Energy Impact Study of the 2003 IECC, 2006 IECC, and
2006 IRC Energy Codes for Nebraska

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

Principal

Vandemusser Design, LLC

Disclaimer: This report was prepared with the support of the U.S. Department of Energy (DOE) Grant #DE-FG48-02R830105.
The findings, conclusions, and recommendations herein are those of the author and do not necessarily reflect the views of DOE.

September 19, 2006

Executive Summary

The focus of this report is annual residential energy consumption under three energy code conditions. The codes compared are:

Nebraska’s current residential energy code, the 2003 International Energy Conservation Code (IECC), and

The 2006 International Energy Conservation Code (IECC) and the 2006 International Residential Code (IRC), which have identical requirements when applied to the homes in this study. These codes allow builders two options for exterior wall construction, both of which are investigated.

These codes were compared in the Nebraska cities of Omaha, Chadron, and Norfolk and for houses with 12% and 18% window to wall area ratio.

2003 IECC performs best in most cases

Homes constructed according to the requirements of the 2003 IECC consumed less energy annually for heating and cooling in the Chadron and Norfolk climates. The 2003 IECC also consumed the least energy in Omaha for houses with 18% window to wall ratio.

Based on the results data, we expect that the 2003 IECC will consume less energy for Omaha houses with window to wall ratios in excess of 15%. At 15% or below, the 2006 IECC/IRC will result in less energy consumption.

Key differences between 2003 and 2006 codes

There are several important differences between the 2003 and 2006 IECC codes. Under the 2003 code, Nebraska consisted of four separate climate zones with different insulation requirements for each. The 2006 code combines the entire state into a single climate zone with uniform requirements. The 2006 component insulation requirements are most similar to the 2003 requirements for Omaha, which has one of the state’s warmer climates. This is why the 2003 code outperforms in the colder cities of Chadron and Norfolk.

A second key difference between the codes is that the 2003 IECC requires more insulation to be used when houses have a larger percentage of windows. This acts both as an incentive for builders to limit the percentage of windows and to partially offset the increased energy consumption that occurs as the amount of window area increases. The 2006 IECC drops this requirement: there is no limit on the window area that can be installed, and there is no longer a requirement to offset the energy consumed by the larger window area with increased insulation elsewhere in the house. This is why the houses in Omaha with 18% window to wall ratio also used less energy under the 2003 IECC.

A third difference is that the 2006 code allows builders to use less insulation in ceilings and floors if the insulation fills the framing cavity. This potentially allows houses to be constructed with much less insulation than the 2003 code would allow. These lower insulation values were not used in this study, but even without using them, the 2003 IECC used less energy in most cases.

About the Study

The study considers the annual energy consumption of houses constructed according to the 2003 and 2006 IECC energy codes. Energy use was modeled for three cities selected to represent climate zones in the state: Chadron, Norfolk, and Omaha. Energy modeling was performed using Energy Plus, a state of the art modeling tool developed by the U.S. Department of Energy.

Four houses were modeled for the study. These include a small ranch style house with 1,453 square feet (sf), a medium ranch style house with 1,852 sf, a medium two story house with 2,103 sf, and a large two story house at 2,932 sf. Each house was modeled with both 12% and 18% window to wall area ratio. Occupancy and usage patterns were based on national data for average use.

The modeling approach and houses used in this analysis were based on those used for a 2003 study of Nebraska energy codes¹. That study investigated the life cycle cost impacts of upgrading Nebraska’s state energy code from the 1983 Model Energy Code to the 2000 IECC. That study concluded that the new energy code would save buyers of new homes between $50 and $295 per year, depending on the size of the house and where they lived. Statewide, the new code was projected to save homeowners $254,000 the first year, and $59.6 million dollars over the life of houses built before 2015. With the passage of LB 888 in 2004, Nebraska homeowners began to experience these savings. Since then, gas rates have increased by 15-20% across the state, rather than the approximately 10% increase that was accounted for in the original cost analysis. Therefore we have reason to believe that adoption of the 2003 IECC has helped soften the blow of these price increases for Nebraska homeowners, and that their actual savings may be greater than the original predictions.

About Energy Codes

Energy codes establish minimum insulation requirements for both commercial and residential buildings. Residential codes benefit homeowners by ensuring that newly constructed homes make use of modern techniques and products that make houses energy-efficient. This results in lower energy bills and often improved thermal comfort for the homeowner, and optimal utilization of fossil fuels and nonrenewable resources for communities. Codes also level the playing field for builders by requiring a basic level of quality in areas that homeowners might not see when they are buying a house, for example, the insulation in the walls.

About the Author

Amy Musser holds a Ph.D. degree in Architectural Engineering and an M.S. degree in Mechanical Engineering. She is also a registered professional engineer in the state of Nebraska, and has been conducting research in the fields of building energy and indoor air quality for approximately 15 years. She completed original Nebraska codes study that investigated the life cycle cost impact of the 2000 IECC for Nebraska while she was a faculty member in the Architectural Engineering Program at the University of Nebraska-Lincoln. She currently holds the position of Principal at Vandemusser Design, LLC, a building energy and air quality consulting firm that she co-founded.

Disclaimer

This report was prepared with the support of the U.S. Department of Energy (DOE) Grant #DE-FG48-02R830105. The findings, conclusions, and recommendations herein are those of the author and do not necessarily reflect the views of DOE.

Introduction

The objective of this study was to compare the energy impact for Nebraska homeowners under the 2003 IECC (International Energy Conservation Code), the 2006 IECC, and the 2006 IRC (International Residential Code). The study compares the modeled energy use of four houses in three Nebraska climates: Omaha, Norfolk, and Chadron. The four houses are based on those used for a previous study of the life cycle cost implications of adopting the 2000 IECC as the state energy code¹. The houses 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. Each house is investigated with both 12% and 18% window to wall area ratio. Occupancy and usage patterns were modeled based on national average usage data.

Selection and specification of houses modeled

House size and type

The four houses studied were based on those used for a previous study of the life cycle cost impact of adopting the 2000 IECC in Nebraska. A 2002 survey of Nebraska building code officials conducted as part of that study determined that the average Nebraska home built that year was 1,870 square feet (sf) in size. Unfortunately, data on floor area were not available for Omaha, where many of the state’s larger homes are likely built. The average new home in Lincoln was approximately 2,200 sf, which supports this assumption. Also, U.S. census data² for 2001 report that the median new home in the area defined as "Midwest" was 1,965 sf, and the average new "Midwest" home was 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 the 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.

The four selected house plans were: 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.

One difference from the previous study is that the four houses were modeled with window to wall area ratios of both 12 and 18%. Previously, the houses were modeled with the actual window area shown on the building plans.

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

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 also found that when records on the type of heating and cooling systems installed were available, 67% of new homes have gas-fired forced air furnaces and central air conditioning systems. All four homes were modeled using this type of heating/cooling system.

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 U.S. 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 contractor³. 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 U.S. houses of “newer, energy efficient construction”⁴. Unless an aggressive air sealing program and independent verification techniques are used, this is the best estimate of the air tightness of a typical home.

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 with one adult and one child who are home during the day, while the other adult and child 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 latent⁵. The two children were modeled as having 75% of this heat gain.

The occupancy schedule is as follows: one adult and one child 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 Handbook⁶ 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.

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. No setback thermostat settings were used.

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)⁵. 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 Construction⁷ 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 code conditions were modeled. These included the 2003 IECC (International Energy Conservation Code) and two different conditions to represent both the 2006 IECC and the 2006 IRC (International Residential Code). Chapter 11 of the 2006 IRC is nearly identical to Chapter 4 of the 2006 IECC, and the prescriptive requirements for the two codes are identical for the houses in this analysis. Thus for the remainder of this report, these cases will be referred to as the “2006 IECC/2006 IRC”.

The 2006 codes contain several major changes. First, the entire state of Nebraska has been included in a single climate zone with uniform requirements throughout the state. Under the 2003 code, Nebraska includes four climate regions with different requirements, with Omaha, Norfolk, and Chadron each falling in a different region. Another change associated with the 2006 codes is that the R-value requirement for wood frame walls for the Nebraska climate may be met using R19 cavity insulation or R13 cavity insulation plus R5 insulated sheathing. Energy consumption was modeled for both of these cases. Another major change associated with the 2006 codes is that their requirements do not change with window to wall area ratio. The 2003 IECC contains more stringent requirements for houses with window to wall ratio exceeding 15%. This means that the component requirements for that code are different for the cases with 12% and 18% window to wall ratio.

Table 3 summarizes the required component values for the code conditions modeled. 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. An exception is the requirement for the 2003 18% window to wall ratio cases, for which the simplified prescriptive track cannot be used. A more detailed prescriptive track with similar tabular values taken from Chapter 5 of that code was used instead.

Component	2003 IECC			2003 IECC			2006 IECC/IRC (case a)	2006 IECC/IRC (case b)
	15% or less window to wall ratio			18% window to wall ratio
	Omaha	Norfolk	Chadron	Omaha	Norfolk	Chadron
Glazing U-factor	0.35	0.35	0.35	0.34	0.33	0.33	0.35	0.35
Glazing SHGC	none	none	none	none	none	none	none	none
Opaque door U-factor	0.35	0.35	0.35	0.35	0.35	0.35	0.35	0.35
Ceiling R-value	38	49	49	49	49	49	38 (note a)	38 (note a)
Wall R-value	18	21	21	22	22	25	19	13+5 (note b)
Floor R-value	21	21	21	19	25	30	30 (note c)	30 (note c)
Basement wall R-value	10	11	11	10	11	15	10/13 (note d)	10/13 (note d)
Forced air furnace (AFUE)	80%	80%	80%	80%	80%	80%	78% (note e)	78% (note e)
Central air conditioning (SEER) (note f)	13.0	13.0	13.0	13.0	13.0	13.0	13.0	13.0

Table 3. Component requirements by building code.

Note a: Both codes allow R30 to be substituted if the uncompressed R30 extends over the top plate at the eaves. In the 2006 Code, R30 may also be used for ceiling areas of up to 500 sf with no attic. The 2003 Code does not limit square feet.

Note b: 13+5 refers to R13 cavity insulation plus R5 insulated sheathing.

Note c: Less than R30 may be used if sufficient to fill the framing cavity; with a minimum of R19.

Note d: R10 may be used if insulation is continuous; R13 must be used if insulation is placed in the framing cavity.

Note e: The “prevailing minimum federal efficiency of 78% is required, but 80% is widely installed and was used for the analysis.

Note f: The 2003 IECC required 10.0 SEER, but is no longer available since the minimum federal efficiency increased to 13.0 is 2006. 13.0 SEER is used for the analysis.

Energy Impact Study of the 2003 IECC, 2006 IECC, and 2006 IRC Energy Codes for Nebraska