Energy analysis of Habitat for Humanity home designs.
Energy analysis tools are standard within the HVAC and design industry for commercial buildings in the early stages of design. Analysis of residential buildings has been less common, however, and since the initial cost of a newly constructed house can be inversely proportional to the energy efficiency, it is often people who live in low-cost housing that are saddled with high energy bills. Habitat for Humanity is an organization that provides affordable housing for low-income families, and it was desired to make sure that the Habitat homes were being constructed as energy-efficiently as possible to make sure they were not costing the families more than necessary. Habitat houses are constructed in part by volunteers, including Habitat clients, although the roofing, mechanical, electrical and plumbing work is contracted out to professionals. Analyses of two of the most commonly built Habitat homes were performed in an energy simulation program and economic analyses were done on several variations of the construction that could be made. These analyses were done as part of a senior capstone design project and the results were provided to the local Habitat chapter to help them make decisions on their construction techniques.
Three house styles were identified by the local Habitat chapter as the most commonly selected for construction: the Cottage, the Parade and the Mount Vernon styles. After some initial modeling was performed, the Cottage and the Parade styles were found to be thermally similar enough in layout and shading that only the Cottage and the Mount Vernon styles were investigated further.
A floor plan of the Cottage style home is shown in Figure 1. The Cottage has 1176 sq. ft. (109 [m.sup.2]) of living space, with three bedrooms, one bathroom and a covered front porch that extends from the house.
A floor plan of the Mount Vernon is shown in Figure 2. The Mount Vernon has 1064 sq. ft. (99 [m.sup.2]) of living space, also three bedrooms, one bathroom and a smaller covered porch with is contained within the overall frame of the house.
Both house models are constructed with identical materials. The walls are framed with two-by-fours and contain R-16 (R-2.8 [m.sup.2]-K/W) insulation. Both house styles both have unconditioned attics, insulated with R-30 (R-5.3 [m.sup.2]-K/W) blow-in insulation above the ceiling of the living area and are roofed with 20-year asphalt shingles. The crawl space of both models is two feet high and conditioned, with 2-inch (5-cm) foam board providing R-11 (R-1.9 [m.sup.2]-K/W) insulation on the concrete bricks. The windows all have a U-value of 0.38.
The mechanical equipment installed is common to both house styles as well. The cooling unit has a 2-ton capacity with an 11 EER rating and the heating unit is natural gas with 54 kBTU (57 MJ) capacity and a 92 AFUE rating. The water heater is also natural gas with a 40-gallon (151 L) capacity and an Energy Factor rating of 0.62.
For the modeling of modifications to the houses, consideration was only given to readily-available modifications. Therefore, no variation on the insulation of the walls of the homes was considered, since with the materials that Habitat has available to them, an increase above R-16 (R-2.8 [m.sup.2]-K/W) would require a two-by-six wall stud, requiring an entire series of additional changes.
In contrast, the amount of blow-in insulation that is added to the ceiling is an easy modification, and one of the things considered was increasing the ceiling insulation value from an R-30 (R-5.3 [m.sup.2]-K/W) to an R-38 (R-6.7 [m.sup.2]-K/W). Increasing the thickness of the foam insulation in the crawl space from an R-11 (R-1.9 [m.sup.2]-K/W) to an R-16 (R-2.8 [m.sup.2]-K/W) was considered as well. The thermal conductivity of the windows was varied from the baseline value of U=0.38 to two levels of high efficiency windows, U=0.29 and U=0.23, as well as a lower efficiency model of window with U=0.49. In the current design, the crawlspace is conditioned and the attic is not, the variations modeled were to add conditioning to the attic, as well as removing the conditioning on the crawlspace to see how much of a difference that makes (as we will see later on, it makes quite a significant difference). In addition, a simulation was run changing the thermostat set point for the cooling season, to see how much of an economic difference that made. Finally, modification to the electrical and gas prices was made to see how an increase in utility rates would affect the economic analysis of the construction material.
Since the internal layout of the houses was not highly complex, the entire living area of the house was modeled as a single area. The attic and crawlspace were separated from the living area by "partitions", which is what the model requires to allow for heat transfer without requiring the spaces be conditioned themselves. The four walls of the homes, foundation, roof, doors, windows and window shading were all modeled from the blueprints provided with insulation values taken from the parameters given or from the variations that were being studied. The thermostat settings for the house were initially taken to be 68[degrees]F (20[degrees]C) in the winter season for the heating unit and 78[degrees]F (25.6[degrees]C) in the summer for the cooling unit, and part of the analysis involved setting back the summer thermostat set point to 80[degrees]F (26.7[degrees]C). The internal loads were modeled from ASHRAE handbook values for a family of four as 155 BTU/h (45.4 W) for latent heat and 245 BTU/h (71.8 W) of sensible heat plus 1600 BTU/h (469 W) for lighting and additional load based on the appliances that are installed in the homes.
The construction was considered "tight" and the infiltration rates were calculated using ASHRAE Handbook guidelines as 49.65 cfm (84.36 [m.sup.3]/h) for Mount Vernon, and 54.88 cfm (93.24 [m.sup.3]/h) for the Cottage in the summer and 69.5 cfm (118 [m.sup.3]/h) for Mount Vernon and 76.8 cfm (130 [m.sup.3]/h) for the Cottage in the winter. For the economic analysis, electricity cost was calculated at $0.1411 per kWh, and natural gas cost was considered $0.67 per therm, which were values provided by the local utility as the current residential rates. The houses are built in or near Evansville, Indiana, which is categorized as ASHRAE climate zone 4A. The solar and temperature data within the simulation was selected for Evansville, IN.
In order to confirm that the computer model was correctly programmed, the output from the computer was compared with alternate predictions for energy use. The peak load from the computer model, which is the maximum amount of heating and cooling that is required throughout the year, was compared with a hand calculation based on the block-load calculation method as detailed in the Fundamentals 2001 Handbook. Figure 3 shows a plot of the peak cooling loads predicted by the model and by the hand calculation for both home styles, and Figure 4 shows the peak heating loads. The computer model matched the hand calculations within 10% for the cooling season and within 5% for the heating season. This was considered acceptable agreement.
The calculation of annual usage from the computer model was compared to an Energy Star audit that was available from Habitat. An audit of this sort is performed on each of their newly constructed homes, and the audit results from a recent home of each construction style were used. The audit produced predictions of annual consumption for cooling and heating energy, based on a blower-door test and using the Home Energy Rating Systems method. The output from the computer model was summed to provide annual usage figures to compare with. However, the results from the computer model were significantly lower than the Energy Star audit predictions for heating season, and significantly higher for cooling season. The annual heating energy consumption predicted is shown in Figure 5, where the computer model is shown as the blue bar on the left and the Energy Star prediction is the green bar on the far right. The annual cooling energy consumption is shown in Figure 6, with the same color coding.
Since the predicted energy consumption from the computer model compared to the Energy Star audit was significantly different, this caused some concern. However, when the internal loads were removed from the computer model, the prediction of energy consumption matched almost identically - 7% difference in heating and 8% difference in cooling. This is shown in the red bar in Figures 5 and 6. No information on the methodology of the Energy Star audit was available, and it was assumed that the internal loads are not included in their audit predictions and therefore the computer model was considered to be in acceptable agreement. However, the internal loads were kept in the computer model for project.
It was desired to be able to compare the computer predictions to metered usage in the actual homes, however because of privacy issues, all that the power company would make available was aggregate electric and gas usage data that was not separated by home style. There was insufficient time during the course of the project to collect permission from the Habitat homeowners to release their data for analysis.
Since the orientation of the home would change for each construction, both of the houses were modeled with the front pointing in each of the cardinal directions: north, south, east and west. The peak heating load did not vary significantly with orientation in either of the home styles. The variation in peak cooling load for the maximum and minimum orientation was only 11% for the Cottage style, with the highest peak cooling energy required when the front door was facing west. For the Mount Vernon style, the variation in minimum and maximum peak load was almost 30%, with the highest demand when the front door was facing west and the lowest when the front door was facing south. Therefore, for the purposes of this study, both house models were orientated with the front door facing west in order to simulate the worst-case scenario for cooling requirements.
The results from modeling an increase in the insulation in the ceiling (between the living space and the attic), and in the crawlspace are shown in Table 1. Shown are the results from the Cottage style home; the results from the Mount Vernon style share the same trends and so for the purposes of this paper will be omitted. The annual energy usage and cost is shown for the baseline insulation, increasing the insulation in the ceiling and crawlspace independently, and increasing them both together. In the right-most column the cost of the additional insulation is shown. As can be seen, the savings are more significant from an increased level of insulation in the ceiling.
Table 1. Energy and Economic Results from Increasing the Ceiling and Crawlspace Insulation Insulation Total Total Total Annual Additional Annual Annual Annual Cost Cost over Energy Energy Energy Savings Current Usage for Usage for Cost over Construction Heating Cooling (USD) Baseline (USD) MMBTU MMBTU Model (GJ) (GJ) (USD) Baseline: 9.8 5.9 $1,086 - - ceiling, (10.3) (6.2) R-30; crawlspace, R-11 R-38 in 8.4 5.5 $1,058 $30 $150 ceiling (8.9) (5.8) R-16 9.1 5.8 $1,078 $8 $165 crawlspace (9.6) (6.1) R-38 in 7.8 5.4 $1,050 $38 $315 ceiling; (8.2) (5.7) R-16 crawlspace
When the windows were considered, the highest efficiency windows with U=0.23 only resulted in a $4 savings during the heating season compared to the current window model of U=0.38. Using the current windows only resulted in a $24 annual savings over the lower efficiency windows (U=0.49). Therefore, the highest efficiency windows were not deemed a cost-effective modification to make to the homes.
Table 2 shows the results from considering the conditioning of the attic and crawlspace. The baseline model was what is being constructed currently: the crawlspace is conditioned, the attic is not. Removing the crawlspace conditioning resulted in a large increase in energy that is required to heat the home in the winter and a small savings in the cooling required for the house. In total, however, there was a net increase in the amount of annual of 25.1 MMBTU (26.4 GJ), a negative "savings"!, equaling approximately $200 annually. This demonstrated that the current practice of conditioning crawlspaces results in more than 250% savings for heating bills. The conditioning of the attic resulted in a small savings in both the heating and cooling seasons. Increasing the thermostat during the cooling season from 78[degrees]F to 80oF (25.6[degrees]C to 26.7[degrees]C) resulted in an annual savings of $26, regardless of the other conditions of the home.
Table 2. Energy Usage Results from Conditioning of Crawlspace and Attic Conditioning Total Total Annual Annual Total Annual Annual Energy Energy Energy Energy Energy Savings Savings Savings Usage for Usage for for for MMBTU Heating Cooling Heating Cooling (GJ) MMBTU MMBTU MMBTU MMBTU (GJ) (GJ) (GJ) (GJ) Baseline: 9.8 5.9 -- -- -- crawlspace (10.3) (6.2) conditioned; attic unconditioned Crawlspace and 36.1 4.7 -26.3 1.2 -25.1 attic (38.1) (5.0) (-27.7) (1.3) (-26.5) unconditioned Crawlspace 34.4 3.5 -24.6 2.4 -22.2 unconditioned; (36.3) (3.7) (-26.0) (2.5) (-23.4) attic conditioned Crawlspace and 9.1 4.8 0.7 1.1 1.8 attic both (9.6) (5.1) (0.74) (1.2) (1.9) conditioned
The modification in insulation for the ceiling and crawlspace was considered from an economic standpoint of a simple payback of the annual energy cost savings (at the present rates) compared to the cost of the modification. Installing R-38 in the ceiling had a simple payback of 5 years, although R-16 in the crawlspace had a payback of 20 years, and making both changes resulted in a simple payback of 8 years. If the electric rates increase by 3 cents, the simple payback for the increasing the insulation in the ceiling is reduced to 4.5 years, and with an increase of 5 cents in the electrical rates it is reduced to 4 years.
In this project, the most popular of the home styles that are selected for construction by Habitat for Humanity clients were analyzed in a computer model, and variations on the present construction were simulated with the model. The material selections for these homes were found to be appropriate for economic considerations in the local climate. The only modification that might be appropriate would be to increase the thickness of the blow-in insulation in the ceiling to an R-38 value over an R-30 value, because this has a simple payback time of 5 years. With a 3-cent increase in the electrical rates, this simple payback period is reduced to 4.5 years and this would be highly recommended. Installing the next highest efficiency windows is not recommended from an economic perspective because the windows do not make enough of a contribution to the total thermal envelope. The present practice of conditioning the crawl space was shown to result in significant energy savings.
A more complete study would allow for the possibility of increasing the wall thickness to see the economics of more wall insulation, since that does contribute significantly to the thermal envelope. However, changing the type of wall stud used would cause a series of other changes to the construction that would need to be carefully considered to get a good picture of the economic effect.
This study is specific to the local Habitat chapter and the local climate. HVAC engineers who regularly perform such analyses on commercial buildings could easily provide some useful volunteer information to Habitat for Humanity chapters in different climates or with different popular home styles by running similar analyses for their local chapters.
Thanks to Professor David J. Ellert, PE, for his help with the thermal analyses that were performed as part of this paper. Also, thanks to Steve Peters, Tina Nuffer and Lori Reed of Habitat for Humanity of Evansville for their assistance and willingness to work with us on this project.
Brandon S. Field, PhD
B. Field is an assistant professor in the Department of Engineering, University of Southern Indiana, Evansville, IN. M. McConnell is a mechanical engineer at Apex Engineering, Mount Vernon, IN.
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|Author:||Field, Brandon S.; McConnell, Matthew|
|Date:||Jan 1, 2013|
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