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Effective Occupied and Vacant Square Footage
in Commercial Buildings in 1992

-- A Useful Benchmark of Commercial Floorspace Vacancy Rates --

One of the major approaches to analyzing energy use in end-use sectors is to relate energy use to measures of the extent of utilization of the sector, either in absolute terms or in terms relative to some maximum utilization level. For example, vehicle miles traveled is a measure of vehicle utilization in the transportation sector. The percent of maximum production capability at which an industry or an individual plant is operating is a measure of industrial capacity utilization in the industrial sector.

For the commercial buildings sector, two concepts that measure how intensely a building is utilized seem to predominate: the number of hours the building is in operation and the amount of floorspace in the building that is occupied (or conversely, the amount that is vacant).

The Commercial Buildings Energy Consumption Survey (CBECS), a national sample survey of commercial buildings, has collected information since 1979 on how energy is used in commercial buildings. Because the amount of energy used in a building is closely related to building characteristics such as the building's size, operating hours and how the building is used, the CBECS also collects information about these characteristics. For more information see the building characteristics.

Historically, CBECS has measured operating hours in some detail, but has provided less detail about the amount of occupied and vacant floorspace in commercial buildings. Two primary measures have been used to estimate occupied and vacant floorspace in the CBECS. However, with the exception of the 1992 CBECS, these two measures are based on building-level classification of vacancy, and do not provide sufficient detail needed to calculate the more precise measures of effective occupied space and effective vacant space. These measures account for the portion of floorspace in an occupied building that is vacant and the portion of floorspace in a vacant building that is occupied.

To address this issue, more detailed information on occupied and vacant floorspace was collected in the 1992 CBECS. Using this more detailed information, approximately, 9 percent of all floorspace in commercial buildings was determined to be vacant in 1992 (Table 1a).

Although this number does not vary much by building characteristics, some building types did show notably high or low vacancy rates. Office buildings and health care buildings had the greatest amount of vacant floorspace, with approximately 14 percent of their respective floorspace vacant. Buildings where the primary activity was religious worship or public order and safety had the least amount of vacant floorspace. Only 2 percent of the floorspace in these types of buildings was vacant (Figure 1). In addition, buildings with ten or more floors had a relatively large percent of vacant floorspace during 1992 (18 percent). (Figure 2).

Other building categories with a relatively large percentage of vacant floorspace were buildings that were not heated (28 percent of floorspace), cooled (18 percent of floorspace) or lit (70 percent of floorspace). It is reasonable to assume that building managers do not want to pay for utilities for the portions of the building that are vacant.

The average measure of effective occupied and vacant floorspace is useful not only to characterize building occupancy, but also to measure the intensity of energy use, since CBECS collects energy use for a calendar year. Ideally, the way to measure energy intensity would be to separate building energy use into two parts, (1) the energy used in occupied floorspace, and (2) the energy used in vacant floorspace, and calculate intensities specific to each. However, CBECS collects energy consumption only for the building as a whole, and there is no known way to separate it by occupancy/vacancy rates. Therefore, the only way, currently, to determine consumption in vacant floorspace is to analyze consumption in buildings that were completely vacant during 1992. This analysis is possible for electricity because it is consumed in virtually all buildings, even those that are totally vacant. (A description of the methodology used to calculate electricity consumption in vacant buildings.

In 1992, approximately 8 thousand Btu of electricity was consumed per vacant square foot in buildings that were totally vacant but still had some electricity supplied to them (Table 2a). These intensities varied by building type: vacant office buildings were the most energy intensive, using about 15 thousand Btu of electricity per vacant square foot; vacant mercantile and service buildings were the least intensive, using 3 thousand Btu of electricity per vacant square foot.

If these intensities are applied to all vacant commercial floorspace in both totally vacantand partially occupied CBECS buildings, electricity consumption can be estimated for all floorspace identified as vacant by the 1992 CBECS. (This process assures that electricity intensity for vacant floorspace in otherwise occupied buildings is the same as its intensity in buildings that were completely vacant during the year, an uncertain equivalence, but the best assumption that can be made.) Then, subtracting this value from total electricity consumption for the sector yields an estimate of electricity consumption specific to occupied floorspace. Finally, dividing the consumption for occupied floorspace by the effective occupied square footage gives an estimate of electricity intensity specific to occupied floorspace.

Approximately 42 thousand Btu of electricity was consumed per occupied square foot in 1992. the amount of electricity used per occupied square foot ranged from 116 thousand Btu in buildings classified as food sales and service to 8 thousand Btu in buildings where the primary activity was religious worship.

For three other energy sources, natural gas, fuel oil and district heat, there were too few completely vacant buildings during 1992 that still had these energy sources supplied to them, to be able to estimate the amount of energy consumed in vacant buildings as a surrogate for the energy intensity specific to vacant floorspace. Therefore, the only occupancy-related intensity measure that can be provided for these energy sources is total consumption per effective square foot This measure has a meaning of its own. It can be thought of as the amount of energy needed per unit of productive commercial space, with vacant square footage treated as nonproductive space.

In 1992, The amount of natural gas consumed per effective occupied square foot was 52 thousand Btu in all commercial buildings using natural gas(Table 3a). The most natural gas intensive buildings were buildings where the primary activity was health care with 133 thousand Btu consumed per effective occupied square foot. Fuel oil and district heat consumption per effective occupied square foot were 22 thousand Btu (Table 4a), and 93 thousand Btu (Table 5a), respectively. For consistency and comparison, comparable measures of total consumption per effective occupied square foot are also provided for electricity (Table 6a).

List of Tables

1. Table 1a. Effective, Occupied, and Vacant Square Footage, 1992

2. Table 1b. Relative Standard Errors for Relative Standard Errors for Effective Occupied, and Vacant Square Footage, 1992

3. Table 2a. Electricity Consumption and Electricity Intensities,
   per Square Foot, Specific to Occupied and Vacant Floorspace, 1992

4. Table 2b. Relative Standard Errors for Electricity Consumption and Electricity Intensities, per Square Foot, Specific to Occupied and Vacant Floorspace, 1992

5. Table 3a. Total Natural Gas Consumption per Effective
   Occupied Square Foot, 1992

6. Table 3b. Relative Standard Errors for Total Natural Gas Consumption per Effective Occupied Square Foot, 1992

7. Table 4a. Total Fuel Oil Consumption per Effective Occupied
   Square Foot, 1992

8. Table 4b. Relative Standard Errors for Total Fuel Oil Consumption per Effective Occupied Square Foot, 1992

9. Table 5a. Total District Heat Consumption per Effective Occupied
   Square Foot 1992

10. Table 5b. Relative Standard Errors for Total District Heat Consumption per Effective Occupied Square Foot 1992

11. Table 6a. Total Electricity Consumption per Effective Occupied Square Foot, 1992

12. Table 6b. Relative Standard Errors for Total Electricity Consumption per Effective Occupied Square Foot, 1992

Links to the Text

• In most CBECS, the issue of occupied or vacant floorspace is measured in two ways. The first way is through building activity classification questions. A building is classified as vacant at the time of the survey if the largest single proportion of the building's square footage is vacant. Otherwise, the building is classified as occupied, with the building activity being the one that takes up the largest proportion of floorspace. In many cases a building has either a single activity, or multiple activities that account for 100% of its floorspace. However, it is often the case that a building classified as vacant has some occupied square footage; and conversely, many buildings classified in an occupied activity category have some vacant space.

  There are two major problems with measuring vacancy rates based solely on building activity classification. First, the actual amount of vacant floorspace in buildings may differ greatly from the amount of floorspace in buildings classified as vacant depending on the amount of vacant space in buildings classified as occupied, and the amount of occupied space in buildings classified as vacant.

  For example, if a building population has an estimated 92 percent of floorspace in buildings classified as occupied, and 8 percent of floorspace classified as vacant, and if the occupied buildings are on average, 95 percent occupied at the time of the survey, and the vacant buildings are, on average, 90 percent vacant, then the actual proportion of vacant space in the population at the time of the survey would be estimated as:

  V = (0.92)(0.05) + (0.08)(0.90) = 0.046 + 0.072 = 0.118, or 11.8 percent

  a value almost 50 percent larger than the 8 percent that is classified as vacant using only the building activity classification.

  A second problem associated with using floorspace in vacant buildings as a surrogate for vacant floorspace is that the CBECS is ordinarily conducted in the fall of the year. If vacancy rates in commercial buildings vary by season, then even the demonstrably flawed measure of floorspace in vacant buildings may itself be biased relative to its true value over time. Thus, the activity status measure is uncertain at best as a measure of the amount of vacancy in buildings.

  A second way occupied and vacant floorspace is measured is through a question that asks whether any space in a sample building was vacant for at least 3 consecutive months during the past year. However, that question alone does not specify the proportion vacant or the time period of the vacancy, and so is not adequate by itself as a measure of the "amount" of vacancy found in a building.

More Details on Occupied and Vacant Floorspace

• In the 1992 CBECS an effort was made to more precisely characterize building occupancy in order to produce a directly-measured value of the amount of vacancy in commercial buildings that would not be subject to seasonal bias. Survey respondents were asked to estimate, month by month during 1992, the percentage of building floorspace that was vacant.
  Calculation of Effective Occupied Square Footage

  Effective occupied square footage was calculated in two different ways, depending on whether or not some portion of the building was vacant for 3 consecutive months.

  Buildings with Three Consecutive Months of Vacancy

  If some parts of a building were vacant for 3 consecutive months, then the percentage of the floorspace that was vacant each month of the year was obtained. To calculate effective occupied square footage, the percent vacant for each month was: (1) summed over all 12 months; (2) divided by 12; and (3) rounded to the nearest integer. This number was subtracted from 100 to get the percent occupied. The total occupied square footage is the percent occupied, multiplied by the total floorspace. Buildings reporting 3 consecutive months of vacancy comprised about 18 percent of the population of commercial buildings.

  This average, while it may not represent the amount of occupied or vacant floorspace of the building at any particular time of the year, is the best measure of the average over the year.

  
  Buildings Not Reporting Three Consecutive Months of Vacancy

  Buildings not reporting 3 consecutive months of vacancy accounted for about 82 percent of all commercial buildings. The occupied square footage in these buildings is equal to the total square footage of the building. Note: The total square footage is an overestimate of occupied square footage for these buildings, because a portion of the buildings will have sections vacant for short periods of time, even though nothing was vacant for 3 consecutive months. However, it is likely that this overestimate is relatively small. For example, if the buildings represented 76 percent of floorspace, and the average building had 10 percent of its floorspace vacant for 1 month during the year, the ignored proportion of vacant floorspace is (0.76)(0.1)(1/12)=0.6 percent of total floorspace.

Description for Calculating Electricity Consumption in Vacant Buildings

• To calculate the electricity consumption per effective vacant floorspace buildings had to meet three criteria: (1) the principal building activity was classified as vacant; (2) electricity was supplied to the building during 1992; and (3) the buildings were not occupied at all during the year. There were 88 of these buildings in the sample. The buildings were then divided into three groups based on their previous or intended use--office buildings, mercantile and service, and all other. Given the data limitations, computing consumption in vacant buildings that are intended to be offices, for example, is the closest to computing electricity consumption in vacant floorspace in office buildings.

  The electricity consumption in vacant square footage is the electricity intensities as calculated above, multiplied by the vacant floorspace in each building category. The electricity consumption in occupied floorspace is the total consumption minus the consumption in vacant floorspace. The intensities for vacant and occupied square footage are equal to the respective consumption divided by the vacant and occupied square footage totals.

Total Consumption per Effective Square Foot Measure

• Energy consumption per occupied floorspace for natural gas, fuel oil and district heat is obtained by summing the energy consumption in each building and dividing it by the total occupied square footage. The resulting figure is the number of Btu (in thousands) consumed per occupied square foot. However, for any building with any vacant floorspace, this number will necessarily be higher than the energy intensity that is calculated using total square footage, because the denominator is smaller. Thus, building categories with the highest percentage of effective vacant floorspace show the largest differences between the energy intensities calculated using total square footage as a denominator and the energy intensities calculated using the effective occupied square footage as a denominator.

  When electricity consumption is calculated in the same way as natural gas, fuel oil and district heat (i.e. relative to occupied square footage rather than total square footage), there is little difference in the consumption of electricity per effective occupied square foot.

  
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