Comfort, Indoor Air Quality, 

and Energy Consumption in 

Low Energy Homes

P. Engelmann, K. Roth, and V. Tiefenbeck

Fraunhofer Center for Sustainable Energy Systems

January 2013
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iii

Comfort, Indoor Air Quality, and Energy Consumption in 

Low Energy Homes

Prepared for:

The National Renewable Energy Laboratory 

On behalf of the U.S. Department of Energy’s Building America Program

Office of Energy Efficiency and Renewable Energy

15013 Denver West Parkway

Golden, CO 80401

NREL Contract No. DE-AC36-08GO28308

Prepared by: 

P. Engelmann, K. Roth, and V. Tiefenbeck

Fraunhofer Center for Sustainable Energy Systems

25 First Street

Cambridge, MA 02141

NREL Technical Monitor: Chuck Booten

Prepared under Subcontract No. KNDJ-0-40345-00

January 2013
iv

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v

Contents

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................ viii

Definitions................................................................................................................................................... ix

Executive Summary.................................................................................................................................... x

Acknowledgments .................................................................................................................................... xii

1 Introduction........................................................................................................................................... 1

1.1 Net Zero Energy Balance.....................................................................................................1

1.2 The Net Zero Energy Home in Stow, Massachusetts..........................................................2

1.3 The Near Zero Energy Home in Townsend, Massachusetts................................................3

1.4 Data Acquisition and Instrumentation .................................................................................4

2 Building Performance .......................................................................................................................... 6

2.1 Yearly Energy Balance ........................................................................................................7

2.1.1 Stow .........................................................................................................................7

2.1.2 Townsend...............................................................................................................10

2.2 Seasonal Energy Balance...................................................................................................13

2.2.1 Stow .......................................................................................................................13

2.2.2 Townsend...............................................................................................................15

2.3 Correlation of Energy Consumption and Ambient Conditions..........................................16

2.4 Summary Energy Consumption.........................................................................................17

3 Thermal Comfort................................................................................................................................. 17

3.1 Static Comfort Models.......................................................................................................18

3.2 Adaptive Comfort Models .................................................................................................19

3.3 Thermal Comfort in Winter ...............................................................................................20

3.3.1 Stow .......................................................................................................................20

3.3.2 Townsend...............................................................................................................22

3.3.3 Summary: Thermal Comfort in the Heating Season..............................................24

3.4 Thermal Comfort in Summer.............................................................................................24

3.4.1 Stow .......................................................................................................................25

3.4.2 Townsend...............................................................................................................26

3.4.3 Summary: Thermal Comfort in the Cooling Season..............................................26

3.5 Summary: Thermal Comfort With Low Energy Consumption .........................................26

4 Indoor Air Quality ............................................................................................................................... 28

4.1 Carbon Dioxide Concentration ..........................................................................................28

4.1.1 Stow .......................................................................................................................28

4.1.2 Townsend...............................................................................................................30

4.2 Volatile Organic Compound and Aldehyde Concentrations .............................................31

4.2.1 Air Sampling Protocol and Analysis......................................................................31

4.2.2 Ventilation Rate Settings .......................................................................................34

4.3.3 Results....................................................................................................................35

4.3.4 Summary for Volatile Organic Compounds and Aldehydes .................................42

5 Conclusions ........................................................................................................................................ 44

References................................................................................................................................................. 45
vi

List of Figures

Figure 1. Graphical illustrations of the net zero energy concept........................................................... 2

Figure 2. The NZEH in Stow, Massachusetts ........................................................................................... 2

Figure 3. Floor plans and position of the mini-split indoor units (red boxes) in the Stow home Left: 

Basement. Right: 1st floor. The positions of the mini-split indoor units are marked by the red 

boxes. .................................................................................................................................................... 2

Figure 4. Floor plans from the Stow home. Left: 2nd floor. Right: Finished attic................................ 3

Figure 5. The NZEH in Townsend Left: Northeast view. Middle: South view. The building is part of 

a low-energy homes development, most of them equipped with PV generators. The room 

above the garage is unconditioned space. Right: Wall-to-window ratios for different 

orientations (without garage/extension). ........................................................................................... 3

Figure 6. Floor plans for the Townsend home Left: 1st floor. Right: 2nd floor. Attic and basement 

are inside the thermal envelope, but are unconditioned spaces. The red boxes mark the 

position of the mini-split indoor units................................................................................................ 4

Figure 7. Left: Both homes were equipped with sensors for ambient temperature and RH. Middle: 

Electric power consumption captured on a circuit level at the breaker box. Right: Electric 

power consumption visualized on a Web portal............................................................................... 5

Figure 8. Left: Loggers used for temporary indoor measurements. Right: The loggers have 

external channels for additional sensors; e.g., for CO2 concentration.......................................... 5

Figure 9. Ambient conditions during the monitoring period as weekly sum (insolation) and weekly 

averages (temperature). The error bars for temperature indicate minimum and maximum 

according to week. Due to sensor errors, there are two gaps in the solar irradiation data. ........ 6

Figure 10. Breakdown of electric energy consumption in Stow by circuits......................................... 7

Figure 11. Breakdown of electric energy consumption in Stow by categories.................................... 7

Figure 12. Monthly energy balance for power consumptionand PV generation.................................. 8

Figure 13. Carpet plot of whole-home average electric power draw throughout the year (5-min 

intervals)................................................................................................................................................ 9

Figure 14. Electricity load pattern for the ductless mini-split system. Power clearly correlates with 

ambient conditions............................................................................................................................... 9

Figure 15. Power draw pattern for the basement outlets circuit............................................................ 9

Figure 16. In April 2011 the user switched the water heater pump to the laundry outlets circuit. 

The reason for the timer schedule change in July 2011 is unknown............................................ 10

Figure 17. Breakdown of the end-use energy in Townsend, including natural gas for DHW........... 10

Figure 18. Breakdown of the end-use energy in Townsend by categories......................................... 11

Figure 19. Monthly balance for electric site energy only. DHW is heated by an instantaneous gas 

heater (no monthly consumption data are available for gas consumption). ............................... 12

Figure 20. Total electric power draw over the 12-month period .......................................................... 12

Figure 21. Power draw of the ductless mini-splits (sum of first- and second-floor units)................ 13

Figure 22. Power draw of the mechanical ventilation system.............................................................. 13

Figure 23. Load patterns for PV generation (orange curves) and power consumption (green 

curves)................................................................................................................................................. 14

Figure 24. Net energy balance for different seasons as averaged daily balance............................... 14

Figure 25. Energy consumption as a function of day of the week....................................................... 14

Figure 26. Daily load curves for different seasons in Townsend: PV production (orange), 

consumption (green curves) ............................................................................................................. 15

Figure 27. Net energy balance in different seasons as averaged daily balance ................................ 15

Figure 28. Energy consumption as a function of weekdays ................................................................ 16

Figure 29. Schematic sketch of ambient conditions and heating/cooling loads in conventional 

(light color) and optimized/low energy homes (dark color)........................................................... 16

Figure 30. Correlation of ambient temperature and average annual specific electric energy 

loadsfor the ductless mini-split in both NZEHs .............................................................................. 17

Figure 31. Ambient temperature in the first weeks of the test period................................................. 20

Figure 32. Plot of indoor against ambient temperatures, divided into daytime and night................ 20

Figure 33. Overlay of indoor temperature curves over the course of the day throughout the year in 
vii

the Stow house ................................................................................................................................... 20

Figure 34. Temperature measurements averaged over the test period (Stow) .................................. 21

Figure 35. Indoor conditions and comfort zones according to ASHRAE 55 (Stow) .......................... 21

Figure 36. Indoor comfort and comfort zones in the master bedroom (Stow) ................................... 22

Figure 37. Correlation of indoor and outdoor temperature in Townsend........................................... 22

Figure 38. Overlay of indoor temperature curves over the course of the day throughout the year in 

the Townsend house.......................................................................................................................... 22

Figure 39. Averaged indoor temperature profile in the living room (Townsend)............................... 23

Figure 40. Comfort limits for the living room in Townsend.................................................................. 23

Figure 41. Comfort rating for a bedroom in Townsend......................................................................... 23

Figure 42. Temperature and RH during the summer months in Stow................................................. 24

Figure 43. Temperature and RH during the summer months in Townsend........................................ 24

Figure 44. Comfort limits and measured temperature and RH conditions in the living room (Stow)25

Figure 45. Comfort limits and measured temperature and RH conditions in the master bedroom 

(Stow)................................................................................................................................................... 25

Figure 46. Comfort limits and measured temperature and RH conditions in the finished attic 

(Stow)................................................................................................................................................... 25

Figure 47. Comfort limits and measured temperature and RH conditions in the living room 

(Townsend).......................................................................................................................................... 26

Figure 48. Comfort limits and measured temperature and RH conditions in a bedroom (Townsend)26

Figure 49.CO2 concentration and RH in the Stow living room during the summer test period ....... 29

Figure 50.CO2 concentration and RH in the Stow master bedroom during the summer test period29

Figure 51. Cross section and picture of the ventilation unit in Stow When the defrost-damper was 

in defrost mode, it blocked the fresh air inlet and opened a bypass for the exhaust air. ......... 29

Figure 52.CO2 concentration and RH in the Stow living room in the winter test period................... 30

Figure 53.CO2 concentration and RH in the Stow master bedroom in the winter test period.......... 30

Figure 54.CO2 concentration and RH in the Townsend living room in winter with different settings 

of the mechanical ventilation system............................................................................................... 31

Figure 55.CO2 concentration and RH in the Townsend master bedroom in winter with different 

settings of the mechanical ventilation system................................................................................ 31

Figure 56. Vials with perfluorocarbon (PMCH and PDCB) and HB emitters ....................................... 33

Figure 57. Active samplers for PFT (blue box) and VOC and aldehydes (peristaltic pump)............. 34

Figure 58. Air exchange profile in m3/h between zones and across the envelope for Townsend.... 35

Figure 59. Indoor formaldehyde concentrations at three air exchange rates (BA1 corresponds to 

Stow, BA2 to Townsend) ................................................................................................................... 42

Unless otherwise indicated, all figures were created by Fraunhofer CSE.
viii

List of Tables

Table 1. Characteristic Data for the Stow NZEH ...................................................................................... 3

Table 2. Characteristic Data for the Townsend NZEH............................................................................. 4

Table 3. Heating and Cooling Degree Days for the Observation Period, Compared to the LongTerm Average for Boston (ASHRAE 2009)......................................................................................... 6

Table 4. Total and Specific Energy Consumption for Every Circuit at the Breaker Box in Stow ....... 8

Table 5. Total and Specific End-Use Energy Consumption in Townsend .......................................... 11

Table 6. Sampling Plan and Measurement Methods ............................................................................. 32

Table 7. Calculated Overall Home Outdoor Air Exchange Rates (Based on PFT Concentrations 

Measured in Bag Samples Collected 2:00 p.m. to 4:40 p.m. Each Friday Afternoon)................ 35

Table 8. VOC Concentrations in Stow and Townsend at Three Ventilation Rate Settings ............... 37

Table 9. Aldehyde Concentrations in Stow and Townsend at Three Ventilation Rate Settings....... 38

Table 10.VOCs Concentrations in Stow at Three Ventilation Rates Based on Passive Sampling 

Data (Data Shown for Four Locations)............................................................................................ 39

Table 11. VOCs Concentrations in Townsend at Three Ventilation Rates Based on Passive 

Sampling Data (Data Shown for Four Locations)........................................................................... 40

Table 12. Aldehydes Concentrations in Stow at Three Ventilation Rates Based on Passive 

Sampling Data (Data Shown for Four Locations)........................................................................... 41

Table 13. Aldehydes Concentrations in Townsend at Three Ventilation Rates Based on Passive 

Sampling Data (Data Shown for Four Locations)........................................................................... 41

Unless otherwise indicated, all tables were created by Fraunhofer CSE.
ix

Definitions

ACH Air changes per hour

ASHRAE American Society of Heating, Refrigerating and Air 

Conditioning Engineers

CO2 Carbon dioxide

COP Coefficient of performance

DHW Domestic hot water

ERV Energy recovery ventilator

HB hexafluorobenzene

HRV Heat recovery ventilator

IAQ Indoor air quality

LBNL Lawrence Berkeley National Laboratory

NZEH Net zero-energy home

OA Outdoor air

PFT Perfluorocarbon tracer

PDCB perfluorodimethylcyclobutane

PMCH perfluoromethylcyclohexane

PMV Predicted mean vote

PPD Percentage of dissatisfaction

PV Photovoltaics

RH Relative humidity

VOC Volatile organic compound
x

Executive Summary

A home with an optimized building envelope, including enhanced insulated walls, highefficiency windows, and airtight construction, can achieve very low energy consumption, 

foremost in cold climates. These measures also have a substantial impact on indoor conditions, 

notably occupant comfort in winter and indoor air quality (IAQ). Specifically, high insulation 

levels produce indoor surface temperatures close to the indoor air temperature, increasing the 

mean radiant temperature in winter and (to a lesser extent) decreasing it in summer. Airtight 

construction prevents uncontrolled ventilation heat losses and protects the building structure, but 

creates the need for mechanical ventilation to ensure sufficient outdoor air supply.

A marketing claim frequently made about high performance homes is that, in addition to being 

extremely energy efficient, they offer increased comfort and IAQ, and are thus healthier than 

conventional homes. This study investigates whether these claims can be supported by measured 

field data.

This report documents the results of an in-depth evaluation of thermal comfort for two potential 

net zero-energy homes (NZEHs) in Massachusetts. We also present the results of an IAQ 

evaluation that was part of a larger study done in conjunction with Lawrence Berkeley National 

Laboratory (LBNL). Even though the homes were monitored and evaluated in great detail, the 

results must be seen as case studies, as occupant behavior has a dramatic impact on energy 

consumption. Nevertheless, research in the United States (Parker 2008) and Europe (Feist et al. 

2001; Passipedia 2012) clearly shows that improvements in energy efficiency measures lead to 

significant reductions in energy consumption. 

The report first gives a brief comparative outline of the two homes and describes the data 

acquisition methods and instrumentation. Section 2 analyzes energy consumption and production 

of both homes over the course of the year, from monthly energy balance down to 5-minute

interval load data. One home did not reach a “zero” balance (over a full year); the other produced

nearly 30% more energy than it consumed.

Section 3 evaluates the homes’ thermal comfort performance. In both homes, residents reported 

no discomfort issues, although indoor temperatures in winter often fell below ASHRAE’s 

comfort range. The heating systems in both homes did not operate at their full capacities, 

suggesting that the heating systems did not limit comfort and the homes seemed to meet 

residents’ comfort preferences. Summer indoor temperatures remained below 82°F (28°C) in 

both homes, thus within ASHRAE’s comfort range. However, higher relative humidity levels 

could cause discomfort.

Section 4 addresses IAQ, focusing on carbon dioxide as indicator for air exchange rates and 

indoor air pollutants. More detailed tests were carried out over three-week periods in 

collaboration with LBNL. The results underline the necessity of mechanical ventilation in 

airtight homes, but also the challenges in commissioning and running these systems, mainly 

because information about “proper” ventilation for occupants and for the builder is lacking.
xi

Overall, the report shows that aggressive NZEH goals are realistic in terms of their energy 

balance at comfort levels that can meet residents’ preferences. Whether (and to what extent) this 

holds true for residents with more typical comfort preferences, especially with respect to indoor 

temperatures in winter, remains to be evaluated. The report also shows the importance of IAQ

testing in these tight constructions to ensure these homes meet the claims of being “healthier” 

than typical homes.

From the detailed analysis of the energy flows we can draw the following conclusions:

• Even with a highly optimized thermal envelope, nearly half the energy is still used for 

space conditioning (dominated by space heating). But both homes have unusually low 

loads for other electricity consumption (appliances, miscellaneous, and plug loads).

• The indoor thermal comfort met user expectations, although the homes as operated by the 

occupants did not always meet ASHRAE comfort standards. 

• Occupants do not fully understand IAQ. As long as there are no odorous pollutants, they

have no sensor for air quality. A sufficient air exchange must be ensured; in low energy 

homes this is typically done by mechanical ventilation.

• The installation, commissioning, maintenance, and operation of mechanical ventilation 

show room for improvement.
xii

Acknowledgments

The authors thank Drs. Henry Willem and Brett Singer of LBNL for their collaboration on this 

project and for composing most of the section on “Volatile Organic Compound and Aldehyde 

Concentrations.” We also thank the Fraunhofer Institute for Solar Energy Systems for enabling 

the continued participation of the lead author after he moved from Fraunhofer Center for 

Sustainable Energy to Fraunhofer Institute for Solar Energy Systems. We also thank Mr. Carter 

Scott of Transformations, Inc. for his support in pursuing in-depth monitoring of the two homes 

his company built. Finally, we thank the owners and occupants of the two homes, who 

enthusiastically welcomed us to monitor their homes.

This project was supported by the U.S. Department of Energy Building America program 

beginning in April 2012. Before that time (beginning in spring 2010), the project was funded 

entirely by Fraunhofer Center for Sustainable Energy.
1

1 Introduction

A home with an optimized building envelope, including enhanced insulated walls, highefficiency windows, and airtight construction, can achieve very low energy consumption, 

foremost in cold climates. These measures also have a substantial impact on indoor conditions, 

notably occupant comfort in winter and indoor air quality (IAQ). Specifically, high insulation 

levels produce indoor surface temperatures close to the indoor air temperature, increasing the 

mean radiant temperature in winter and (to a lesser extent) decreasing it in summer. Airtight 

construction prevents uncontrolled ventilation heat losses, but creates the need for mechanical 

ventilation to ensure sufficient outdoor air (OA) supply.

Furthermore, a marketing claim frequently made about high-performance homes is that, in 

addition to being extremely energy efficient, they offer increased comfort and IAQ, and are thus 

healthier than conventional homes. 

Achieving widespread deployment of very low-energy homes is a core goal of the Building 

America program. To date, however, very few such homes have been built. In this project, we 

performed an in-depth evaluation of two potential net zero-energy homes (NZEHs)recently 

constructed in Massachusetts to provide data to help evaluate three questions that need to be 

affirmatively addressed to achieve broad market acceptance of such homes:

• Can they consume much less energy than conventional homes?

• Can they provide superior—or at least acceptable—occupant comfort relative to 

conventional homes?

• Can they provide superior—or at least acceptable—IAQ relative to conventional homes?

Even though the homes were monitored and evaluated in great detail, the results must be seen as 

case studies, as occupant behavior has a dramatic impact on energy consumption and acceptable 

comfort conditions. 

1.1 Net Zero Energy Balance

The NZEH concept is to greatly reduce energy consumption through energy efficiency and to 

meet the remaining energy needs with renewable sources (see Figure 1). These buildings are 

normally connected to the power grid and achieve a net zero energy balance over the course of a 

year. Different ways of defining a net zero balance, balance boundaries, and type of energy 

(primary energy, site energy, etc.) is discussed by Torcellini et al. (2006).
2

Figure 1. Graphical illustrations of the net zero energy concept

Left: Efficiency measures (enhanced building envelope, energy saving appliances, etc.) reduce 

energy consumption. Right: The remaining demand is supplied by renewable energy sources 

supply the remaining demand.

1.2 The Net Zero Energy Home in Stow, Massachusetts

The Stow home (see Figure 2 through Figure 4) was constructed in spring 2010; monitoring 

started in mid-July, and the new homeowners completely moved in by the end of July. The house 

is all electric; space heating and cooling are provided by a 10.5-kWth (36,000-Btu/h) ductless 

mini-split with one outdoor and four indoor units (one on every floor). Potable hot water was 

initially provided by an electric resistance heater; in October 2010, it was changed to an airsource heat pump (see Table 1).

Figure 2. The NZEH in Stow, Massachusetts

Left: Southeast view. Middle: South and northwest views.

Right: Wall-to-window ratio for the orientations.

Figure 3. Floor plans and position of the mini-split indoor units (red boxes) in the Stow home

Left: Basement. Right: 1st floor. The positions of the mini-split indoor units are 

marked by the red boxes.
3

Figure 4. Floor plans from the Stow home. Left: 2nd floor. Right: Finished attic.

Table 1. Characteristic Data for the Stow NZEH

Characteristics Values

Floor Area 2964 ft2 (275 m2)

Calculated Demand for Space Heating 12 kBtu/ft2

/yr (37.8 kWh/m2/yr)

Insulation (ft2

·°F·h/Btu) (W/m2K)

Exterior Walls to Ambient R65 0.09

Exterior Walls to Ground R25 0.23

Roof R63 0.09

Windows R5.1 1.11

Ventilation Mechanical, with heat recovery, ηHRV≅60%

Photovoltaic (PV) Generator 6.3 kWp

HERS Index 9

The retail contract cost on the Stow home was approximately $307,000, including the PV 

system, or $104/ft2. The home has four floors and no garage, which brings down specific cost.

1.3 The Near Zero Energy Home in Townsend, Massachusetts

The Townsend home (see Figure 5 and Figure 6) was completed in summer 2010. The new 

owners moved in during July and August 2010. Two ductless mini-splits, each with a single 

indoor unit, meet space heating and cooling loads (3.5 kWth/12,000 Btu/h each). Unlike the 

NZEH in Stow, this house has an instantaneous gas heater for water heating. Despite similar 

exterior dimensions, the conditioned floor area is smaller relative to the house in Stow because 

attic and basement are conditioned spaces there. The room above the garage is not conditioned to 

date, but was prepared as room for expansion (see Table 2).

Figure 5. The NZEH in Townsend

Left: Northeast view. Middle: South view. The building is part of a low-energy homes development, 

most of them equipped with PV generators. The room above the garage is unconditioned space.

Right: Wall-to-window ratios for different orientations (without garage/extension).
4

Figure 6. Floor plans for the Townsend home

Left: 1st floor. Right: 2nd floor. Attic and basement are inside the thermal envelope, but are 

unconditioned spaces. The red boxes mark the position of the mini-split indoor units.

Table 2. Characteristic Data for the Townsend NZEH

Characteristics Values

Floor Area 1835 ft2(170 m2)

Calculated Demand for Space Heating 19.2kBtu/ft2

/yr(60.8 kWh/m2/yr)

Insulation (ft2

·°F·h/Btu) (W/m2K)

Exterior Walls to Ambient R47 0.12

Exterior Walls to Ground R 24 0.23

Roof R 63 0.09

Windows R 4.8 1.2

Ventilation Mechanical, with heat recovery, ηHRV≅60%

PV Generator 7.14 kWp

HERS Index 2

The cost of the Townsend home was $245,000, including the PV system, or $134/ft2

. This is a 

two-story home and has a garage.

1.4 Data Acquisition and Instrumentation

We installed fixed components with continuous data recording to collect data in the two homes. 

We also used temporary sensors and data loggers for short-term and seasonal tests.

Fixed installations included small weather stations for ambient conditions (onsite measurement 

of ambient temperature and relative humidity (RH) (see Figure 7) at each site. Data were 

captured and stored in a battery- and solar-powered data logger. The second permanent 

installation was an electric power meter that used split-core current transformers to measure the 

electric power consumption of each circuit at the breaker box, as well as the PV generation, at 1-

min intervals. The device, an eMonitor from Powerhouse Dynamics, uploads the data to a Web 

portal to be processed and visualized for the user (see Figure 7). The electricity consumption 

values measured were apparent power values at the beginning of the study. In January 2011, the 

manufacturer conducted a firmware update to measure real power instead. Except for a bathroom 
5

circuit in one home (which reported a continuous consumption of 4–5 W before the upgrade and 

then changed to 0 W), the data did not change significantly, and nothing occurred that would 

have drastically affected the overall energy balance. In both homes the occupants had access to 

the Web portal that shows the energy consumption (aggregated and in real time).

Figure 7. Left: Both homes were equipped with sensors for ambient temperature and RH. 

Middle: Electric power consumption captured on a circuit level at the breaker box. 

Right: Electric power consumption visualized on a Web portal.

For temporal measurements, small data loggers with integrated or attached sensors were used. 

These loggers, from the “HOBO” series from onset computers, measured indoor temperature and 

RH. For IAQ tests they were extended with a carbon dioxide (CO2) sensor (see Figure 8).

Figure 8. Left: Loggers used for temporary indoor measurements. 

Right: The loggers have external channels for additional sensors; e.g., for CO2 concentration.
6

2 Building Performance

The analysis of the energy consumption and building performance is based on 12 months of data 

for both homes. The measurement period in Stow was September 1, 2010 to August 31, 2011; 

for Townsend it was November 1, 2010 to October 31, 2011. Figure 9 gives a graphical 

overview of the ambient conditions for 2010 and 2011; Table 3 lists characteristic values 

(heating and cooling degree days) for the observation period (data from September 1, 2010 to 

August 31, 2011).

Figure 9. Ambient conditions during the monitoring period as weekly sum (insolation) and 

weekly averages (temperature). The error bars for temperature indicate minimum and maximum

according to week. Due to sensor errors, there are two gaps in the solar irradiation data.

Table 3. Heating and Cooling Degree Days for the Observation Period, 

Compared to the Long-Term Average for Boston (ASHRAE 2009) 

(°C)

2010/2011 Average, Boston

Heating Degree Days 65 3,476 3,123

Cooling Degree Days 65 803 417

The focus of this section is to analyze the overall energy use for various end uses. The question 

is whether other categories (domestic hot water [DHW] or miscellaneous loads) become more 

important because building energy efficiency has improved. Another obvious question is whether 

the net zero-energy balance is achieved.

As mentioned in Section 1.4, we measured only the electric energy consumption. As a heat pump 

system is used for heating and cooling, a comparison of measured energy consumption (electric) 

and loads for space heating(thermal) is difficult, because the coefficient of performance (COP, 

the ratio of electric energy input to thermal energy output) changes with the ambient 

temperatures. With the given instrumentation we cannot evaluate the performance of the minisplit. Although this is an important question, the space heating loads remain dominant in these 

buildings.
7

2.1 Yearly Energy Balance

2.1.1 Stow

As described in Section 1.2, the home is all electric: space heating is provided by a heat pump.

DHW was heated with an electric resistance heater in the storage tank at the beginning, later by 

an air-source heat pump (in the basement), combined with the resistance heater for peak loads.

Figures 10, 11 and 12 show the breakdown of electric energy consumption over 12 months. The 

consumption is clearly dominated by the mini-split system, which consumes nearly half the 

energy. DHW is “hidden” in different circuits. Only the direct resistance heater has a dedicated 

circuit; the heat pump water heater was plugged into different circuit over the course of a year,

mainly to basement outlets and later switched to the laundry outlet (see also Figure 15 and 

Figure 16).One difficulty for assessing and comparing the data was an unusually high occupation 

density from spring 2011 onward. Because of long-term visitors, the house was occupied by 

eight people instead of four. 

Figure 10. Breakdown of electric energy consumption in Stow1 by circuits

Figure 11. Breakdown of electric energy consumption in Stow by categories

Even though the building is super-insulated, nearly 50% of the electric energy consumption is 

used for the mini-split. On the other hand, the specific use is very low: 16.5 kWhel/m2 for heating 

and cooling (see Table 4). The mini-split still dominates the overall consumption because the 

occupants are extremely aware of energy consumption. The electric energy, excluding heating, 

cooling, DHW, and refrigerator, is approximately 3500 kWh; the national average in the U.S.

Northeast is 6,793 kWh (DOE 2010).

 1

nomenclature: out is used for outlets.

MiniSplit, 45%

basement out, 

12%

radon fan, 5%refrigerator, 5% DHW el, 5%

HRV, 5%

laundry, 5%

well filter, 4%

living room, 

3%

stove, 3%

mudroom out well pump

small bedroom

dishwasher

attic

kitchen

basement light

upstairs hall

mudroom light

hood

kitchen Lights

bathroom out

outside out

Other, 18%
8

Table 4. Total and Specific Energy Consumption for Every Circuit at the Breaker Box in Stow

kWh kWh/m2yr kWh/ft2yr % of total

Total 10,120 36,8 3,4

Mini-Split 4,545 16.5 1.5 45%

Basement Outlets 1,237 4.5 0.4 12%

Radon Fan and Smoke Detectors 539 2.0 0.2 5%

Refrigerator 515 1.9 0.2 5%

DHW Direct Electric 500 1.8 0.2 5%

Heat Recovery Ventilator (HRV) 478 1.7 0.2 5%

Laundry 471 1.7 0.2 5%

Well Filter 449 1.6 0.2 4%

Living Room 322 1.2 0.1 3%

Stove 293 1.1 0.1 3%

Mudroom Outlets 169 0.6 0.1 2%

Well Pump 126 0.5 0.04 1%

Small Bedroom 105 0.4 0.04 1%

Dishwasher 100 0.4 0.03 1%

Attic 77 0.3 0.03 1%

Kitchen 60 0.2 0.02 1%

Basement Light 33 0.1 0.01 <1%

Upstairs Hall 33 0.1 0.01 <1%

Mudroom Light 20 0.1 0.01 <1%

Hood 19 0.1 0.01 <1%

Kitchen Lights 13 0.0 0.00 <1%

Bathroom Outlets 10 0.0 0.00 <1%

Outside Outlets 7 0.0 0.00 <1%

kWh kWh/kWp

PV Generation 7,778 1,235

The breakdown for the mini-split into heating and cooling is based on the daily average of 

ambient temperatures; the turnover point is set to 59°F (15°C): on days with an average ambient 

temperature lower than 59°F (15°C) the power consumption was accounted as heating, above 

59°F (15°C) as cooling.

Figure 12. Monthly energy balance for power consumption2and PV generation

 2

Six circuits with the highest consumption are shown separately, remaining loads are lumped together in “other”

0

400

800

1200

1600

Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug

PV MiniSplit

basement out. smoke det./radon fan

DHW el. refrigerator

HRV other [kWh]
9

Figure 13 through Figure 16 depict specific and typical patterns in the overall electricity loads, as 

well as in dedicated circuits. In these carpet plots, the value of electric power (in Watts) is colorcoded and the time dimension is plotted against the two axes; the x-axis represents the course of 

the year, the y-axis the 24 hours of a day. 

Figure 13. Carpet plot of whole-home average electric power draw throughout the year

(5-min intervals)

Figure 14. Electricity load pattern for the ductless mini-split system.

Power clearly correlates with ambient conditions.

Figure 15. Power draw pattern for the basement outlets circuit3

 3 The base load of this circuit stems from home office equipment in the basement. In September 2010, the 

homeowner installed a heat pump for heating DHW (also located in the basement). To avoid acoustic noise 

problems, the heat pump is controlled by a timer that generates the repetitive patterns seen.
10

Figure 16. In April 2011 the user switched the water heater pump to the laundry outlets circuit. 

The reason for the timer schedule change in July 2011 is unknown.

2.1.2 Townsend

As described in Section 1.3, the Townsend home was built and occupied a few months later than 

the Stow home. The 12-month period for the energy balance is November 1, 2010 to October 31, 

2011. Unlike the Stow home, it relies on two energy sources: electricity for space heating 

(ductless mini-split) and appliances, and natural gas for DHW. The following numbers represent 

site energy consumption; we did not convert the value to primary or source energy.

The numbers for the total consumption include two smaller and a large data gap: from December 

23, 2010 to January 12, 2011 the eMonitor was switched off, so no data were recorded during 

that period. From February 19 to February 27, 2011, the eMonitor was sent to the manufacturer 

for an upgrade; from March 6 to March 14 an unknown error interrupted its operation. Table 5

includes an estimate of the consumption during that time, based on average values and outdoor

temperature correlations.

Figure 17. Breakdown of the end-use energy in Townsend, including natural gas for DHW

minisplit; 47%

nat gas; 18%

fridge; 9%

smoke 

sensors, bedroom, 5%

living room, 5%

oven, 3%

DHW system

HRV, 3%

eMonitor

kitchen outlets

dishwasher

microwave

basement lights

left bedroom

clothes dryer

kitchen lights

bathroom 2nd

middle bedroom

laundry

bathroom 1st

garage out

other; 13%
11

Figure 18. Breakdown of the end-use energy in Townsend by categories

The picture for a breakdown of energy consumption (based on circuits or by category) shows a 

similar picture as in Stow: by far the biggest load is the mini-split. But again the consumption for 

household appliances and miscellaneous loads is very low.

The PV generation exceeds the consumption by approximately 1500 kWh (end-use energy 

balance), even though the specific output of the PV system is rather low: 960 kWh/kWp is a low 

yield given the geographic location (compared to Stow: 1,235 kWh/kWp). This is mainly caused 

by local shading conditions (see Table 5 and Figure 19 through Figure 22; see also Section 2.2.2

and Figure 26).

Table 5. Total and Specific End-Use Energy Consumption in Townsend

(Due to data gaps in the heating period, an estimate of the 

real consumption is added in parentheses.)

kWh1 kWh/m2/yr kWh/ft2/yr

% of 

Total

Total

Electric: 

4364 (4868)

Incl. Gas: 

5321(5825)

Electric: 

25,7 (28,6)

Incl. Gas: (34,3)

Electric: 

2,4 (2,7)

Incl. Gas: (3,2)

Ductless Mini-split 2498 14.7 1.4 47

%

Refrigerator 503 3.0 0.3 9%

Smoke Sensors, 

Bedroom 244 1.4 0.1 5%

Living Room 245 1.4 0.1 5%

Oven 170 1.0 0.1 3%

DHW System 

(Pumps) 126 0.7 0.1 2%

Hrv 140 0.8 0.1 3%

Outlet eMonitor 84 0.5 0.05 2%

Kitchen Outlets 72 0.4 0.04 1%

Dishwasher 54 0.3 0.03 1%

Microwave 48 0.3 0.03 1%
12

kWh1 kWh/m2/yr kWh/ft2/yr

% of 

Total

Basement Lights 43 0.3 0.02 1%

Left Bedroom 35 0.2 0.02 1%

Clothes Dryer 34 0.2 0.02 1%

Kitchen Lights 30 0.2 0.02 1%

Bathroom 2nd 16 0.1 0.01 0%

Middle Bedroom 7 0.0 0.00 0%

Laundry 6 0.0 0.00 0%

Bathroom 1st 1 0.0 0.00 0%

Garage Out 9 0.1 0.01 0%

Natural Gas 

(DHW) 957 5.6 0.5 18

%

kWh kWh/kWp

PV Generation 6860 961

Figure 19. Monthly balance for electric site energy only. DHW is heated by an instantaneous gas 

heater (no monthly consumption data are available for gas consumption).

Figure 20. Total electric power draw over the 12-month period
13

Figure 21. Power draw of the ductless mini-splits (sum of first- and second-floor units)

Figure 22. Power draw of the mechanical ventilation system4

2.2 Seasonal Energy Balance

In addition to the yearly balance, this section also presents differences in building energy 

performance for the summer, winter, and shoulder seasons.

2.2.1 Stow

Figure 23 shows loads for PV generation and consumption as average values, based on a threemonth time period. Values for the same time of the day were averaged over three months and 

plotted as a daily load curve. Interesting observations of the loads in Stow include:

• PV peak loads at noon do not largely differ among seasons.

• Energy consumption is very low and consistent in summer and the shoulder seasons.

• In winter, heating loads nearly double the overall load.

 4 In May 2011 the system was set to constant operation because of IAQ measurements (see Section 4.2). In autumn 

2011 the users switched to automated timer-controlled settings, and from October 2011 onward to 24/7 operation.
14

Figure 23. Load patterns for PV generation (orange curves) and 

power consumption (green curves)

In Figure 24, consumption versus generation is plotted as a three-month-cumulative energy 

balance over the course of a day. Only in the summer months (July and August) does the PV 

system provide enough energy for a net-zero balance, or even a small PV surplus. In shoulder 

seasons the energy balance is slightly negative, and in winter the lower PV yield (in combination 

with higher loads) leads to a larger overall net import of energy from the grid.

Figure 24. Net energy balance for different seasons as averaged daily balance

We also investigated whether load patterns differ over the course of a week. For Stow there is no 

evidence for a correlation between electric power consumption and day of the week. Figure 25

shows the accumulated energy consumption for each weekday. The five circuits consuming the 

most energy are shown separately; the other loads are combined.

Figure 25. Energy consumption as a function of day of the week

-4

-3

-2

-1

0

1

2

3

4

00:00 04:00 08:00 12:00 16:00 20:00

Jun Jul Aug

Mar Apr May

Sep Oct Nov

Dec Jan Feb

Jun Jul Aug

Mar Apr May

Sep Oct Nov

Dec Jan Feb

[kW]
15

2.2.2 Townsend

The comparison of average loads versus average PV generation between seasons (Figure 26) 

shows different characteristics from the home in Stow: PV production clearly decreases during 

winter, mostly because of topography: south of the house a hill and trees block the sun in the 

early afternoon during the winter.

Figure 26. Daily load curves for different seasons in Townsend: PV production (orange), 

consumption (green curves)

As the accumulated daily net balance shows (see Figure 27), the energy balance is positive most 

of the year. In the winter months (December, January, and February), consumption exceeds the 

PV generation by about 1000 kWh, mainly because the PV array is shaded.

Figure 27. Net energy balance in different seasons as averaged daily balance

In contrast with the Stow house, there is a slight correlation between energy consumption and 

day of the week; i.e., energy consumption on Sundays is typically somewhat higher. This is

caused by higher loads in the living room outlets and kitchen (including oven), but the other 

loads are quite stable. Figure 28 shows the sum of the five biggest consumers; the other circuits 

are subsumed in sum rest.
16

Figure 28. Energy consumption as a function of weekdays

2.3 Correlation of Energy Consumption and Ambient Conditions

A key question is how much energy is used to maintain comfortable conditions (primarily stable 

indoor temperatures). The graphical analysis of the correlation of ambient temperature and loads

for space conditioning can be used to answer this. Reduced heat exchange with the environment,

combined with solar and internal gains, drastically reduces sensible heating and cooling loads. In 

theory, the temperature range where a building stays comfortable with passive measures widens 

and the temperature slope (load correlation) has a flatter angle (see Figure 29).

Figure 29. Schematic sketch of ambient conditions and heating/cooling loads in conventional 

(light color) and optimized/low energy homes (dark color)

Figure 30 shows specific daily average loads of the ductless mini-split, correlated to daily mean 

values of the ambient temperature. The correlation plots for both homes confirm that the 

dominating loads are caused by low ambient temperatures in winter. Cooling loads in summer 

are small or occur for only a limited part of the day, resulting in low daily average values (hardly 

more than 2 W/m2electric).
17

Figure 30. Correlation of ambient temperature and average annual specific electric energy 

loads5

for the ductless mini-split in both NZEHs

2.4 Summary Energy Consumption

The main results from the overall analysis of the energy consumption are:

• Even though the loads for space conditioning are drastically reduced (specific electric 

energy consumption for the mini-splits: 15 and 16 kWh/m2/yr) remain the biggest loads 

in terms of percentage from the total consumption.

• The remaining loads (appliances and miscellaneous electric loads) are very low. This is 

mostly because the occupants of both homes decided to buy energy efficient houses and 

tried to use as little energy as possible. They did nothing extraordinary to achieve a low

consumption level. They used compact fluorescent lamps for lighting and switched off

devices that were not in use (partly by using switchable power strips). It shows bigger 

differences in user behavior, even though the energy consumption is quite similar.

• The measured performance of these buildings proofs them as very energy efficient. 

Without knowing the COP of the mini-split, a comparison of measured consumption and 

calculated demand is difficult. Assuming an average COP of 2.5 (manufacturer manual: 

COP for heating = 3.9), both buildings perform as well as, or better than, calculated. A 

comparison of energy for space heating:

o Stow: calculated demand: 37.8 kWh/m2/yr; measured consumption: 

38.0 kWh/m2/yr

o Townsend: calculated demand: 60.8 kWh/m2/yr; measured consumption: 

35.1 kWh/m2/yr

o The discrepancy between measurement and calculation in Townsend can be 

explained by lower indoor temperatures during the heating season (see Section 3).

Without having data on the performance of the mini-split it is hard to tell how much additional 

energy savings might be possible by optimizing the heat pump system. To further reduce energy 

consumption for space conditioning, either the heat loss could be further reduced: the weakest

components are the ventilation units with a heat recovery efficiency of only about 60%. Or the 

COP of the mini-split could be enhanced.

 5

X-axis: daily average of ambient temperature, Y-axis: daily average of electric load from mini-split, divided by 

conditioned floor space.
18

3 Thermal Comfort

Apart from energy consumption, another key question for the performance of super-insulated 

buildings is indoor thermal comfort in winter and summer. This is particularly of interest for both 

homes because each uses a point heat/cooling source (only one mini-split indoor unit) on each 

floor. We measured temperature and RH in different rooms in the summer and winter periods 

and assessed the thermal comfort rating according to ASHRAE 55(2010). The comfort models 

(static and adaptive) are explained in Sections 3.1 and 3.2.

For the NZEH, the adaptive model of EN 15251 (2008) is not directly applicable because the 

homes have air conditioning. The adaptive model in ASHRAE 55 is difficult for short-term 

measurement because indoor values are compared to a monthly mean for the ambient 

temperature: for a three-month measurement period the values can be compared to only three 

reference points. Instead we decided to use the static comfort model of ASHRAE 55that defines 

comfort zones for indoor temperature and RH and is based on the EN ISO 7730:2005 PMV 

comfort model.

3.1 Static Comfort Models

The static approach to thermal comfort (ISO 7730) is derived from the physics of heat transfer 

and is combined with an empirical fit to sensation (predicted mean vote, PMV and predicted 

percentage of dissatisfaction, PPD) (Fanger 1970). The required four environmental input 

variables are air and mean radiant temperature, air speed, and RH. The two personal variables are 

clothing and metabolic heat production. The PMV is probably the index of thermal comfort most 

widely used for assessing moderate indoor thermal environments. It rests on steady-state heat 

transfer theory, obtained during a series of studies in climatic chambers, where the climate was 

held constant. It predicts the comfort vote of occupants on the ASHRAE scale of subjective 

warmth (–3 cold to +3 hot) as well as the PPD for a certain indoor condition.

Thermal comfort requirements in ISO 7730 rest on the heat balance approach (Fanger 1970) and 

are distinguished into a summer and a winter season. The ranges of temperature that occupants 

find comfortable are merely influenced by the characteristic heat insulation of clothing. 

Therefore, the defined comfort criteria are generally applicable for all rooms independent of the 

building technology for heating, cooling, and ventilation.

θo,c,summer = ° 24.5 C for summer season

θo,c,winter = ° 22 C for winter season

The criterion for thermal comfort is stipulated as an average operative room temperature of 

76.1°F (24.5°C) for the summer and 71.6°F (22°C) for the winter, with a tolerance range 

depending on the predicted PPD: ±33.8°F, ±34.7°F, and ±36.5°F (±1.0°C, ±1.5°C, and ±2.5°C)

(classes I, II, and III).

Fanger’s thermal comfort model requires the input variables metabolic rate and the insulation 

level of clothing. For the winter period, a clo of 1.0 is assumed, which represents typical winter 

clothing with long-sleeved shirt and long pants. The summer period is described with a clo factor 

of 0.5, representing a light, short-sleeved shirt and light pants. The prevailing ambient conditions 
19

are not considered. Therefore, it is not explicit whether Fanger’s model refers to summer or 

winter conditions. 

3.2 Adaptive Comfort Models

Since the publication of the PMV equation in the 1970s, many studies have been published on 

thermal comfort in buildings under operation. Some have given support to PMV; others have 

found discrepancies. It has become apparent that no individual field study can adequately 

validate PMV for everyday use in buildings (Humphreys and Nicol 2002). The fundamental 

assumption of the adaptive approach is expressed by the adaptive principle: “if a change occurs 

such as to produce discomfort, people react in ways which tend to restore their comfort” (Nicol

and Humphreys 2002; Nicol and McCartney 2002). EN 15251:2007-08 and ASHRAE 55 

describe the adaptive approach that includes the variations in the outdoor climate and the 

person’s control over interior conditions to determine thermal preferences. It is based on findings 

of surveys on thermal comfort conducted in the field. Data about the thermal environment were 

correlated to the simultaneous response of subjects under real working conditions. The thermal 

response is usually measured by asking occupants for a comfort vote on a descriptive scale such 

as the ASHRAE or Bedford scale (Nicol and Humphreys 2002; Nicol and McCartney 2002; 

Nicol and Roaf 2005). Based on field studies, de Dear and Brager (1998) proposed new thermal 

comfort standards for naturally ventilated buildings, leaving PMV as the standard for airconditioned buildings.

The adaptive comfort model takes into account the thermal sensation of the occupants, different 

actions to adapt to the (changing) thermal environment (e.g., change of clothes, opening 

windows) as well as variable expectations with respect to outdoor and indoor climate, seeking a 

customary temperature. The underlying assumption is that people are able to act as meters of 

their environment and that perceived discomfort is a trigger for behavioral responses to the 

thermal environment. Although these behavioral phenomena cannot yet be described 

theoretically in full detail, a model was derived from results of field studies that represents limits 

for the operative temperature as a function of the outdoor temperature. This simplified approach 

also avoids difficulties occurring with the assumption of appropriate clo- and meteorological 

values, as it has to be done with the PMV approach. They are included in the resulting accepted 

temperature as part of the adaptation.

As mentioned in the beginning of this section, for the adaptive comfort model 

ASHRAE 55defines thermal comfort for naturally ventilated buildings with reference to the 

monthly mean ambient air temperature θe,month for the adaptive model as this is generally 

available from meteorological stations.

θ θ o,c = °+ ⋅ 17.8 C 0.31 e,month

The tolerance range is determined by dependence on occupant satisfaction, namely ±4.5°F 

(±2.5°C) for 90% acceptance and ±6.3°F (±3.5°C) for 80% acceptance.
20

3.3 Thermal Comfort in Winter

3.3.1 Stow

The indoor conditions were recorded from February to May 2011. Temperature loggers were 

placed in the living room and in the master bedroom on the second floor. 

Figure 31. Ambient temperature in the first weeks of the test period

Figure 32. Plot of indoor against ambient temperatures, divided into daytime and night

Even though about 13% of the hourly mean indoor temperatures were below 68°F (20°C), the 

occupants felt comfortable and had no complaints about low temperatures (to the contrary, the

homeowners considered it to be “the warmest house we ever lived in”). This can be confirmed 

by the space heating capacity, i.e., the occupants were always able to set the system to higher set

points to increase the runtime of the heating system, but did not do that.

Figure 33. Overlay of indoor temperature curves over the course of the day

throughout the year in the Stow house

16

17

18

19

20

21

22

23

24

-15 -10 -5 0 5 10 15 20 25 30

[°C] indoor, living room 11PM-6AM 7AM-10PM

outdoor [°C]

16

17

18

19

20

21

22

23

24

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00

[°C] indoor temperature, living room
21

Figure 33 shows the indoor temperature over the course of the day (for the entire measurement 

period). The temperature is very stable: the deviation over a day is rarely more than 3.6°F (2°C).

In Figure 34 these curves are averaged to a median temperature. The median and the mode are 

close together, indicating a very stable indoor environment.

Figure 34. Temperature measurements averaged over the test period (Stow)

To put the data in the context other comfort analyses, we also evaluated the temperature and RH

relative to ASHRAE 55-2010. The underlying comfort model is the Fanger comfort model, 

defined in ISO 7730. The “clothing rate” “clo 1.0” is referring to long trousers and a sweater, 

“clo 0.5” represents a T-shirt and shorts.

Figure 35. Indoor conditions and comfort zones according to ASHRAE 55 (Stow)

Figure 35 shows that indoor conditions are usually in a comfortable range (assuming an 

equivalent clothing rate), but for some time temperatures fell below the comfort minimum.

The picture in the master bedroom is similar. Outdoor temperatures lower than 50°F (10°C)

cause indoor temperatures to fall below 68°F (20°C). Again, the occupants reported no

discomfort.

16

17

18

19

20

21

22

23

24

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00

mode

median

[°C] indoor temperature, living room

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, living room

indoor temperature [°C] 

clo 1.0

clo 0.5
22

Figure 36. Indoor comfort and comfort zones in the master bedroom (Stow)

3.3.2 Townsend

The measurement for indoor comfort in winter took place at the same time as in Stow (February

to May 2011).

Figure 37. Correlation of indoor and outdoor temperature in Townsend

Figure 37 shows that indoor temperatures were usually below 68°F (20°C). However, occupants

neither complained nor set the thermostat to higher set points, indicating that their preferences 

deviated from the ASHRAE standard. 

Figure 38. Overlay of indoor temperature curves over the course of the day 

throughout the year in the Townsend house

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, master bedroom

indoor temperature [°C] 

clo 1.0

clo 0.5

12

14

16

18

20

22

-15 -10 -5 0 5 10 15 20 25 30

[°C] indoor, living room

outdoor [°C]

10

12

14

16

18

20

22

24

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00

[°C] indoor temperature, living room
23

Figure 38 shows combined indoor temperature characteristics in the living room. Again, the 

temperature level is usually below 68°F (20°C). However, instead of a rather flat curve, two 

spikes occur: in the morning around 6:00 a.m. and in the afternoon after 5:00 p.m. It shows that 

the occupants systematically set the set points for space heating higher only when they needed

more warmth.

Figure 39. Averaged indoor temperature profile in the living room (Townsend)

According to the comfort limits defined in ASHRAE 55, the house is almost always too cold. 

The occupants compensate by using slightly higher clothing rates and accepting lower 

temperatures. The RH values for the living room and the master bedroom are different because 

the master bedroom is used by only one person and there are other moisture sources (plants and

cooking) in and near the living room.

Figure 40. Comfort limits for the living room in Townsend

Figure 41. Comfort rating for a bedroom in Townsend

10

12

14

16

18

20

22

24

00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 00:00

mode

median

[°C] indoor temperature, living room

0

20

40

60

80

100

12 14 16 18 20 22 24 26 28 30

[%] relative humidity, living room

indoor temperature [°C] 

clo 1.0

clo 0.5

0

20

40

60

80

100

12 14 16 18 20 22 24 26 28 30

[%] relative humidity, bedroom

indoor temperature [°C] 

clo 1.0

clo 0.5
24

3.3.3 Summary: Thermal Comfort in the Heating Season

The results from two single-family homes depend heavily on occupant behavior and can be 

hardly used to draw general conclusions. The indoor conditions in Stow are very stable and 

although temperature drops to levels that are generally rated as uncomfortable, the occupants are 

satisfied. Whether this is due to individual preferences or to building characteristics (no drafts

because of the airtight construction and/or high indoor surface temperatures because of the high 

insulation levels) cannot be answered without data from a larger number of homes or families.

The results from Townsend clearly indicate the influence of occupant acceptance. In that case the 

occupants were eager to save energy, and did not seem to mind temperatures that many people 

would consider to be too cold.

The use of a single heat source on each floor seemed to be sufficient. The occupants in both 

homes stated that they felt no need for an additional heat source in specific rooms. The measured 

temperatures confirm that: we did not see large differences among rooms.

3.4 Thermal Comfort in Summer

Besides indoor conditions and comfort in winter, we analyzed the performance of the building in 

the summer. The key question is to what extent the building stays comfortable without additional 

cooling (which is provided by the ductless mini-split).The test period was from July to 

September 2011; the ambient conditions for Stow and Townsend are shown in Figure 42 and 

Figure 43. Indoor conditions were recorded at10-min intervals, the weather data at 5-min

intervals.

Figure 42. Temperature and RH during the summer months in Stow

Figure 43. Temperature and RH during the summer months in Townsend

0

5

10

15

20

25

30

35

40

07/22 08/05 08/19 09/02

[°C] ambient temperature, Stow

0

20

40

60

80

100

0 10 20 30 40 50

[%] relative humidity, Stow

ambienttemperature [°C] 

0

5

10

15

20

25

30

35

40

07/22 08/05 08/19 09/02

[°C] ambient temperature, Townsend

0

20

40

60

80

100

0 10 20 30 40 50

[%] relative humidity, Townsend

ambienttemperature [°C] 
25

3.4.1 Stow

For comfort analysis in summer, we use the ASHRAE 55 comfort limits to analyze the indoor 

conditions.

Figure 44. Comfort limits and measured temperature and RH conditions 

in the living room (Stow)

Figure 45. Comfort limits and measured temperature and RH conditions 

in the master bedroom (Stow)

Figure 46. Comfort limits and measured temperature and RH conditions 

in the finished attic (Stow)

These figures show a similar picture: indoor temperatures never exceed 82.4°F (28°C), and 

periods with values exceeding 78.8°F (26°C) are limited. Potential discomfort would rather be 

due to high RH. The mechanical ventilation introduces (rather than reduces) additional latent 

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, living room

indoor temperature [°C] 

clo 1.0

clo 0.5

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, master bedroom

indoor temperature [°C] 

clo 1.0

clo 0.5

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, attic

indoor temperature [°C] 

clo 1.0

clo 0.5
26

loads: the heat recovery has the benefit of cooling OA (we measured temperature drops of

41°–46.4°F [5°–8o

C]), but the moisture content remained the same.

The overall temperature difference between the floors was minimal (we expected the top floor 

would be warmer). 

3.4.2 Townsend

The indoor conditions in Townsend were measured over the same time period as in Stow. The 

picture for summer comfort is similar: indoor temperatures typically do not exceed 

uncomfortable limits, but the RH is high.

Figure 47. Comfort limits and measured temperature and RH conditions

in the living room (Townsend)

Figure 48. Comfort limits and measured temperature and RH conditions

in a bedroom (Townsend)

3.4.3 Summary: Thermal Comfort in the Cooling Season

In contrast to the winter measurements, the results for the summer comfort in Stow and 

Townsend are rather similar: even though the occupants rarely used the ductless mini-split for

space cooling, the temperatures remained in a comfortable range. A potential cause for 

discomfort is the high RH.

3.5 Summary: Thermal Comfort With Low Energy Consumption

The analysis shows that good indoor comfort can be achieved with passive measures (high 

insulation levels, air tightness) and the complexity for technical components for space 

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, living room

indoor temperature [°C] 

clo 1.0

clo 0.5

0

20

40

60

80

100

18 20 22 24 26 28 30

[%] relative humidity, small bedroom

indoor temperature [°C] 

clo 1.0

clo 0.5
27

conditioning is drastically reduced. This brings up the opportunity to simplify the systems—a 

chance to reduce investment costs (smaller units) and running costs (low energy consumption). 

A single-point heat source is sufficient in the winter, because additional internal gains (people 

and/or solar gains) are enough to keep indoor temperatures stable. But the installation separate 

from occupied rooms (in the hallway in the second floor, no direct connection to the bedrooms) 

causes problems in the summer, because additional heat gains; e.g., from people, are undesirable 

and can cause rising temperatures and discomfort.
28

4 Indoor Air Quality

Besides energy consumption and thermal comfort, the monitoring focused on IAQ. Both 

buildings have airtight envelopes. Blowerdoor tests were conducted in both homes, with an ACH 

at 50 Pa n50 = 0.5 h-1 in Stow and n50 = 0.7 h-1 in Townsend. As described in Table 1 and Table 

2, both homes have mechanical ventilation. A main construction difference is the routing of the 

airflow: in Stow the airflow is fully ducted. Exhaust air is taken from the bathrooms and the 

kitchen and fresh air is blown into the living room and the bedrooms. For cost optimization, the 

ductwork in Townsend is simplified: exhaust air is taken from the bathrooms as well, but the 

fresh air is blown into the central hallway and staircase. To increase air exchange between floors

and bedrooms, large grilles were installed in the walls, and the kitchen and living room on the 

first floor are open to the staircase (see Figure 6).

The Stow home has a Lifebreath 155ECM energy recovery ventilator, with an estimated OA

volume flow rate of 70–80 cfm (“medium” low speed), consistent with the 70 cfm value used in 

the Passive House Planning Package. This translates into an air exchange rate of about 0.3 h-1

. 

Although we did not measure the airflow rates directly, this is generally consistent with the ACH 

inferred from perfluorocarbon tracer (PFT) emitter measurements (see Table 7). The Townsend 

home has a Fantech Model SHR 1504, with an estimated outdoor air volume flow rate of 

80–100 cfm. To meet ASHRAE Standard 62.2 (ASHRAE 2010), both homes would require OA

volume flow rates of about 50 cfm. 

The analysis focuses on two main topics: CO2 concentration and indoor pollutants. Whereas 

CO2 concentration measurements were conducted over several weeks with equipment owned by 

Fraunhofer Center for Sustainable Energy, the tests for indoor pollutants (VOCs, aldehydes) 

were performed in collaboration with Lawrence Berkeley National Laboratory (LBNL) over a 

three-week test period.

4.1 Carbon Dioxide Concentration

CO2 can be considered as a proxy for air exchange rates relative to occupancy. The gas is 

odorless and nontoxic. The question about acceptable indoor concentrations remains 

controversial. We used the limits stated in ASHRAE 62.1 (2007a); i.e., 700 ppm above the 

outdoor concentration (1050–1200 ppm). The results are shown in a graphical analysis, where 

the CO2 concentration is plotted against the indoor RH. In that way, two IAQ parameters are 

plotted in one graph, along with the limits for a comfortable indoor environment. The limit for 

RH is set from 30% to 70%, following ASHRAE 55. The range is marked as “comfort area” with 

dashed lines, similar to the graphs used to present thermal comfort.

4.1.1 Stow

We placed two CO2 sensors in the Stow home: one in the kitchen/living-room area and one in the 

master bedroom. We also recorded temperature and RH.

4.1.1.1 Summer Period

The first test, conducted from August 13 to October 4, 2010, show two notable points:

• The RH was rather high (>60%) most of the time.

• The CO2 concentration was unexpectedly high, especially in the master bedroom.
29 

Figure 49.CO2 concentration and RH in the Stow living room during the summer test period

Figure 50.CO2 concentration and RH in the Stow master bedroom during the summer test period

A likely explanation for these values was found in the ventilation unit: the flap for the frost 

protection was in the wrong position. This flap was built in to prevent icing in the heat 

exchanger, by closing the fresh air inlet and opening a bypass. Instead of being exchanged with 

fresh air, exhaust air was recirculated (see Figure 51). The issue was fixed in October 2010. 

Figure 51. Cross section and picture of the ventilation unit in Stow

When the defrost-damper was in defrost mode, it blocked the fresh air inlet and 

opened a bypass for the exhaust air.

4.1.1.2 Winter Period

The CO2 measurement was repeated between February 28 and May 20, 2011. As expected for 

the winter period, the RH levels were lower, but CO2 concentrations were still very high, 

especially for a building with continuously running mechanical ventilation. One obvious reason 

was a high occupant density. Most of the time during the test period, the home was occupied by 

eight people—twice the number the ventilation system was designed for. Also, tests performed 

0

400

800

1200

1600

2000

2400

0 10 20 30 40 50 60 70 80 90 100

[ppm] CO2 living room

relative humidity [%] 

0

400

800

1200

1600

2000

2400

0 10 20 30 40 50 60 70 80 90 100

[ppm] CO2 bedroom

relative humidity [%] 
30

during the indoor pollutant measurements (see Section 4.2) show air exchange rates (n ≈ 0.3 –

0.4 h-1

) that are suitable for “normal” occupation, but too low for that level of occupancy.

Figure 52.CO2 concentration and RH in the Stow living room in the winter test period

Figure 53.CO2 concentration and RH in the Stow master bedroom in the winter test period

4.1.2 Townsend

As in Stow, two CO2 sensors were deployed in Townsend: one in the kitchen/living room area, 

the other in the master bedroom. Due to a sensor error, data were available only for the winter 

period.

4.1.2.1 Winter Period

Because of the air quality tests performed in collaboration with LBNL (see next section), the 

CO2 test can be split into two operation modes: the time when the HRV was operated manually 

(switched on and off by the occupants) and constant operation during the LBNL test.

Figure 54 and Figure 55 show the influence of continuous ventilation: while the CO2

concentration in the living room hardly reached critical limits (even with manual operation of the 

mechanical ventilation), the bedroom showed insufficient air exchange rates. When the HRV 

was switched to constant ventilation mode, the CO2 concentration was always acceptable. The 

occupants switched off the ventilation because of acoustic noise and to reduce electricity

consumption.

0

400

800

1200

1600

2000

2400

0 10 20 30 40 50 60 70 80 90 100

[ppm] CO2 living room

relative humidity [%] 

0

400

800

1200

1600

2000

2400

0 10 20 30 40 50 60 70 80 90 100

[ppm] CO2 bedroom

relative humidity [%] 
31

Figure 54.CO2 concentration and RH in the Townsend living room in winter with 

different settings of the mechanical ventilation system

Figure 55.CO2 concentration and RH in the Townsend master bedroom in winter with 

different settings of the mechanical ventilation system

4.2 Volatile Organic Compound and Aldehyde Concentrations6

The main goal of these tests was to evaluate chemical indoor pollutants and understand the 

impact of ventilation rates on the chemical concentrations and emission rates. The study was 

conducted simultaneously in both homes from May 6 today 27, 2011). During the study, the 

average daytime weather condition was cool (48.2°–69.8°F [9°–21°C]) and relatively dry 

(40%–62% RH) with occasional precipitation events (0–0.3 in.).

4.2.1 Air Sampling Protocol and Analysis

Air was sampled during weekly visits, on Friday afternoons, encompassing three consecutive 

one-week periods between May 6 and May 27, 2011. These site visits were executed by 

Fraunhofer CSE staff, henceforth referred to as the field team based on a sampling protocol that 

was developed with LBNL researchers. The sample and data analyses were carried out at LBNL 

between May and July 2011. Table 6 summarizes the sampling plan and methods for both study 

homes. VOCs and aldehydes were actively sampled every Friday at two indoor locations 

(selected near the center of each floor) and one outdoor location. The programmable PFT

sampling systems were also placed at the same indoor locations. Up to 10 passive air samplers 

were distributed around the house for passive time-integrated VOC, aldehyde, and PFT 

sampling. 

 6 Section 4.2 was composed primarily by Dr. Henry Willem of LBNL. 

0

400

800

1200

1600

2000

2400

0 10 20 30 40 50 60 70 80 90 100

[ppm] CO2 living room HRV manually on/off HRV 24/7

relative humidity [%] 

0

400

800

1200

1600

2000

2400

0 10 20 30 40 50 60 70 80 90 100

[ppm] CO2 bedroom HRV manually on/off HRV 24/7

relative humidity [%] 
32

Table 6. Sampling Plan and Measurement Methods

Target 

chemicals

Sampling 

Method

Sampling 

Media

No. of 

Location Sampling Plan Instrument

VOCs 

(40)

Active Tenax TA

3 (2 indoor 

and 

1 outdoor)

40 min per sample 

(every Friday 

afternoon)

Peristaltic 

pump

Passive Tenax TA

10 (9 indoor 

and 

1 outdoor)

Continuously for 

one week per sample 

(samples collected 

every Friday)

–

Aldehydes 

(3)

Active

DNPHcoated 

cartridge

3 (2 indoor 

and 1 

outdoor)

40 min per sample 

(every Friday 

afternoon)

Peristaltic 

pump

Passive

DNPHcoated 

cartridge

10 (9 indoor 

and 

1 outdoor)

Continuously for 

one week per sample 

(samples collected 

every Friday)

–

PFTs 

(3)

Active 

(PMCH, 

PDCB)*

Bagsets 2 (both 

indoor)

2 h and 40 min per

bag (daily, afternoon 

between 2:00 p.m.

and 4:40 p.m.)

Programmable 

air sampler

Passive 

(HB)** Tenax TA 2 (both indoor)

Continuously for 

one week per sample 

(samples collected 

every Friday)

–

*PMCH = perfluoromethylcyclohexane; PDCB = perfluorodimethylcyclobutane

** HB = hexafluorobenzene

Before the study period started, the field team installed small vials to emit PFTs throughout the 

homes. These tracers were used to determine the whole-house ventilation rates and air exchange 

rates between zones. At the start of each sampling week the team set a ventilation condition and 

installed passive samplers for time-integrated measurements of VOCs, aldehydes with low 

molecular weight formaldehyde and acetaldehyde), and PFT concentrations. The samples were 

collected before the ventilation setting was changed. 
33

Figure 56. Vials with perfluorocarbon (PMCH and PDCB) and HB emitters

In parallel to the passive sampling method, the team also used active sampling techniques to 

measure the same parameters. The team installed programmable sampling pumps to collect air 

samples in bags between 2:00 p.m. and 4:40 p.m. each afternoon. These samples were analyzed 

for PFT concentrations to determine daily variations of ventilation rates at the specified time. Of 

note, the PFT types measured in conjunction with bag sampling were different from the PFT 

collected using passive samplers. After the ventilation setting had been kept continuously for six 

days, the team returned to the study homes on Friday afternoons to collect air samples. The 

samples were then analyzed for VOCs and aldehydes. These air sampling visits coincided 

roughly (but not directly) with ventilation rate data collected from the 2:00 p.m. to 4:40 p.m. PFT 

bag samples. 

One challenge in the study was to achieve and maintain three levels of ventilation rates in each 

home. To reduce daily variance, the residents were asked to keep windows closed as much as 

possible throughout the study period, particularly starting from the night before the VOC and 

aldehyde sampling. Also, the ERV units operated continuously and thermostatic control was set 

at 72°F±2°F (22.2°C±1.1°C). The environmental control was necessary to maintain a near 

steady-state condition. Despite these efforts, daily PFT measurements indicated that up to 20% 

variation was detected, which could be related to external weather conditions, wind direction, 

windows opening, and occupant behavior. 

VOCs and aldehydes were sampled on Tenax TA sorbent tubes (P/N 012347-005-00; Gerstel) 

and silica gel cartridges coated with 2,4-dinitrophenylhydrazine (DNPH XPoSure Aldehyde 

Sampler; Waters Corporation) with ozone scrubbers. A peristaltic sampling pump was used to 

draw the air through the sampling media. Mean VOCs sampling flow rates were 93.2 mL/min 

(S.D.: 10.7 mL/min) and Aldehyde sampling flow rates were 1030 mL/min (S.D.: 60 mL/min). 

The simultaneous sampling of both VOCs and aldehydes took 40 min.

VOC samples were analyzed using a gas chromatography/mass spectrometry system. The 

samples were thermally desorbed with cryogenic inlet system. Forty target chemicals were 

selected for analysis. The list was representative of various sources of indoor VOCs and included

25 target chemicals shortlisted by the California Department of Public Health. Aldehyde 

samplers were analyzed for formaldehyde and acetaldehyde, using the high-performance liquid 

chromatography system.
34

Three types of liquid PFTs were distributed throughout the homes. The team used 2-dram vials 

for the liquid PMCH and PDCB; and 2-mL vials for liquid HB. All vials were capped with a 

septum material on the cap and inverted when in use. PFT vials were distributed in both homes 

one week before air sampling started to allow the concentration to reach pseudo steady-state. To 

understand the air exchange characteristics in these multilevel homes, the team divided the 

ventilated spaces into two vertical zones for PFT distribution. In Stow, the finished basement and 

first floor were designated as zone 1; the second floor and finished attic were zone 2. In the 

Townsend home, the first floor was zone 1 and the second floor was zone 2. In both homes, the 

team distributed five vials in zone 1 and five PDCB vials in zone 2. HB vials were distributed at 

all locations where a PMCH or PDCB vial was placed. We used air bags to sample both PMCH 

and PDCB, i.e. using the active sampling method. HB was sampled on the passive VOC 

samplers.

Figure 57. Active samplers for PFT (blue box) and VOC and aldehydes (peristaltic pump)

The mass of each target chemical represented by the area of total-ion current chromatogram was 

quantified based on calibration curves prepared from its pure chemical. Analytical blanks were 

included in the analysis. Travel blanks were transported, stored, and analyzed along with all 

other samples from the same sampling event. A total of 10% duplicate samples were collected 

with at least one sample for each sampling method every week. Start and stop sampling flow 

rates and the total sampling duration were recorded for each sample and were used in the 

calculation of chemical concentrations. Relative precision of the results in terms of sampling and 

analysis was ±10% for most of the target chemicals. The uncertainty associated with the 

analytical method of PFTs was ±5%.

4.2.2 Ventilation Rate Settings

The team maintained three levels of OA ventilation rates in both homes by adjusting the flow 

rate setting of the ERV units. Changes were made every Friday after the active sampling was 

completed and the passive samples were collected. Ventilation rates were calculated from air 

concentrations of PMCH and PDCB in both zones using a two-zone mass balance model. There

were more air exchanges between zones in the Stow home than in the Townsend home, which is 

likely related to the ducted air distribution system in Stow. Table 7 summarizes the whole-house 

ventilation rates for both homes, which were determined from the net amount of air infiltration.

The range of ventilation provided in Townsend was substantially higher than that of the Stow 

home. Altering the flow rate settings of the ERV units—particularly in Stow—did not appear to 

increase ventilation rate significantly. The passive sampling over a one-week period estimated 

consistently higher ventilation rates in both homes, indicating that occupants’ activities and 
35

behaviors could increase ventilation rates. Ventilation rates in the bedrooms of both homes were 

higher than other locations by up to 40% (data not shown here). 

4.3.3 Results

Figure 58 shows a typical air exchange profile in m3/h between zones and across the envelope. 

There was indication of vertical stack effect from zone 1 to zone 2 and higher infiltration rate on 

the first (lower) zone of the house. The rate of air exchange was determined from the net 

infiltration or exfiltration across the envelope estimated for both zones.

Figure 58. Air exchange profile in m3/h between zones and across the envelope for Townsend

Table 7. Calculated Overall Home Outdoor Air Exchange Rates 

(Based on PFT Concentrations Measured in Bag Samples Collected 

2:00 p.m. to 4:40 p.m. Each Friday Afternoon)

ERV Setting Stow

(h-1)

Townsend

(h-1)

Low 0.25 0.41

Medium 0.36 0.56

High 0.45 0.69

Table 8 and Table 9 list the VOC and aldehyde concentrations in both homes at three ventilation 

rates. The results were obtained from an active sampling method. Results from the passive 

sampling are shown in Table 10 to Table 13. Only data collected from main rooms or areas of 

the house were included as the variations between measurements on the same floor were 

relatively small. Blank cells indicate that concentration data were below the lower limit of 

quantitation. Aldehyde concentrations data obtained from passive sampling were generally lower 

than the active sampling results. This was consistent with the PFT results, which suggested 

higher integrated air exchange rates over a one-week period and thus better dilution of indoorgenerated chemical. The same effects were, however, not clearly seen when comparing active 

and passive VOC results. 

VOC and aldehyde concentrations were relatively constant between locations in both homes. 

Most variations observed were insignificant. These indicated reasonably good mixing in both 

homes. There was, however, a trend of lower concentrations in the bedrooms, which could be 

related to the higher ventilation rates measured in these enclosed areas. The predominant 

chemicals in both homes were similar: hexanal, α-pinene, d-limonene, nonanal, octanal, 

3-carene, and 2-butoxyethanol. Increasing ventilation rates appeared to lower the concentrations 
36

of most VOCs and aldehydes, except decanal, nonanal, benzene, and decamethylcyclopentasiloxane. Combining data from active and passive sampling methods, measured indoor 

formaldehyde concentrations in Stow were in the ranges of 28–44 μg/m-3 at low, 24–34 μg/m-3

at medium, and 20–32 μg/m-3at high ventilation settings. In Townsend, the ranges were 

27–49 μg/m-3 at low, 25–47 μg/m-3 at medium, and 10–24 μg/m-3 at high ventilation rates. 
37

Table 8. VOC Concentrations in Stow and Townsend at Three Ventilation Rate Settings

Class Target Chemicals

(μg/m3)

Stow Level 1 Stow Level 2 Townsend Level 1 Townsend Level 2

Low Med High Low Med High Low Med High Low Med High

Alcohol 2-Ethyl-1-hexanol 9.2 6.1 6.6 10.0 5.8 7.6 9.0 4.6 1.2 7.8 3.4 2.5

Aldehyde

Benzaldehyde 8.6 6.9 5.6 9.1 6.6 5.6 7.1 5.0 6.2 5.7 4.1 8.8

Butanal 7.4 7.3 – 9.0 7.7 – 5.4 5.1 – 5.5 4.4 –

Decanal 9.2 11.9 12.4 12.9 11.9 13.4 5.1 4.6 2.2 5.6 3.2 5.8

Heptanal 16.9 13.6 9.3 19.9 13.4 9.5 7.7 5.5 1.2 7.1 4.3 3.0

Hexanal 171 147 79.9 203 145 86.7 102 75.0 6.9 102 59.4 29.1

Nonanal 39.8 33.6 36.5 47.1 32.5 36.4 20.8 14.6 6.3 20.8 11.6 16.4

Octanal 31.2 26.3 19.9 37.0 25.5 21.4 18.6 12.5 3.4 17.3 9.5 7.6

Alkane

Decane 2.8 2.2 1.6 3.1 2.1 1.7 6.4 5.1 3.0 6.0 4.1 4.9

Dodecane 8.6 5.8 3.8 9.4 5.7 4.1 1.9 1.3 – 1.7 0.9 1.0

Heptane 5.7 4.5 2.2 6.5 4.4 2.3 9.7 8.6 1.0 8.6 6.5 2.0

Hexadecane 4.2 2.4 2.6 4.6 2.2 2.5 2.6 1.2 0.7 2.8 1.0 1.6

Octane 3.5 2.4 1.8 3.7 2.4 1.9 1.5 1.6 0.6 1.5 1.3 1.2

Tetradecane 4.6 2.6 1.9 4.8 2.5 2.2 4.8 2.7 0.9 4.5 2.1 2.1

Undecane 7.6 5.8 3.2 8.9 6.0 3.8 7.0 4.3 2.2 6.1 3.1 4.7

Aromatic

1,2,3-

Trimethylbenzene 0.5 0.3 – 0.5 0.3 – 0.5 0.4 – 0.4 0.3 –

1,2,4-

Trimethylbenzene 1.2 0.8 0.3 1.3 0.8 0.4 1.8 1.6 – 1.6 1.2 0.6

Benzene 2.8 0.6 0.7 3.0 0.6 0.8 1.9 1.7 1.9 0.8 1.4 2.0

Ethylbenzene 1.5 1.0 0.5 1.6 1.0 0.6 2.0 2.1 – 1.8 1.6 0.7

o-Xylene 4.0 4.8 2.0 4.7 4.7 2.2 2.8 3.5 0.6 2.6 2.6 1.1

m/p-Xylene 3.6 2.6 1.3 4.0 2.5 1.6 5.3 6.4 1.0 4.9 4.6 1.9

Toluene 18.0 9.9 4.5 18.1 9.7 9.0 17.3 17.0 2.8 15.5 12.2 4.9

Ester TXIB (di-isomer) 12.1 6.3 5.3 9.7 6.2 6.8 0.9 0.4 – 0.8 0.4 0.5
38

Class Target Chemicals

(μg/m3

)

Stow Level 1 Stow Level 2 Townsend Level 1 Townsend Level 2

Low Med High Low Med High Low Med High Low Med High

TXIB (monoisomer) 2.9 2.1 2.3 2.8 1.8 2.4 3.4 2.2 - 3.0 1.5 2.0

2-Butoxyethanol 22.3 17.6 14.0 26.1 17.3 15.8 6.7 4.9 - 6.4 3.5 2.5

Siloxane

Decamethylcyclopentasiloxane 6.0 4.7 24.0 5.6 4.8 11.3 33.1 30.6 1.2 37.3 26.9 3.3

Octamethylcyclote

trasiloxane 1.2 0.6 0.5 1.0 0.6 0.5 1.3 0.8 – 1.6 0.8 –

Hexamethylcyclotrisiloxane 5.8 3.2 3.5 4.5 4.2 3.5 4.4 3.6 0.7 5.9 4.3 4.0

Terpene

3-Carene 25.7 18.7 7.4 28.1 18.4 7.8 24.3 21.6 1.8 22.5 16.4 4.6

a-Pinene 101 70.1 29.8 116 72.6 32.1 37.7 22.9 3.5 32.6 17.7 6.5

a-Terpineol 1.0 0.6 0.3 1.1 0.6 0.4 0.5 0.2 – 0.4 0.2 –

d-Limonene 84.6 16.8 6.7 95.7 16.7 7.6 98.7 20.8 – 58.1 14.7 1.9

g-Terpinene 0.7 0.5 – 0.8 0.5 – 13.4 0.4 – 6.2 – –

Table 9. Aldehyde Concentrations in Stow and Townsend at Three Ventilation Rate Settings

Target Chemicals

(μg/m3)

Stow Level 1 Stow Level 2 Townsend Level 1 Townsend Level 2

Low Med High Low Med High Low Med High Low Med High

Formaldehyde 44.0 32.8 25.5 41.9 31.5 20.2 38.8 28.7 10.0 35.3 25.7 18.6

Acetaldehyde 85.9 52.7 27.7 83.0 51.4 21.6 40.5 29.2 5.7 37.6 27.4 10.2
39

Table 10.VOCs Concentrations in Stow at Three Ventilation Rates Based on Passive Sampling Data 

(Data Shown for Four Locations)

Class Target Chemicals

(μg/m3)

Stow Living Room Stow Kitchen Stow Main 

Bedroom

Stow Common 

Bedroom

Low Med High Low Med High Low Med High Low Med High

Alcohol 2-Ethyl-1-hexanol 9.3 7.9 7.5 10.4 9.3 5.9 7.0 7.0 5.0 9.1 15.3 9.1

Aldehyde

Benzaldehyde 7.4 6.6 8.3 7.7 8.0 7.5 6.3 6.9 7.0 9.1 8.5 10.4

Butanal 10.9 10.3 10.7 10.4 10.2 9.1 7.1 8.7 8.3 10.7 10.9 13.5

Decanal 9.2 16.5 10.7 11.7 18.0 10.0 19.6 14.2 16.1 11.4 9.7 17.8

Heptanal 15.0 17.3 17.4 16.4 16.5 14.6 15.9 19.6 17.9 23.0 23.8 28.8

Hexanal 223 211 221 228 207 172 166 177 167 241 235 289

Nonanal 32.6 43.6 38.2 40.1 43.5 34.4 37.3 40.7 38.3 42.8 45.6 52.2

Octanal 24.4 28.0 29.7 27.3 29.3 25.0 29.2 34.2 33.4 39.2 40.1 48.4

Alkane

Decane 4.2 3.0 2.9 4.1 3.0 2.7 3.3 2.6 2.5 3.6 3.2 3.5

Heptane 5.1 7.2 6.4 6.6 7.7 6.7 2.9 4.0 3.3 3.8 5.2 5.1

Hexadecane 3.8 5.1 2.8 3.9 4.1 2.4 3.3 3.4 2.2 3.5 3.5 2.6

Octane 3.5 3.8 3.1 3.9 3.6 3.2 2.5 2.6 2.2 3.3 3.9 4.1

Tetradecane 4.1 4.4 2.7 4.2 4.0 2.4 3.2 3.2 2.4 4.0 3.6 2.8

Undecane 8.1 7.8 6.3 8.6 6.8 5.8 7.0 6.9 5.5 10.8 7.2 9.7

Aromatic

1,2,3-Trimethylbenzene 0.7 0.5 0.4 0.8 0.5 0.4 0.4 0.3 - 0.5 0.4 -

1,2,4-Trimethylbenzene 1.7 1.3 0.9 1.9 1.3 1.0 1.1 0.9 0.7 1.2 1.0 0.8

Benzene 27.1 10.1 1.0 - 3.8 0.9 0.0 21.9 1.1 13.2 22.7 1.6

m/p-Xylene 5.4 3.6 3.2 5.6 3.8 3.0 3.8 2.8 2.2 5.8 4.0 4.5

o-Xylene 3.8 3.6 4.2 3.6 3.2 3.9 2.4 2.4 2.7 3.7 3.3 4.5

Toluene 16.6 17.8 14.3 16.5 18.1 14.0 10.5 10.9 8.6 15.4 18.0 19.6

Ester

TXIB (di-isomer) 7.7 11.0 5.6 7.2 8.8 4.0 3.9 3.8 2.8 6.4 6.1 5.5

TXIB (mono-isomer) 8.8 7.5 6.6 7.7 6.8 5.2 4.6 5.2 4.3 12.5 11.9 12.4

2-Butoxyethanol 19.8 15.5 20.5 23.1 17.7 17.0 16.7 15.9 16.1 23.4 19.7 23.4

Terpene

3-Carene 23.6 25.2 21.6 25.1 25.1 20.4 14.6 16.2 12.6 25.0 27.4 26.0

a-Pinene 100 91.2 90.4 88.7 88.2 81.8 54.0 64.4 52.4 90.3 94.3 89.2

a-Terpineol 0.8 0.8 0.7 0.9 0.8 0.6 0.6 0.9 0.5 0.9 0.8 0.8

d-Limonene 35.8 101 35.1 38.3 112 33.6 22.5 26.1 19.8 31.8 35.0 30.5

g-Terpinene 0.4 0.8 0.8 0.9 0.9 0.7 0.4 - - - - 0.5
40

Table 11. VOCs Concentrations in Townsend at Three Ventilation Rates Based on Passive Sampling Data 

(Data Shown for Four Locations)

Class Target chemicals

(μg/m3

)

Townsend Living 

Room

Townsend Kitchen Townsend Main 

Bedroom

Townsend Common 

Bedroom

Low Med High Low Med High Low Med High Low Med High

Alcohol 2-Ethyl-1-hexanol 8.5 6.6 6.2 9.5 7.5 8.5 8.2 6.2 6.8 7.1 6.2 5.9

Aldehyde

Benzaldehyde 7.6 7.7 6.6 8.9 8.3 8.0 6.7 6.1 6.7 6.1 5.7 6.1

Butanal 10.8 11.3 7.1 11.5 12.9 7.8 8.4 8.3 5.7 7.9 8.4 5.4

Decanal 5.3 4.1 3.4 5.4 4.0 4.9 5.0 4.4 4.8 5.1 4.8 3.4

Heptanal 10.6 9.9 6.8 11.9 10.9 8.4 8.0 7.4 6.4 7.6 7.9 5.7

Hexanal 174 171 120 201 188 142 150 136 117 138 136 94.3

Nonanal 23.1 19.6 15.4 26.9 21.0 22.0 19.5 16.9 16.5 20.7 18.9 14.9

Octanal 21.1 18.4 13.2 23.9 20.9 17.4 16.8 15.1 13.4 15.6 14.8 11.2

Alkane

Decane 11.5 10.2 5.0 13.1 11.4 6.9 7.8 7.7 4.2 7.8 8.2 3.7

Heptane 19.2 19.8 10.7 23.0 26.4 15.4 9.6 10.3 6.2 10.3 11.5 6.1

Hexadecane 2.2 1.4 1.9 2.1 1.4 2.1 3.7 2.3 2.9 2.6 1.9 2.6

Octane 3.0 3.1 1.6 3.5 3.7 2.0 2.1 1.9 1.1 2.3 2.1 1.1

Tetradecane 4.7 3.7 4.0 6.1 4.5 5.9 5.1 3.6 3.9 4.1 3.1 3.6

Undecane 10.1 8.0 4.9 11.7 8.9 6.5 6.9 6.0 3.9 6.7 6.3 3.3

Aromatic

1,2,3-Trimethylbenzene 0.8 0.6 0.4 0.8 0.7 0.4 0.5 0.4 0.3 0.5 0.4 –

1,2,4-Trimethylbenzene 3.2 2.4 1.4 3.4 2.9 1.8 1.9 1.7 1.2 1.8 1.7 1.0

Benzene 2.8 4.5 25.4 1.8 5.0 4.6 21.6 2.8 14.9 21.1 3.0 13.6

m/p-Xylene 12.7 11.1 4.6 13.9 13.3 6.0 7.0 6.4 3.5 7.1 7.2 3.2

o-Xylene 5.0 5.4 2.2 5.4 6.3 2.7 3.2 3.5 1.9 3.1 3.7 1.5

Toluene 35.3 34.7 16.7 39.4 41.6 21.9 20.6 19.4 12.4 20.7 21.3 11.0

Ester

TXIB (di-isomer) 0.7 0.7 0.5 0.8 1.0 1.0 0.6 1.1 - - 0.4 0.6

TXIB (mono-isomer) 8.4 8.6 5.7 9.2 9.5 9.1 6.7 7.0 6.1 5.6 7.4 5.4

2-Butoxyethanol 6.1 7.4 6.4 8.1 8.2 9.8 6.1 6.8 6.9 4.6 6.9 7.0

Terpene

3-Carene 41.9 35.5 18.3 45.5 42.3 23.2 26.0 22.4 14.9 25.6 24.7 12.9

a-Pinene 46.1 40.7 26.3 48.6 46.6 27.7 41.1 31.2 26.2 32.5 29.7 17.8

a-Terpineol 0.4 – – 0.6 0.5 0.5 0.4 – 0.4 – – –

d-Limonene 46.8 28.6 19.3 58.6 35.1 27.2 25.4 20.7 15.8 26.6 22.6 13.7

g-Terpinene 0.8 – 0.5 1.1 0.5 1.5 – – 0.5 – – –
41

Table 12. Aldehydes Concentrations in Stow at Three Ventilation Rates Based on Passive Sampling Data 

(Data Shown for Four Locations)

Target Chemicals

(μg/m3)

Stow Living Room Stow Kitchen Stow Main Bedroom Stow Common Bedroom

Low Med High Low Med High Low Med High Low Med High

Formaldehyde 39.2 34.2 32.2 35.2 31.4 30.8 29.1 27.1 25.7 28.2 23.7 20.3

Acetaldehyde 61.4 54.6 51.8 65.4 54.8 51.9 41.8 40.9 40.6 68.6 64.1 61.7

Table 13. Aldehydes Concentrations in Townsend at Three Ventilation Rates Based on Passive Sampling Data 

(Data Shown for Four Locations)

Target Chemicals

(μg/m3)

Townsend Living 

Room

Townsend

Kitchen

Townsend Main 

Bedroom

Townsend Common 

Bedroom

Low Med High Low Low Med High Low Low Med High Low

Formaldehyde 39.3 33.2 32.9 40.5 37.0 34.0 49.6 46.7 38.3 27.1 26.4 24.9

Acetaldehyde 50.9 39.7 27.3 55.8 46.3 27.5 43.1 31.9 23.1 37.1 34.4 18.3
42

4.3.4 Summary for Volatile Organic Compounds and Aldehydes

The indoor formaldehyde concentrations in both homes exceeded the chronic and 8 h reference 

exposure levels, which were set at a stringent limit of 9 μg/m-3 by the Office of Environmental 

Health Hazard Assessments (2008). The measured levels were lower than acute (1 h) REL of 

55 μg/m-3. That said, most new homes have indoor formaldehyde concentrations that exceeded 

this recommended guideline and the levels measured are still within the typical range measured 

in relatively new homes. For residential, formaldehyde concentrations up to 60 μg/m3 are still 

regarded as acceptable. Offermann (2009) reported a median indoor formaldehyde concentration 

of 36 μg/m3 based on data from 108 new homes in California. Mean concentration of

formaldehyde from a large-scale study of 300 homes in three cities in the eastern United States

was 21.6 μg/m3 (Weisel et al. 2005). Based on all the reviewed studies, the typical formaldehyde 

levels in residences seem to fall within the range of 20–50 μg/m3. The data are skewed toward 

higher concentrations for newer homes, as expected. Pressed-wood products, including furniture, 

some flooring systems, and resins, are some major sources of formaldehyde emissions (Raw et 

al. 2004; Salthammer and Mentese 2010). Wood flooring, wallboards, laminated furniture, and 

cabinetry were some of the main sources of formaldehyde uniformly present in both homes. 

Figure 59 demonstrates a very clear association between indoor formaldehyde concentrations 

and ventilation rates for the Stow and Townsend homes. Increasing ventilation by a nominal 0.1 

ACH would lead to a reduction of formaldehyde concentration by 25% and 18% in Stow and 

Townsend, respectively. The positive impact of ventilation rate on formaldehyde concentration is 

consistent with an earlier report by Hodgson et al. (2000), who conducted an experiment in a 

single home by switching off the HRV system to alter the ventilation rate. Gilbert et al. (2008)

used cross-sectional data of measurements in 96 homes in Canada to develop a linear regression 

model of formaldehyde concentration as a function of ventilation rates. Lower concentrations 

were associated with higher ventilation rates following a linear function. Aubin et al. (2010) 

reported the preliminary findings from a field study of 84 homes of asthmatic children in 

Canada. Indoor formaldehyde concentrations were lower in homes with higher ventilation rates 

for data collected in the winter period.

Figure 59. Indoor formaldehyde concentrations at three air exchange rates

(BA1 corresponds to Stow, BA2 to Townsend)

The concentration ranges of VOCs, formaldehyde, and acetaldehyde compounds were of similar 

magnitudes between both homes. This similarity indicated that the IAQ was affected by similar 

and uniform sources from structural and interior materials installed by the builder. Logue et al.
43

(2011) identified nine contaminants as the priority hazards for residential chronic exposure. 

Compared to this list, the chemicals with notable concentrations in both homes were

formaldehyde and acetaldehyde. Compared to the summary in Brown et al. (1994) of weighted 

average (geometric mean) concentrations for VOCs commonly encountered in residences, 

α-pinene, d-limonene and hexanal had notably elevated concentrations. These odorous VOCs 

may be off-gassed from various indoor sources such as wood products for α-pinene; and surface 

treatment and cleaning products for d-limonene and hexanal. With the exception of limonene and 

hexanal, concentrations of most VOCs with indoor sources were generally low considering that 

these homes were about one year old. The observed levels were comparable to relatively new 

(three- to five-year–old) homes in California (Offermann 2009). This suggests that using lowemitting paints, flooring, and other materials may have lowered the initial emissions from indoor 

surfaces, thus reducing the concentrations. The elevated levels of limonene and hexanal may

have come from cleaning products or air fresheners, i.e., products not related to the building/ 

construction materials.

The chemical exposures could be further reduced with additional ventilation, as 82% of the 

detected compounds varied with ventilation.
44

5 Conclusions

This report documents the results of an in-depth evaluation of the energy production and 

consumption, thermal comfort, and IAQ for two potential NZEHs in Massachusetts over one 

year. It investigates whether claims about energy efficiency, comfort, and health benefits can be 

supported by measured field data. 

One home did not reach a zero balance (in terms of a whole-year period); the other produced 

nearly 30% more energy than it consumed. It is noteworthy that (1) both homes have unusually 

low loads for electricity consumption other than heating, and (2) that even with a highly 

optimized thermal envelope, nearly half the energy is still used for space conditioning 

(dominated by space heating). 

The indoor thermal comfort in summer and winter met occupants’ expectations, although indoor 

temperatures did not always meet comfort standards (per the ASHRAE55 comfort range) in 

winter. Yet occupants reported no discomfort issues and the heating systems in both homes did 

not operate at their full capacity, indicating that the homes seemed to meet residents’ comfort 

preferences. Summer indoor temperatures remained within ASHRAE’s comfort range for 

temperature; a potential cause of discomfort could be relatively high RH levels.

The tests of the air exchange and indoor air pollutants carried out in collaboration with LBNL 

underline the necessity of mechanical ventilation in airtight homes, but also the challenges in 

commissioning and effectively running these systems. As long as there are no odorous 

pollutants, occupants have no sensor for air quality. The installation, commissioning, 

maintenance, and operation of mechanical ventilation show room for improvement. In particular, 

the tests carried out in collaboration with LBNL revealed a mechanical problem that caused 

recirculation of the exhaust air in one home. A second test half a year later revealed that although 

the home was sufficiently ventilated for its projected occupancy, the air exchange was not 

sufficient by then, as twice as many occupants were staying in the home as the ventilation system 

was designed for.
45

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DOE/GO-102013-3746 ▪ January 2013

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