope leakage was 475 cubic feet per 

minute at 50 Pascal (cfm50) (224 L/s @ 50 Pa) for the H2 and 625 cfm at 50 Pa (295 L/s @ 50 Pa) for the 

H1. Both houses easily met the design target of 1200 cfm at 50 Pa (566 L/s @ 50 Pa), even with the 

partially open window in the H1. After taking altitude into account, the measured values of cubic feet per 

minute at 50 pascals converted to annual average infiltration rates of approximately 0.12 ach for H2 and 

0.15 ach for H1. 

A tracer gas monitoring system was installed in each house from Friday afternoon (December 24) until 

Monday morning (December 27) to measure the net air exchange rates with and without ventilation. By 

the time the tracer gas test started, the motorized window in the H1 had been closed. Figure 2 shows the 

measured hourly average air exchange rate expressed as air changes per hour (ach). During the test period, 

the winds were calm (0-3 mph) (0-1.3 m/s), but outdoor temperatures were frequently in the single digits oF 

(-18oC to -12oC). The occasional data gaps in Figure 2 occurred when tracer gas was being injected into 

the houses to maintain minimum concentration levels. During these periods, the ACH could not be 

calculated. 

From hour 0000 until hour 2200 on Saturday, the ventilation systems were operated in both houses. 

The air exchange rate in H1 with the intermittent supply ventilation system running at a 33% duty cycle 

was usually between 0.20 and 0.25 ACH. The air exchange rate in H2 with the heat recovery ventilator 

(HRV) operating was generally between 0.27 and 0.33 ACH. At 2200 on Saturday, both ventilation 

systems were turned off. Natural infiltration was typically between 0.05 ACH and 0.12 ACH for both 

houses, even when the inside-outside temperature difference was as high as 60oF (33oC). 

The ventilation rate recommended by ASHRAE Standard 62.2 (ASHRAE 2004) was 42 cfm (20 L/s) 

continuously or 84 cfm (40 L/s) at 50% duty cycle in the case of an intermittent system like the one in H2. 

Tracer gas measurements indicated that the increase in air exchange rate due to operation of the ventilation 

system in H1 was about 0.13 ach, or 29 cfm (14 L/s) based on an estimated conditioned volume of 13,400 

ft3

 (379 m3), including the conditioned crawlspace. The net ventilation rate of the HRV in the H2 prototype 

was about 0.21 ACH, or 47 cfm (22 L/s). Based on these measurements, it appeared that the H2 ventilation 

rate was consistent with ASHRAE 62.2, while the H1 was ventilated at a lower rate by design. 

Temperature Stability and Uniformity 

Comparisons of the thermal comfort in H1 and H2 were made from Friday afternoon through Monday 

morning. Of special interest was the difference in mean radiant temperature for the forced air heating 

system compared to the radiant slab heating system. Shielded and black globe temperature sensors were 

temporarily installed in the living room, master bedroom, and west bedroom of each house. Figure 3 shows 

a typical installation of the shielded and black globe sensors. The sensors were located 4 ft (1.2 m) above 

floor level near the center of the room. The shielded sensors represented the temperature of the air in the 

room without the influence of radiant heat transfer. The black globe sensors responded to both the room air 

temperature by convective heat transfer and the mean radiant temperature by exchanging heat through 

radiation with all surfaces in the room. The black globe temperature was meant to approximate the thermal 

comfort of a person in the room. It is important to note that black globe temperatures can vary significantly 

within a single room because the view factors to hot and cold surfaces sensors change depending on 

location. 

Figures 4 and 5 show the 5-minute average shielded air temperatures from December 26 (Sunday) 

through December 27 (Monday) for both houses. The temperatures in H1 (Figure 4) exhibited short-term 

variations as the forced air furnace cycled on and off in response to the thermostat. The short-term 

variation in shielded air temperature for each location in H1 ranged from 2oF to 4oF (1.1oC to 2.2oC), 

depending on the time of day and the room. The room-to-room temperature variation in H1 was as high as 

 3
6oF (3.3oC). This room-to-room non-uniformity was likely a result of the single thermostat attempting to 

satisfy disparate and dynamic thermal zones, and to insufficient air balancing and mixing among the rooms. 

The five-minute average shielded air temperatures in H2 (Figure 5) were significantly different from 

those in H1. H2 temperatures exhibited almost none of the short-term and room-to-room variation 

observed in H1. Three separate thermostats controlled the hydronic system in H2 and, as a result, much 

more uniform temperatures were achieved compared to H1. However, the H2 exhibited a more pronounced 

long-term temperature increase in response to late afternoon passive solar gains. The temperatures in 

rooms with high solar gains rose from 3oF to 5oF (1.7oC to 2.8oC) above their nominal set points. This was 

an interesting contrast to H1, which had nearly identical passive solar gains. It appeared that the concrete 

floor in H2 was less able to store the solar gains and moderate the air temperatures because it was already 

warm from hydronic heating. In other words, a greater fraction of incoming solar radiation may have 

reflected off the warm slab, instead heating the walls, ceiling, and indoor air. 

We next examined the difference between the shielded temperatures and the black globe temperatures 

during the time period shown in Figures 4 and 5. The black globe temperatures were typically less than the 

shielded temperatures at night and slightly greater than the shielded temperatures during daytime hours 

when passive solar gains were present. In the H1, the difference between black globe and shielded 

temperature was frequently greater than 1.5oF (0.8oC). In the H2, the difference was rarely greater than 

0.5oF (0.3oC), suggesting a potentially noticeable improvement in thermal comfort attributable to the 

radiant floor heating system. These results also suggested that the H2 thermostat could be set about 1oF 

(0.6oC) lower than the H1 during the heating season yet achieve approximately the same comfort level 

based on mean radiant temperature. Such a thermostat adjustment would translate to approximately 1% 

whole-house energy savings if acted upon by the occupants. 

Solar Water Heating Performance 

The solar collector heat flows were calculated by measuring the flow rate and temperature difference 

of the glycol mixture entering and leaving the collector. The collector efficiency at noon on sunny days 

during the test period was about 40%, which was fairly consistent with our expectations. 

However, there appeared to be significant reverse thermo-siphoning in the collector loop at night. 

There was a potential for this flow to establish itself at night because the hot storage tank was at a lower 

elevation than the cold solar collector. A check valve to prevent reverse flow was specified and installed 

but did not appear to be functioning correctly. This thermo-siphoning effect resulted in the loss of a 

substantial portion of the thermal energy collected during the day. 

A second check valve was installed in the system on Friday, March 25, to prevent reverse 

thermosiphoning. Figure 6 shows the subsequent change in hourly average collector inlet and outlet 

temperature. Before Friday, the collector inlet temperature dropped to around 50oF (10oC) at night while 

the collector outlet temperature rose to around 86oF (30oC). After Friday, the collector inlet and outlet 

temperatures stayed around 77oF (25oC), which was the temperature in the mechanical closet. It therefore 

appears that the thermosiphon problem was resolved. 

Space Heating Performance 

Figure 7 shows a graph of hourly average thermal energy flows for space heating in the H2 measured 

using the long-term monitoring system. The boiler and radiant space heating energy flows were nearly 

identical because there was no DHW use during the short-term test period, and the solar hot water system 

did not provide a significant contribution toward the space-heating load. Very little active heating was 

required on Saturday and Sunday afternoons, indicating that the passive solar design virtually eliminated 

the heating load during sunny periods even when the outside temperature was below 40oF (4oC). The 

maximum space heating occurred at about 2100 even though the maximum indoor-outdoor temperature 

difference was typically at about 0600. This behavior was probably due to the thermal capacitance of the 

radiant floor heating system, which could have been slow to respond to rapidly changing heating loads. 

PV System Performance 

The H2 prototype was equipped with a nominal 1.6 kW (5540 Btu/hr) grid-tied photovoltaic system 

mounted on a south-facing roof surface at an angle of 45 degrees from horizontal. On December 23–24, 

2004, we ran 18 i-V (current-voltage) traces over a range of solar and temperature conditions. Key 

 4
parameters at Standard Test Conditions (STC) calculated using the calibrated model were compared to 

those provided by the manufacturer (BP Solar 2003). Figure 8 shows a single i-V curve measured near 

STC compared to the curve from the manufacturer. At the maximum power point, the measured power 

output at STC was 11% lower than predicted using the manufacturer’s data. However, it is not surprising 

that the measured performance of the PV array was on the order of 10% below manufacturer’s 

specifications because the manufacturer only guarantees 90% of the rated performance for the first 12 years 

of operation (BP Solar 2003). 

Based on the measured AC output of the inverter, and applying the manufacturer’s inverter efficiency 

curve, the hourly PV array efficiency (η) was calculated. The hourly back-of-module temperature was 

calculated based on measurements at the lower left corner of the arrays, adjusted by 18oF (10oC) to better 

reflect the temperature near the middle of the array. This adjustment was determined by examining the 

front-of-module temperatures under near-constant solar and temperature conditions using a small infrared 

sensor attached to an extension pole held over the front of each module. Although the infrared sensor 

readings were biased because of radiation reflected from the surface of the array, the relative temperature 

distribution was probably accurate enough for our purposes. 

Using the measured array efficiencies (η) and effective back-of-module temperatures (Tmod), the 

efficiency at STC (η0) and the temperature coefficient of efficiency (γ) were calculated by performing 

linear regression analysis using Equation 1, which is a simple model of PV array efficiency as a function of 

the effective back-of-module temperature (Tmod) for the array: 

η = η0*(1+γ*[Tmod-T0]) (1) 

where 

η = PV array efficiency 

η0 = PV array efficiency at Tmod = T0 

γ = temperature coefficient of efficiency 

Tmod = effective back-of-module temperature for the array (oC) 

T0 = 23oC (corresponding to a cell temperature of about 25oC) 

A significant amount of shading of the PV array was observed in the morning, caused by the roof on 

the east of the array. The effect of this shading on PV power for a typical sunny winter day is evident in 

Figure 9, which compares the measured AC output from the inverter to the unshaded irradiance on a clear 

day in January. 

BUILDING SIMULATIONS 

PV Simulations 

Using the short-term measurements discussed in the previous section and the procedure described by 

Barker (2003), a calibrated model for use in TRNSYS (Klein 2000) was developed. The efficiency of the 

inverter was modeled using the efficiency curve published by the manufacturer. To account for shading 

effects from the east roof, a correlation was developed to calculate the effective beam shading fraction as a 

function of time of day and time of year. 

The calibrated TRNSYS model predicts an annual energy production of 2320 kWh (7.92 MBtu) based 

on TMY2 data. This result is about 82% of what would be predicted using the manufacturer’s 

specifications. If the PV array had been installed in a location with no shading, the predicted annual energy 

production would be about 10% higher, or 2558 kWh (8.73 MBtu) per year. 

Solar Hot Water and Space Heating Simulations 

Based on the final configuration and measured performance of the solar hot water system, we 

developed a TRNSYS model to estimate annual energy savings and solar fraction. Space heating loads 

were taken from DOE-2.2 whole-house simulations (which will be discussed in the next section), and hot 

water demand profiles were based on the BA benchmark. Because of the configuration of the system, it 

was not possible to differentiate between solar energy used for water heating and solar energy used for 

 5
space heating. Therefore, the combined load for space and water heating was used in the analysis of the 

solar system. 

Figure 10 shows the predicted monthly energy provided by the solar system compared to the combined 

space and water-heating load. It is evident that the solar system will meet a large fraction of the DHW load 

during the summer months but is not likely to make a noticeable contribution toward the space-heating 

load, especially during the coldest months when even less useable solar energy is collected. Over the 

course of a typical year, the TRNSYS model predicts that a solar fraction of about 34.6% of the combined 

water and space heating load can be achieved. This is consistent with 94 therms (2800 kWh) of energy 

savings, or about $73/year based on an estimated natural gas cost of $0.78/therm for Carbondale. This does 

not account for the small increase in electricity cost (~$4) associated with the pump and controls. 

Whole-House Energy Simulations 

DOE-2.2 simulations were performed for both the H1 and H2 using the detailed design specifications, 

combined with measurements from short-term field-testing. Three zones were used in the model, 

corresponding to the three heating control zones in the H2 test house. Because the H1 had only one control 

zone, it was modeled using three zones held at the same temperature. In keeping with the standard BA 

performance analysis procedures (Hendron et al. 2004), the whole-house energy use of each test house was 

compared to the Building America Benchmark, Regional Standard Practice (RSP), and Builder Standard 

Practice (BSP). Key specifications for each of these base cases are listed in Table 2. 

Table 2. Key features of the Building America Benchmark, RSP, and BSP 

Same as Test House 

Except: Benchmark Regional Standard Practice (RSP) 

Builder Standard Practice 

(BSP) 

Building Shell: 

 

Wall 

R-19.2 hr·ft2

·oF/Btu 

(R-3.4 m2

·K/W) 

R-13 hr·ft2

·oF/Btu 

(R-2.3 m2

·K/W) 

R-19 hr·ft2

·oF/Btu 

(R-3.3 m2

·K/W) 

Ceiling/roof 

R-38.5 hr·ft2

·oF/Btu 

(R-6.8 m2

·K/W) 

R-27 hr·ft2

·oF/Btu 

(R-4.8 m2

·K/W) 

R-27 hr·ft2

·oF/Btu 

(R-4.8 m2

·K/W) 

Windows 

U-Value = 0.36 Btu/hr·ft2

·oF 

(2.0 W/m2

·K) 

SHGC = 0.32 

Double pane, clear Double pane, low-e 

Crawlspace wall 

R-16.7 hr·ft2

·oF/Btu 

(R-2.9 m2

·K/W), 

unvented 

Uninsulated, vented 

R-10 hr·ft2

·oF/Btu 

(R-1.8 m2

·K/W), unvented 

Infiltration rate 0.39 ACH 0.39 ACH 0.35 ACH 

HVAC: 

Air conditioner SEER 10 SEER 10 SEER 10 

Gas furnace (H1 only) 78 AFUE furnace 78 AFUE furnace 90 AFUE furnace 

Gas boiler (H2 only) 80 AFUE boiler 80 AFUE boiler 90 AFUE furnace 

Duct location Crawlspace Crawlspace Crawlspace 

Gas DHW 0.54 EF 0.54 EF 0.56 EF 

Ventilation Continuous exhaust fan Continuous exhaust fan Continuous exhaust fan 

Lighting 90% incandescent 90% incandescent 90% incandescent 

 6
The predicted whole-house source energy savings compared to the benchmark are 20% for the H1 and 

52% for the H2, as shown in Tables 3 and 4. In the H1 house, the space heating end-use was reduced the 

most, followed by lighting. The ventilation system showed negative savings because the large central 

furnace fan is used to draw in and distribute the ventilation air. For the H2 house, improvements in space 

heating, domestic hot water, lighting, and site generation (PV system) were the largest contributors to 

whole-house energy savings. There was again an energy penalty for the ventilation system, but in this case 

the HRV reduced the impact of ventilation on space conditioning energy, so the net effect was actually an 

overall reduction in energy use. It should be pointed out that the HRV had certain additional features that 

tended to increase the fan energy requirements, including HEPA filtration and air recirculation, resulting in 

approximately twice the energy consumption as the HRV that was originally specified. It should also be 

noted that Building America analysis requires the use of a SEER 10 air conditioner when modeling a 

prototype with no cooling system if there is any cooling load at all. This approach credits the energy 

savings associated with a reduced cooling load but does not credit energy savings resulting from the 

absence of an air conditioner. 

Table 3. Predicted End-Use Energy Consumption of the H1 Prototype. 

Source Energy Savings for H1 

Annual Source Energy Percent of End-Use Percent of Total 

Bench RSP BSP H1 Bench RSP BSP Bench RSP BSP 

End-Use MBTU/yr MBTU/yr MBTU/yr MBTU/yr % % % % % % 

Space Heating 76 79 81 57 24% 28% 29% 11% 13% 14% 

Space Cooling 4 7 2 1 65% 81% 28% 1% 3% 0% 

DHW 24 24 24 21 13% 13% 13% 2% 2% 2% 

Lighting 19 19 19 10 44% 44% 44% 5% 5% 5% 

Appliances + Plug 42 42 42 37 12% 12% 12% 3% 3% 3% 

OA Ventilation 2 2 2 6 -276% -276% -276% -3% -3% -3% 

Total Usage 166.7 173.3 170.1 134.1 20% 23% 21% 20% 23% 21% 

Site Generation 0 0 0 0 0% 0% 0% 

Net Energy Use 167 173 170 134 20% 23% 21% 20% 23% 21% 

 The "Percent of End-Use" columns show the change in each end-use category. 

 The "Percent of Total" columns show the overall energy savings associated with each end-use. 

 7
Table 4. Predicted End-Use Energy Consumption of the H2 Prototype 

Source Energy Savings for H2 

Annual Source Energy Percent of End-Use Percent of Total 

Bench RSP BSP H2 Bench RSP BSP Bench RSP BSP 

End-Use MBTU/yr MBTU/yr MBTU/yr MBTU/yr % % % % % % 

Space heating 75 79 81 35 54% 56% 57% 24% 26% 27% 

Space cooling 4 7 2 2 60% 78% 18% 1% 3% 0% 

DHW 24 24 24 12 53% 53% 53% 8% 7% 8% 

Lighting 19 19 19 10 44% 44% 44% 5% 5% 5% 

Appliances + 

plug 42 42 42 37 12% 12% 12% 3% 3% 3% 

OA ventilation 2 2 2 8 -343% -343% -343% -4% -3% -3% 

Total usage 166 173 170 103 38% 40% 39% 38% 40% 39% 

Site generation 0 0 0 -24 14% 14% 14% 

Net energy use 166 173 170 79 52% 54% 53% 52% 54% 53% 

 The "Percent of End-Use" columns show the change in each end-use category. 

 The "Percent of Total" columns show the overall energy savings associated with each end-use. 

The energy savings associated with specific packages of energy-efficiency measures in the H1 and H2 

are summarized in Tables 5 and 6. These tables provide estimates of energy cost savings but do not include 

the first cost or maintenance cost of individual measures. Because such costs are very difficult to quantify 

in the context of a prototype house, we did not try to evaluate the overall cost effectiveness of the energysaving measures. For the H1, the most important improvements were higher insulation levels, highperformance windows, very tight envelope, an efficient lighting and appliance package, and efficient space 

and water heating equipment. For the H2, there were obviously many key features that contributed to the 

52% energy savings. The largest contributors were the PV system, efficient radiant heating, solar hot 

water, efficient lighting and appliances, orientation-specific glazing, and tight building envelope. It can 

also be seen that the HRV in the H2 is expected to provide a very significant whole-house source energy 

savings of about 7% compared to the central-fan integrated supply ventilation system used in the H1 and 

3% energy savings compared to the simple exhaust ventilation fan assumed for the benchmark. 

 8
Table 5. Predicted Savings for Energy Efficiency Measures in the H1 Prototype 

National Average Builder Standard (Local Costs) 

Site Energy Source Energy Energy Cost Energy Cost Measure Package

Increment kWh therms MBTU Savings % $/yr 

Savings 

% $/yr 

Savings 

% 

Value 

($/yr) 

Savings 

($/yr) 

Base 

(BA) 6098 1021 166.7 $ 1,505 $ 1,291 

Base 

(RSP) 6562 1039 173.3 -4% $ 1,563 -4% $ 1,342 

Base 

(BSP) 5907 1074 170.1 -2% $ 1,539 -2% $ 1,316 

Base + 

 imp. wall insulation 5857 1046 166.7 0% $ 1,508 0% $ 1,290 2% $ 26 $ 26 

Base + 

 imp. ceiling ins 5784 1016 162.9 2% $ 1,473 2% $ 1,261 4% $ 29 $ 55 

Base ++ 

 improved windows 5846 966 158.5 5% $ 1,431 5% $ 1,227 7% $ 34 $ 89 

Base ++ 

 infiltration 5802 902 151.5 9% $ 1,366 9% $ 1,174 11% $ 53 $ 142 

Base ++ 

 ventilation 6745 902 161.1 3% $ 1,448 4% $ 1,250 5% $ (77) $ 66 

Base ++ 

 improved EF DHW 6745 871 158.0 5% $ 1,419 6% $ 1,226 7% $ 24 $ 90 

Base ++ 

 improved heating system 6159 793 144.0 14% $ 1,293 14% $ 1,118 15% $ 108 $ 198 

H1 Prototype 

 lighting, appl., and plug 4819 830 134.1 20% $ 1,211 20% $ 1,038 21% $ 80 $ 278 

Notes: 

"Source Energy Savings %" and "National Average Energy Cost Savings %" compared to the BA base case, whereas the "Energy Cost 

Savings %" and the "Package savings $/yr" are compared to the BSP case. 

National average electric cost: 0.0874 $/kWh 

National average gas cost: 0.952 $/therm 

Colorado electric cost: 0.0814 $/kWh 

Colorado gas cost: 0.778 $/therm 

 9
Table 6. Predicted Savings for Energy Efficiency Measures in the CORE H2 Prototype 

National Average Builder Standard (Local Costs) 

Site Energy Source Energy Energy Cost Energy Cost Measure Package

Increment kWh therms MBTU 

Savings 

% $/yr 

Savings 

% $/yr 

Savings 

% 

value 

($/yr) 

savings 

$/yr 

Base 

(BA) 6098 1015.8 166.1 $1,500 $1,286 

Base 

(RSP) 6562 1034 172.7 -4% $1,558 -4% $1,338 

Base 

(BSP) 5907 1075 170.2 -2% $1,540 -3% $1,317 

Base + 

 imp. wall insulation 5857 1045 166.6 0% $1,507 0% $1,289 2% $27 $27 

Base + 

 imp. ceiling ins 5784 1015 162.8 2% $1,472 2% $1,260 4% $29 $57 

Base ++ 

 improved windows 5822 953 156.9 6% $1,416 6% $1,215 8% $45 $102 

Base ++ 

 infiltration 5780 890 150.0 10% $1,352 10% $1,163 12% $52 $154 

Base ++ 

 ventilation (central fan) 6357 890 155.9 6% $1,403 6% $1,210 8% $(47) $107 

Base ++ 

 ventilation (HRV) 6299 788 145.0 13% $1,301 13% $1,126 15% $84 $191 

Base ++ 

 improved EF DHW 6299 691 135.1 19% $1,208 19% $1,050 20% $159 $267 

Base ++ 

 improved heating 

system 6345 569 123.1 26% $1,096 27% $959 27% $91 $358 

Base ++ 

 solar hot water 6392 475 114.0 31% $1,011 33% $890 32% $69 $427 

Base ++ 

 lighting, appl., and plug 5025 505 103.0 38% $920 39% $802 39% $88 $515 

H2 prototype 

 including PV 2705 505 79.3 52% $717 52% $613 53% $189 $704 

Notes: 

"Source Energy Savings %" and "National Average Energy Cost Savings %" compared to the BA base case, whereas the "Energy Cost 

Savings %" and the "Package savings $/yr" are compared to the BSP case. 

National Average Electric Cost: 0.0874 $/kWh 

National Average Gas Cost: 0.952 $/therm 

Colorado Electric Cost: 0.0814 $/kWh 

Colorado Gas Cost: 0.778 $/therm 

 10
LONG-TERM MONITORING OF THE H2 PROTOTYPE 

During the short-term test period, we installed long-term monitoring equipment to collect energy 

consumption data in the H2 prototype for one year, beginning in January 2005. Our interest in long-term 

monitoring was to document the actual performance of the house after it became occupied and to evaluate 

the long-term performance of the PV and energy efficiency systems. It is particularly useful to have 

detailed performance data if the utility bills indicate that energy use is significantly different from initial 

expectations. The data can be used to determine whether the lighting, heating, cooling, and hot water 

systems are consuming more or less energy than expected and whether the PV and solar DHW systems are 

producing more or less than expected. About eight months of data have been collected at the present time, 

and we will continue collecting data until at least January 2006. 

Solar Water Heating 

Figure 11 shows a graph of the monthly average measurements of solar energy collected for each hour 

of the day for the period from January through August 2005. The reverse thermosiphoning described 

earlier in this paper caused the apparent positive (but actually negative) heat flow that occurred at night 

from January through March. The turbine in the flow meter was actually moving in the reverse direction 

from its intended flow direction, but the counting mechanism could not discriminate between forward and 

reverse turbine rotation. 

Hourly measured collector efficiency values from January through August 2005 were compared to the 

corresponding rated efficiency curve derived from Solar Rating and Certification Corporation Document 

OG100 (SRCC 1995). The results of this comparison suggested that the measured efficiency was 

consistent with the rated efficiency within the accuracy of our instrumentation. 

PV System 

The measured average electricity output of the PV system through August 2005 is shown in Figure 12. 

The PV output is consistent with our expectations for a nominal 1.6 kW (5500 Btu/hr) system. Over the 

first eight months of 2005, an average of 64% of electrical energy use has been met by the PV system on a 

net-energy basis. 

Other Monitored Results 

The natural gas usage during the first few months of occupancy struck many observers as very high 

considering the level of energy savings that was anticipated. The measured space heating load was 20 

million Btu for the time period from January through April. The DOE-2.2 simulations were repeated using 

the actual weather conditions and occupant behavior patterns (internal loads, thermostat settings, and hot 

water usage). Indeed, the simulations predicted that the space heating load should have only been about 14 

million Btu (4100 kWh), or 30% less than the actual measured load. Heat losses to the ground were the 

most likely cause of the discrepancy, although we have not yet verified this as the cause because our 

current model cannot perform reliable analysis of interactions between the heated slab, the crawlspace, the 

ground, and the rest of the house. Because of limitations with DOE-2.2, certain effects are difficult to 

model accurately: (1) the radiative exchange between the heated slab and the crawlspace floor, (2) air 

movement between the crawlspace and the house, and (3) ground coupling effects over time. We decided 

that direct measurements of heat loss to the ground would be the best way to quantify losses through the 

crawlspace and subsequently installed two heat flux transducers in the ground to provide an estimate of this 

heat flow. The Community Office for Resource Efficiency (CORE) is planning to install rigid insulation 

above the crawlspace in January 2006, and we will monitor the effect on ground heat loss over the second 

half of the winter. 

It was also noticed that the base level of electricity use was relatively high for the H2 prototype, 

remaining close to 400 W (1400 Btu/hr) throughout the night. The HRV selected by the builder was not 

the same unit originally specified, and it consumed a relatively large amount of electricity, approximately 

170 W (580 Btu/hr) continuously. The high electricity use of the HRV was associated with functionality 

that may not be necessary for typical homes, including HEPA filtration and recirculation of indoor air. In 

addition, it was observed that the boiler pump was operating 24 hours/day, contributing approximately 80 

W (270 Btu/hr) to the overnight electricity use. (This does not include the increased gas usage associated 

with transferring heat from the boiler to the solar tank during the night when there is no call for heating.) 

 11
The remaining 150 W (510 Btu/hr) could be attributed to fairly typical continuous loads from the 

refrigerator, clocks, electrical standby losses, and perhaps some lighting. Modifications to the pump 

controls were completed in late June 2005, and subsequent data indicated a significant decrease in base 

electricity load during summer nights. We also expect the HRV to be replaced with a less energy intensive 

model by the end of 2005. 

SUMMARY AND DISCUSSION 

We were able to draw several conclusions about the performance of the H1 and H2 prototype houses 

based on the short-term test results, whole-house and solar energy simulations, and preliminary data from 

long-term monitoring: 

• Both the H1 and H2 prototypes met the design target of 1200 cfm at 50 Pa (566 L/s @ 50 Pa) 

with room to spare. Blower door testing indicated leakage rates of 475 cfm at 50 Pa (224 L/s @ 

50 Pa) for the H1 and 625 cfm at 50 Pa (295 L/s @ 50 Pa) for the H2. The estimated annual 

average infiltration rate, adjusted for altitude, was 0.12 ACH for the H1 and 0.15 ACH for the 

H2. 

• Despite very cold weather during the test period, the natural infiltration as measured by a tracer 

gas was between 0.05 ach and 0.12 ach for both prototypes. 

• Based on tracer gas measurements, the ventilation rate provided by the HRV in the H2 prototype 

was about 0.21 ach, or 47 cfm (22 L/s), meeting the ASHRAE 62.2 recommendation of 42 cfm 

(20 L/s). The intermittent supply ventilation system operating at 33% duty cycle in the H1 

prototype provided an average of about 0.13 ach, or 29 cfm (14 L/s), which was less than the 

recommended level in ASHRAE 62.2. 

• Relatively large room-to-room temperature variations were measured in the H1 prototype, which 

had a forced air heating system with a single thermostat. In contrast, room-to-room temperatures 

were very uniform in the H2 prototype, which had a radiant floor heating system with three 

control zones. 

• Short-term temperature fluctuations were evident in the H1 prototype due to the cycling of the 

forced air heating system. No such short-term fluctuations were present in the H2 prototype, but 

passive solar gains that could not be stored in the heated slab resulted in a relatively large 

temperature rise of about 4oF (2.2oC) on sunny afternoons. 

• A comparison of temperatures measured by shielded sensors and black globe sensors indicated 

that an equivalent comfort level could be achieved by the radiant floor system in the H2 

prototype with a thermostat setting about 1oF (0.6oC) lower than the forced air system in the H1. 

• A reverse thermosiphoning phenomenon was observed in the H2 solar water heating system 

during short-term testing and subsequent monitoring. The problem was fixed on March 25 when 

a second check valve was added to the system. 

• The predicted annual net solar contribution toward water heating and space heating in the H2 is 

34.6%, saving about $73/year. This estimate is based on typical occupant behavior (as 

represented by the benchmark operating conditions) and TMY2 weather data and does not 

account for the small increase in electricity associated with the solar DHW system (~$4). The 

actual solar contribution will be evaluated after a year of measured data is collected. 

• The current estimate of annual PV output is about 2320 kWh (7.92 MBtu) based on a TRNSYS 

model calibrated with field measurements. This represents about 46% of the predicted total 

electricity use based on typical occupants and TMY2 weather data. The actual fraction of the 

electricity load met by the PV system during the first few months of 2005 was even higher at 

64%. The main reason the actual value was better than predicted was because the occupants 

used less electricity than a typical family in a three-bedroom house. 

• Shading of the PV array on the H2 during the morning hours caused by the east section of the 

roof is predicted to have a noticeable effect (~10%) on annual PV output. 

 12
• Whole-house energy savings for the H2 is estimated to be 52% compared to the BA benchmark, 

exceeding the design goal of 45% by a significant margin. Whole-house energy savings for the 

H1 is expected to be about 20%. 

• Because the H2 was intended to be a showcase for advanced energy efficiency technologies, the 

package would not be cost effective to a builder in a typical production environment without 

large subsidies and cost sharing. In contrast, the H1 included a well-established combination of 

measures that have proven to be cost effective in other BA projects. Unfortunately, we did not 

have access to sufficient builder cost data to substantiate cost-effectiveness in the context of this 

particular project. 

• The expected reduction in CO2 emissions over the estimated 30 year life of the energy efficiency 

measures is approximately 262,000 lb for the H1 and 697,000 lb for the H2. 

• Preliminary monitoring of electricity and DHW consumption in the H2 did not indicate any 

major performance issues, with the exception of the boiler pump and the large HRV fan, both of 

which operated continuously and led to an unusually large base electric load. The pump controls 

have since been modified, and the pump now only operates when auxiliary heat is needed. The 

HRV may be replaced by a less energy intensive model in the near future. 

• The actual measured space heating load is higher than our simulations predict. The most likely 

cause is that we have underestimated the winter energy loss through the crawlspace. Because 

this effect cannot be modeled with a high level of accuracy using our DOE-2.2 model, we will 

be performing some additional tests during the remainder of the monitoring period to help 

identify the cause of this issue with greater certainty. 

ACKNOWLEDGMENTS 

The authors would like to express their appreciation to the homeowners of both test houses, and our 

partners at Building Science Corporation, Community Office for Resource Efficiency, Novy Architects, 

and Fenton Construction for their generous assistance during the short-term test period and for several 

months thereafter. We would also like to thank Ed Pollock and George James of the US Department of 

Energy for supporting this project financially through Building America. 

REFERENCES 

Barker, Greg. 2003. Predicting long-term performance of photovoltaic arrays using short-term test data 

and an annual simulation tool. ASES, Boulder, Colorado. 

BP Solar. 2003. Specification sheet for the BP 3125 photovoltaic module. Document #4025E-1. BP 

Solar, London. 

BSC. 2004. Base case house vs. prototypes–40% performance evaluation. Building America deliverable 

report #KAAX-3-32443-06.A.3. Building Science Corporation, Westford, Massachusetts. 

Hendron, R. 2005. Building America research benchmark definition. National Renewable Energy 

Laboratory, Golden, Colorado 

http://www.eere.energy.gov/buildings/building_america/pdfs/benchmark_2005.pdf. 

Hendron, R., R. Anderson, R. Judkoff, C. Christensen, M. Eastment, P. Norton, P. Reeves, and E. Hancock. 

2004. Building America performance analysis procedures, Revision 1. National Renewable Energy 

Laboratory, Golden, Colorado. 

King, D.L. 1997. Photovoltaic module and array performance characterization methods for all system 

operating conditions. Proceeding of NREL/SNL Photovoltaics Program Review Meeting, November 

18-22, 1996, Lakewood, CO. New York: AIP Press. 

Klein, S. 2000. TRNSYS: A Transient System Simulation Program – Reference Manual. Madison, WI: 

Solar Energy Laboratory, University of Wisconsin. 

Perez. R.R., P. Ineichen, and E.L. Maxwll. 1992. Dynamic global-to-direct irradiance conversion models. 

ASHRAE Transactions 98(1):354-369. 

SRCC. 1995. Operating guidelines for certifying solar collectors. Document OG100. Solar Rating and 

Certification Corporation, Cocoa, Florida. 

 13
FIGURES 

Figure 1. H1 (right) and H2 (left) prototype houses (viewed from the southwest). 

Figure 2. Measured air exchange rates during tracer gas tests. 

 14
Figure 3. Shielded air temperature sensor (left) and black globe temperature sensor (right) in master 

bedroom. 

Figure 4. Average shielded air temperatures in the H1 at five-minute intervals (December 26-27, 

2004). 

 15
Figure 5. Average shielded air temperatures in the H2 at five-minute intervals (December 26-27, 

2004). 

Figure 6. Collector inlet and outlet temperatures before and after the thermosiphoning issue was 

resolved. 

 16
Figure 7. Space heating thermal energy flows during test period. 

Figure 8. i-V curves comparing the manufacturer’s curve to the measured curve and scaled to STC 

at 25oC (77oF) and 1000 W/m2

. 

 17
Figure 9. Comparison of AC output from the PV system to irradiance for a typical winter day, 

illustrating the effect of morning shading. 

Figure 10. Predicted contribution of solar water heating system toward meeting the domestic hot 

water and space heating loads, calculated using a TRNSYS model and measured performance data. 

 18
Figure 11. Monthly average solar thermal energy collected during the first eight months of 2005. 

Figure 12. Average PV system output during the first eight months of 2005. 

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Evaluation of Affordable Prototype Houses at Two Levels of Energy 

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14. ABSTRACT (Maximum 200 Words)

Two high performance prototype houses were built in Carbondale, Colorado, as part of the U.S. Department of 

Energy’s Building America (BA) Program. Each prototype was a 1256 ft2 (117 m2), 1-story, 3-bedroom house, and 

met the local requirements for affordable housing. The National Renewable Energy Laboratory (NREL) performed 

short-term field testing and DOE-2.2 simulations in support of this project at the end of December 2004. We also 

installed long-term monitoring equipment in one of the houses, and are currently tracking the performance of key 

building systems under occupied conditions. One of the houses (designated H1) included a package of cost-effective 

energy efficiency features that placed it well above the Energy Star level, targeting a Home Energy Rating System 

(HERS) score of 88-89. The other (designated H2) was a BA research house, targeting a HERS score of 94-95, and 

45% whole-house energy savings compared to the BA Benchmark. Preliminary results from the field evaluation 

indicate that the energy savings for both houses will exceed the design targets established for the project, although 

the performance of certain building systems, including the ventilation and foundation systems, leave some room for 

improvement. 

15. SUBJECT TERMS

Building envelope; heat recovery units; radiant heating equipment; residential buildings; solar equipment; Building 

America; U.S. Department of Energy 

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