Introduction

The objective of this research was to investigate the life cycle cost impact of upgrading Nebraska’s residential energy code to the 2000 IECC (International Energy Conservation Code). Two other code conditions were used for comparison: the 1983 MEC (Model Energy Code), the current statewide minimum, and the Nebraska average currently being required by jurisdictions in the state. This average condition was determined by a 2002 survey of Nebraska code officials conducted by the Nebraska Energy Office. The study considered the reduction in energy costs associated with upgrades to the energy code, and compared those savings against any increases in costs of construction required to meet the code. Weather conditions, construction costs and utility rates were considered for four cities: McCook, Omaha, Norfolk and Chadron.

Computational models of four houses were developed for the study. These include a ranch style house at the 20^th percentile size being constructed in Nebraska, a ranch style house and a two story house at the median home size, and a two story house at the 80^th percentile size. Occupancy and usage patterns were modeled based on national average usage data. The impact of setback thermostats and an alternate occupancy profile were also investigated.

Selection and specification of houses modeled

House size and type

Based on the survey of Nebraska building code officials, the calculated average Nebraska home built in 2002 was 1,870 square feet (sf) in size. Unfortunately, data on floor area are not recorded in Omaha, and we believe many of the state’s larger houses are built in its larger communities. The average new home in Lincoln was approximately 2,200 sf, which supports this assumption. Also, average house sizes have been rising, so a larger area of 2,100-2,200 sf is also relevant as an estimate of the “average” Nebraska home.

These data agree well with published U.S. census data¹. For 2001, the median new home in the area defined as "Midwest" had 1,965 sf, and the average new "Midwest" home had 2,209 sf (very large homes skew the average higher). The census data also include some information on the distribution of sizes. This was used to estimate the 20^th and 80^th percentile house sizes for this study. The 20^th percentile Nebraska home is larger than 20 percent of new homes built in Nebraska. Likewise, the 80^th percentile home is larger than 80 percent of new Nebraska homes. By interpolation of the census data, the 20^th percentile home in the "Midwest" is approximately 1,450 sf, and the 80th percentile is about 2,900 sf.

Four houses were modeled using these sizes: a ranch house at the 20^th percentile, a ranch house at the mean size determined by the survey of Nebraska code officials, a two story house between the median and average sizes for Midwest homes according to the U.S. Census data, and a two story house at the 80^th percentile. Plans and estimating kits were supplied by Design Basics, an Omaha building plan service that supplies plans for 15,000 houses per year. The actual houses modeled, their square footages, and other characteristics are shown in Table 1.

The decision to model both smaller homes as ranches was based on the survey of code officials, which identified 69% of new homes built in the state as ranch style. The split entry style, which is also likely to be used for smaller homes accounted for only 13% of the total. Two story homes accounted for 18% of the statewide total, and undoubtedly are more common for larger homes. The larger “average” home and the 80^th percentile home were both modeled having two stories.

House	Plan area	Style	Ceiling height (range, ft)	Above grade exterior wall area (sf)	Door area (sf)	Window area (sf)
20^th percentile	1,453 sf	ranch	7.5-10.0	1,530	42	160
Surveyed mean	1,852 sf	ranch	7.5-10.0	2,070	70	160
Midwest mean	2,103 sf	2 story	7.5-9.0	2,620	88	229
80^th percentile	2,932 sf	2 story	7.5-12.7	2,540	86	477

Table 1. Characteristics of houses modeled.

According to the survey, 92% of Nebraska houses have basements, and 26% of these are finished basements. All four houses were modeled with conditioned basements.

The survey found that when records on the type of heating and cooling systems installed are recorded, 67% of new homes have gas-fired forced air furnaces and central air conditioning systems, 24% reported “electric heat and air conditioning,” and only about 4% reported using heat pumps of various types. We suspect that the “electric heat and air conditioning” category may actually contain both electric resistance heating and heat pumps. Because both were in the minority, all four homes were modeled using forced air heating with gas-fired furnaces and central air conditioning.

An air infiltration rate of 0.5 air change per hour was used in modeling the above ground portions of all four houses under all three code conditions. Basements located below grade are modeled with 0.2 air change per hour to reflect their reduced tendency toward air exchange with the outdoors. Air infiltration rates in US houses vary by up to a factor of 10, and have been shown to vary by approximately 15% in identical houses constructed at the same time by the same contractor². The rate of 0.5 air change per hour was selected for the model because it is the median annual infiltration value measured in a study of 312 US houses of “newer, energy efficient construction”³.

Occupant information

Occupant behavior and heat gains associated with people and their activities influence the energy required for heating and cooling. This study assumes a family of four living in each house, and two different occupancy conditions were modeled. In the first, one adult and one child are home during the day while the other adult and child are away from home during the workday. The second condition assumes that both adults work full-time outside the home and both children are away from home during the workday. The heat gain from each adult occupant was modeled as 250 Btu/hr sensible and 200 Btu/hr latent³. The two children were modeled as having 75% of this heat gain.

Two occupant schedules were used. In the first, one adult and one child are away from home during the day for work or school and a second adult and child are home during the day. The first two occupants are modeled as being away from home between 8:00 a.m. and 6:00 p.m. on weekdays and between 10:00 a.m. and 5:00 p.m. on weekends. This occupancy schedule was specified to produce the number of “at home” hours as are recommended by the Environmental Protection Agency’s Exposure Factors Handbook⁵ for a working American adult. The other two occupants are assumed to have the same weekend activities as the others and to spend two hours each weekday outside the house. Their schedule places them away from home between 2:00 p.m. and 4:00 p.m. on weekdays and between 10:00 a.m. and 5:00 p.m. on weekends.

The second occupant schedule changes in that all four occupants are modeled as being away from home for work or school during the day. Their activities follow the same schedule as for the first two occupants described above.

Thermostat settings

Occupants’ use of setback thermostats also influences heating and cooling energy consumption. This model assumes a thermostat setpoint of 70°F in the winter and 76°F in the summer. These conditions are within the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) comfort ranges for people seasonally dressed. Simulations were conducted with two sets of conditions. The first assumes that thermostat setbacks are not used. The second assumes that the thermostat setting is reduced to 62°F between 10:00 p.m. and 6:00 a.m. in the winter for both occupancy scenarios. For the occupancy condition in which no one is home during the day, this scenario also assumes a setback to 62°F between 8:00 a.m. and 5:00 p.m. in the winter and an increase to 80°F for these same hours during the summer.

Appliance loads

Sensible internal heat gains include the occupants themselves (discussed above), appliances, and lighting. Heat gains for some appliances, such as refrigerators, are generally independent of occupant activities. The usage of other appliances, such as televisions, depends on occupant activity. Sensible loads for appliances were computed primarily based on national residential statistics published by the Energy Information Administration (EIA)⁴. This report shows that the average American home consumes approximately 34.6 million Btu annually for appliances that contribute to internal heat gain. These gains were broken into two categories: those related to occupants and their activities, and those that are nearly constant. The occupancy-related sources account for 18.2 million Btu, and are (in decreasing order of magnitude): hot water, lighting, clothes dryers, color televisions, cooking, dishwashers, microwave ovens, personal computers, VCRs, clothes washers, stereos, and laser printers. Sources that are independent of occupancy account for 16.4 million Btu and are (in decreasing magnitude): refrigerator, freezer, waterbed heaters, ceiling fans, aquariums, answering machines, battery chargers, cordless phones, fax machines, and residual items. The contribution of each item to energy use is weighted to account for their frequency of occurrence in the nation’s housing stock.

Internal heat gains are also related to house size. The EIA reports median energy expenditures based on number of rooms. These were divided by the median national household energy expenditure to obtain a factor that was used to scale the non-occupancy related heat gains. The occupancy related heat gains are more likely to be related to the number of occupants than the size of the house, so they were not scaled.

To coincide with occupant activities, the occupancy-related sources were scheduled to occur from 6:00 a.m. to 8:00 a.m. and from 6:00 p.m. to 10:00 p.m. on weekdays, and from 8:00 a.m. to 10:00 a.m. and 5:00 p.m. to 10:00 p.m. on weekends, for a total of 2,288 hours per year. Heat sources that are independent of occupancy were modeled as constant over the entire year. Table 2 summarizes the internal heat gain values used for the analysis.

House size (sf)	# of rooms	% US average energy cost	Occupant related gains (Btu/hr)	Non-occupant related gains (Btu/hr)
1,453	5	96	7,955	1,790
1,852	6	111	7,955	2,069
2,103	8	143	7,955	2,668
2,932	9	182	7,955	3,413
U.S. Average	N/A	100	7,955	1,872

Table 2. Internal sensible heat gains from equipment.

Latent loads also contribute to a home’s cooling energy consumption. For an average family of four, Canada’s Institute for Research in Construction⁶ recommends the following latent loads: respiration from the occupants themselves, 5,760 Btu/day for occupancy related activities (including showering, bathing, dishwashing, cooking, and cleaning), and 5,760 Btu/day from other sources (including construction moisture, seasonal storage, basements and crawlspaces, rain penetration and unknown sources). Latent loads from the occupants themselves were modeled according to the occupancy schedules. To achieve the daily rates above, latent loads from occupant activities were modeled using the same schedule as for occupancy-related sensible loads at a rate of 960 Btu/hr. The other latent loads were modeled as constant throughout the day at a rate of 240 Btu/hr.

Codes

Three different energy codes were modeled. These included the 1983 Model Energy Code (the current Nebraska Building Energy Conservation Standard), the 2000 International Energy Conservation Code, and the Nebraska average being enforced across the state. This average was calculated based on the Nebraska Energy Office’s survey of state code officials, concluded in December 2002. Table 3 summarizes the required component values for the three cases.

Component	1983 MEC (note a)	2000 IECC (note b)	Nebraska average
Glazing U-factor	Note a	0.40 - 0.35	0.45
Ceiling R-value	25	38 - 49	33
Wall R-value	5.6	18 - 21	11
Floor R-value	12.5	21	21
Opaque door U-factor	Note a	0.35	0.25
Basement wall R-value	0	10 - 11	8
Glazing SHGC	none	none	none
Forced air furnace	78% AFUE (note c)	80% AFUE	80% AFUE
Central air conditioning	6.8 SEER (note c)	10.0 SEER	10.0 SEER

Table 3. Component requirements by building code.

Note a: R-values for walls and ceilings using the 1983 MEC are to include the effects of windows, doors, and skylights. Consistent with the published rules of LB755, the statewide average of 6,500 degree days is used to determine the 1983 MEC’s requirements for the entire state.

Note b: The ranges shown reflect the fact that Nebraska includes four of the degree day ranges specified in the 2000 IECC, and requirements vary across the state.

Note c: Although 78% Annual Fuel Utilization Efficiency (AFUE) and 6.8 Seasonal Energy Efficiency Ratio (SEER) are the minimum requirements for the 1983 MEC, 80% AFUE and 10.0 SEER that are locally available and widely installed. These higher values were used for the 1983 MEC case for the energy and cost analysis.

There is no Solar Heat Gain Coefficient (SHGC) requirement for glazing in climates with more than 3,500 degree days. For modeling, a default SHGC of 0.66 was used. This is the default value found in Table 102.5.2(3) of the 2000 IECC for double glazed clear fenestration with operable metal frames or fixed nonmetal frames.

The 1983 MEC wall insulation values are for the composite wall, and include the effects of doors and windows. In our model, we used door and window U-factors equal to those required by the Nebraska average code, and then calculated the required wall insulation R-value, which varied from 7 to 9 depending on the house. For 2000 IECC and the Nebraska average code condition, the R-values shown for ceilings, walls, and floors apply to insulation only, and the codes specify that other parts of the wall section, including framing, drywall, sheathing, and siding are not to be counted toward this value. From this perspective, these updated codes are actually easier to apply than the 1983 MEC, since less calculation is needed. In the model, resistances of other wall components were specified based on materials indicated in the plans.

In the model, basements were considered conditioned space. The 2000 IECC requirement is for conditioned basement walls to be insulated. If the basement wall is not a conditioned space, the 2000 IECC allows for the insulation to be placed in the floor cavity between the basement and first floor.

The requirements shown above in Table 3 are associated with the “simplified prescriptive track” of each code, which is the easiest and most often used means of code compliance. The codes also contain “performance tracks” that allow homeowners to trade off upgraded components in one area to allow flexibility in other areas. Therefore, the actual codes can be more flexible than is implied by the table, but the simplified prescriptive track is used by most builders.

Climates

Four cities were chosen to represent the climate variation in Nebraska. These cities were chosen to represent the heating degree day categories used in the 2000 IECC to specify required thermal performance of envelope components. The National Oceanic and Atmospheric Administration (NOAA) publishes a list of annual degree days that includes approximately 140 cities and towns in the state of Nebraska. The heating degree days (65°F base) in the state range from 5,552 to 7,862. This includes four categories specified in the 2000 IECC, and one city from each of these categories was chosen for modeling. Table 4 summarizes the degree day categories, the selected cities, and their actual numbers of degree days. Note that the state’s second largest city, Lincoln, has nearly the same climate as Omaha (6,119 vs. 6,153 degree days). Numbers of degree days for other code jurisdictions not shown can be found in Table A4 in the appendix to this report. Also shown in Table 4 are the component criteria for thermal performance in each of the four climate zones.

Degree day range	City	Annual degree days	Max. glazing U-factor	Min. ceiling R-value	Min. wall R-value	Min. floor R-value	Min. basement wall R-value
5,500-5,999	McCook	5,967	0.40	38	18	21	10
6,000-6,499	Omaha	6,153	0.35	38	18	21	10
6,500-6,999	Norfolk	6,766	0.35	49	21	21	11
7,000-8,499	Chadron	7,021	0.35	49	21	21	11

Table 4. Cities, their climates, and 2000 IECC component criteria.

Cost analysis

RS Means Residential Cost Data⁷ were used to determine installed cost for the building components considered in the study. This step required that the code-specified U-factors and R-values be translated into defined building components for which costs could be compared. Only the costs for components that differ between energy codes were included in the construction cost calculation. In some instances, Means did not provide as much detail as was needed to differentiate the components (for example, window types), so quotes from local vendors were used to supplement the estimates, as described below. The total price for each component includes purchase price, installation, overhead, and profit. This is the total installed cost to the customer. Local cost adjustment factors from Means were then used to adjust each of the costs to the four locations: Omaha, Norfolk, McCook, and Chadron.

The calculated energy cost for each house includes electricity used by the HVAC system fan year round, electricity needed for cooling, and gas used for heating. Rates were obtained in May 2003 from local utilities serving the four geographical areas (see Tables 9 and 10).

Windows

Base prices for windows were taken from Means values for premium quality vinyl clad windows. A few of the windows listed on the house plans were not exactly the same size as those found in Means, and in these cases the next largest size or a window with the same glass area was used.

The window U-factors required by the different codes vary. U-factor is usually decreased by adding either an insulated air or argon space, adding a low-e coating, or improving frame performance. However, the Means data does not include cost differences for these upgrades. The Means prices are for double glazed insulating glass with ½ inch air space and no coating, a combination that provides U = 0.50. We then obtained costs from local vendors to upgrade to argon fill and low-e coating. An upgrade to low-e glass with e = 0.40 was estimated at $25.00 per window, and an upgrade to low-e glass with e = 0.15 was priced at $30 per window. Argon gas fill requires an additional cost of $15.00 per window. Table 5 below shows the four window U-factor requirements of the various codes and the glass type and coating combinations needed to comply with each. U-factors of various combinations were obtained from a thermal engineering text⁸.

U-factor (Btu/hrft²ºF)	Glass type	Coating
0.50	Double glazed, ½” air space	none
0.45	Double glazed, ½” air space	Low-e (0.40)
0.40	Double glazed, ½” air space	Low-e (0.15)
0.35	Double glazed, ½” argon fill	Low-e (0.15)

Table 5. Types of windows used to meet U-factor requirements.

Exterior wall insulation

Wall insulation base prices were also obtained from Means⁷ and insulating values not specified by Means taken from a thermal engineering text⁸. All four house plans have 2 by 4 stud walls, but some of the higher wall insulation requirements could not be obtained using only 3 ½ inch batt insulation. In these cases, the R-value requirement was met by placing 3 ½ inch batt insulation between the studs and a layer of rigid insulation used as sheathing.

The houses with R-11 or lower walls have an outer layer of plywood to which the

Life Cycle Cost Analysis
of the
2000 International Energy Conservation Code
for Nebraska

Amy Musser, Ph.D., P.E.

Assistant Professor

University of Nebraska-Lincoln

August 6, 2003

Executive Summary

Savings in the Thousands

Hundreds of Thousands of Dollars Saved Statewide

Savings Are Compounded

About the Study

Introduction

Selection and specification of houses modeled

House size and type

Occupant information

Thermostat settings

Appliance loads

Codes

Climates

Cost analysis

Windows

Exterior wall insulation

Life Cycle Cost Analysis of the 2000 International Energy Conservation Code for Nebraska

Amy Musser, Ph.D., P.E.

Assistant Professor

University of Nebraska-Lincoln

August 6, 2003

Executive Summary

Savings in the Thousands

Hundreds of Thousands of Dollars Saved Statewide

Savings Are Compounded

About the Study

Introduction

Selection and specification of houses modeled

House size and type

Occupant information

Thermostat settings

Appliance loads

Codes

Climates

Cost analysis

Windows

Exterior wall insulation

Life Cycle Cost Analysis
of the
2000 International Energy Conservation Code
for Nebraska