Ventilation and IAQ in New California Homes

May 01, 2019
Spring 2019
A version of this article appears in the Spring 2019 issue of Home Energy Magazine.
Click here to read more articles about Indoor Air Quality

Starting in the mid-2000s, the California Energy Commission funded several research studies (for example, Price, et al., 2007, and Offermann, 2009) that aimed to evaluate the potential indoor air quality (IAQ) impacts associated with envelope air sealing, and the potential to mitigate these through the use of mechanical ventilation systems. These studies found (1) that a majority of the households in new California homes reported not opening windows regularly for ventilation in some seasons, and a substantial minority of households reported not using windows to ventilate during any season; (2) that actual, measured ventilation rates in many homes were below target minimum levels; and (3) that the median measured formaldehyde concentration across study homes was 4 times the chronic reference exposure level set by the California Office of Environmental Health Hazard Assessment.

Measuring exhaust airflow.

Exhaust ventilation.

To address these issues, the state building code (Title 24) has required mechanical ventilation in new homes since 2008, using a California-specific version of ASHRAE Standard 62.2. This includes requirements for exhaust from kitchens, bathrooms, and laundry rooms, as well as whole-house ventilation.

From 2016 to 2018, Lawrence Berkeley National Laboratory and partners conducted a field study, Healthy, Efficient New Gas Homes (HENGH), to assess whether installed ventilation equipment meets code requirements and to measure key pollutants in homes built to this standard to determine if the ventilation requirements provide acceptable IAQ. Partners included the Gas Technology Institute (GTI), the Pacific Gas and Electric Company (PG&E), and the Southern California Gas Company (SoCalGas). The study also included assessments of energy and IAQ consequences of envelope air-tightening requirements and approaches for mechanical ventilation requirements. The results of the complete study can be found in Chan, et al. (2018). This article focuses on the results of the field measurements.


The field study characterized 70 homes built between 2011 and 2017. All the homes had at least one gas appliance (cooker, furnace, or water heater). Forty-eight homes were located in PG&E and 22 homes in SoCalGas service territories. Each home was monitored over roughly one week with the central mechanical ventilation system operating and with the occupant pledging not to leave windows open for ventilation.

Pollutant measurements included time-resolved fine particulate matter (PM2.5) indoors and outdoors; and formaldehyde, nitrogen dioxide (NO), and CO2 indoors. Time-resolved sampling gives us a particle concentration reading every minute, as distinct from time-integrated sampling, where the particles are added up over an extended period of time—typically by collecting a sample on a filter over a period of several days.

Time-integrated measurements were made for formaldehyde, NO2 and nitrogen oxides (NOX) indoors and outdoors at all homes. Activity-monitoring devices were installed on the cooktop, range hood and other exhaust fans, and the heating-and-cooling system. The IAQ measurement devices used in this study are listed in Table 1. In addition, the field team monitored equipment use by the occupants by, for example, measuring temperature on a cooktop and by mounting small vane anemometers to monitor fan flows. Participants also kept daily logs of activities that could affect IAQ.

Table 1. IAQ Measuring Devices


Measurement Device




MetOne ES-642


1 minute

MetOne BT-645

Indoor (central)

1 minute

CO2, T, RH

Extech SD-800

Indoor (central,
master & other bedrooms)

1 minute


Aeroqual NO2 Monitor

Indoor (central)

1 minute

Passive Ogawa

Indoor (central)

1 week


GrayWolf (Shinyei) FM-801

Indoor (central,
master bedroom)

30 minutes 

Passive SKC UMEx-100 Outdoor, Indoor
(central, master bedroom)

1 week

T, RH  Onset HOBO U23 Pro v2 Outdoor, Indoor (master & other bedrooms)  1 minute
Onset HOBO UX100-011 Indoor (central)  

1 minute

Diagnostic tests of envelope and duct leakage were made in each home, together with airflow measurements of the ventilation systems. The most difficult measurement to make was for supply system airflows. Because the supply duct terminated into the central forced-air system, it was necessary to measure airflows either by using pitot tube-style devices inserted into ducts that were very hard to access and had insufficient straight length to meet any reasonable accuracy requirement, or by measuring the air flow at exterior inlets that were also very difficult to access. In the future, supply air systems need to be designed and installed such that their airflows can be reasonably verified.


The homes were typical of new California construction: Most homes were 2,000–3,500 square feet, with two to three and a half bathrooms and three to five bedrooms. There was a mix of one-story and two-story houses, with a solitary three-story home. The majority of homes were between one and three years old at the time of monitoring. The homes averaged one occupant for every 1,000 square feet.

Most homes had envelope leakage between 3 ACH50 and 6 ACH50, with a median of about 4.5 ACH50. Only four homes had envelope leakage less than 3 ACH50, the level required for compliance with the 2018 International Energy Conservation Code (ICC 2018).

The various mechanical ventilation approaches are broken down in Figure 1 by ventilation system type, operation mode, and location of exhaust or supply fan (if any). Sixty-four of the 70 homes had exhaust ventilation; the other 6 had supply ventilation. Of the six supply systems, four used a supply fan ducted to the central forced-air duct system, and the other two relied on operation of the central forced-air system blower. These two systems were wired for control by a programmable thermostat, but the ventilation function was not programmed at either home. As a result, these two homes were tested with the exhaust fan in the laundry room operating continuously during the one-week monitoring period to provide mechanical ventilation at a rate that met or exceeded the minimum code requirement.

Whole House Mechanical Ventilation System Type and Fan Location (N=70)

Whole House Mechanical Ventilation System Type and Fan Location (N=70)
Figure 1. Sixty-four of the 70 homes had exhaust ventilation; the other 6 had supply ventilation.

Three homes had a single exhaust fan located remotely in the attic and connected to all bathrooms; when installed correctly and set to operate continuously, this configuration satisfies both bathroom exhaust and whole-dwelling mechanical ventilation airflow requirements. These homes had no switch inside the house that occupants could use to turn the fan on or off. In all three cases, the field team observed installation problems. In one of the homes, the exhaust vent was detached from the roof. In the other two homes, the exhaust fan was not plugged in. In one of these two homes, the exhaust fan did not work and had to be replaced. Study participants contacted the builder, and the repair occurred prior to the one-week monitoring in all three cases.

In all but two cases, the whole-house ventilation exceeded the minimum requirements, and on average the installed airflow was 50% greater than the minimum (as shown in Figure 2). This is similar to the results in Stratton, Walker, and Wray (2012) for previous tests of new (built in 2010–11) California homes. This is primarily because ventilation systems are not being designed or installed to operate at the minimum flow rate. There are fixed flow rates available from ventilation equipment manufacturers, and it appears that many homes have the next size up from the minimum installed. This would appear to be the simplest and lowest-risk approach.

Comparison of Measure Fan Flow for Whole House Ventilation (N=56, exhaust only) and Title 24 Minimum Flow

Comparison of Measure Fan Flow for Whole House  Ventilation (N=56, exhaust only) and Title 24 Minimum Flow
Figure 2. In all but two cases, the whole-house ventilation exceeded the minimum requirements.

An important finding of the study is that the whole-dwelling mechanical ventilation systems were operating in only one quarter of the homes when the field teams first arrived (Figure 3). The most common reasons that systems were not operating were that occupants were unaware that the system existed or that they did not understand the control switch for the whole-dwelling mechanical ventilation system—which typically was not labeled. Field teams also found that programmable controls were not always set correctly: In half the homes with these controllers the fans did not operate. In the two homes where the thermostat was the controller, the fan was turned off in both cases. Both Title 24 and ASHRAE Standard 62.2 require that the controller of a whole-house ventilation system have an identifying and informative label. ASHRAE Guideline 24 provides the following example language for labeling:

Manual switches associated with a whole-building ventilation system should have a clear label such as, “This controls the ventilation system of the home. Leave on except for severe outdoor contamination.” In addition, guidance on operations and maintenance procedures should be provided to occupants.
Whole House Ventilation System Controller Types and Labels (N=70)

Whole House Ventilation System Controller Types and Labels (N=70)
Figure 3. The most common reasons that systems were not operating were that occupants were unaware that the system existed or that they did not understand the controls.

The Title 24 Residential Compliance Manual also provides suggested labeling language, such as “Ventilation Control,” “Operate whenever the house is in use,” or “Keep on except when gone over 7 days.” It also recommends using more-detailed labeling for intermittent systems to provide occupants with basic information on how to operate the timer. However, no specific wording is mandated in Title 24. The wording of the whole-house ventilation system label, like the choice of the system installer, has a direct impact on the understanding of the study participants. For example, in the three homes that had the vague and difficult to interpret message “Continuous Duty,” all three systems were turned off, however, of the 12 homes with a label that identified the control switch, 7 were operating. Improved labeling requirements should be a priority, not just in California, but in any home with mechanical ventilation.

The kitchen ventilation equipment in many homes appears to meet most but not all of the Title 24 requirements: moving ≥ 100 CFM at a setting with a certified sound rating of ≤ 3 sones. While most homes had a range hood or over-the-range microwave exhaust fan (OTR) that met the 100 CFM minimum airflow requirement, many of the range hoods and most of the OTRs did so only at a medium or high speed that was likely to be louder than 3 sones. Some OTRs did not meet the airflow requirement even at the highest speed setting. An important caveat to this finding is that the OTR airflows could be biased low based on the measurement method, which required taping over the air inlets provided at the front top of some OTRs.

Measuring supply airflow.

Most bathroom exhaust fans met the requirement of 50 CFM minimum airflow for an intermittently operated fan. About 90% of the master bathroom fans met this requirement. The field team observed that in approximately two-thirds of homes the main exhaust fan in the master bathroom had a humidistat control. The most common setting was 80% relative humidity for 20-minute run time.

We estimated the total house air exchange rate using these measured mechanical system airflows and their corresponding run times together with an estimate of natural infiltration (using the advanced model in the ASHRAE Handbook of Fundamentals). A typical air exchange rate for these homes was about 0.35 ACH, with most values between 0.20 and 0.61 ACH. These air exchange rates were higher than those measured by Offermann (2009) (the California New Home Study or CNHS) before mechanical ventilation was required in the 2008 Title 24 building code. Offermann reported median air exchange rates of 0.26 ACH for 107 homes measured during a single monitoring day and 0.24 ACH for 21 homes measured over a two-week period.

Because PM2.5 is one of the most important contaminants of interest from a health perspective, we determined the type of air filter used in the central forced-air systems. Almost all filters (96%) were rated MERV 8 or higher, and 30% were rated MERV 11 or higher. This presents a reasonable level of filtration, but filters are only effective if the central forced air system is operating. In shoulder seasons with little or no heating or cooling operation, these filters will not be removing any particles.

Comparisons of indoor formaldehyde, NO2, and PM2.5 from HENGH with CNHS suggest that contaminant levels are lower in recently built mechanically ventilated homes (Figure 4).

Comparison of Mean Indoor Concentrations Measured by HENGH and Results from a Prior Study

Comparison of Mean Indoor Concentrations Measured by HENGH and Results from a Prior Study
Figure 4. Comparisons of indoor formaldehyde, NO2, and PM2.5 with the California New Home Study suggest that contaminant levels are lower in recently built homes.

California’s regulation to limit formaldehyde emissions from composite wood products, combined with increased air exchange rates, appears to have substantially lowered concentrations in new homes; the mean in the HENGH study is about 20 ppb, or about 45% lower than the mean in the pre-2008 CNHS homes. Formaldehyde levels for both studies were higher than California guidelines, but lower than other international guidelines.

The indoor NO2 concentrations were slightly higher for HENGH than those reported in CNHS (6.2 ppb versus 5.4 ppb), while median outdoor levels were similar in the two studies. All of the measured NO2 concentrations were well below the US EPA 53 ppb annual ambient air quality standard for NO2. The primary source of NO2 in homes is combustion of natural gas for cooking. In HENGH, all homes had natural-gas cooking appliances, whereas all of the CNHS homes had electric cooktops, so we might expect higher NO2 in the HENGH homes. The finding of relatively low time-averaged NO2 concentrations in this study is significant. It suggests that the combination of kitchen exhaust and whole-house ventilation provides reasonable control of NO2 in new California homes.

Air quality measurement.

learn more

Chan, W.R., et al. Ventilation and Indoor Air Quality in New California Homes with Gas Appliances and Mechanical Ventilation. Final Project Report (number pending) prepared for California Energy Commission, 2018.

Offermann, F. Ventilation and Indoor Air Quality in New Homes, CEC-500-2009-085. California Energy Commission, 2009.

Price, P. N., et al. Ventilation Practices and Household Characteristics in New California Homes, CEC-500-2007-033. Final Report ARB Contract 03-326 prepared by California Energy Commission, 2007.

Singer, B.C., et al. “Measured Performance of Filtration and Ventilation Systems for Fine and Ultrafine Particles and Ozone in an Unoccupied Modern California House.” Indoor Air 27, no. 4 (2017): 780–90.

Stratton, J.C., Walker, I.S., and Wray, C.P. Measuring Residential Ventilation System Airflows: Part 2 - Field Evaluation of Airflow Meter Devices and System Flow Verification, LBNL-5982E. Berkeley, California: Lawrence Berkeley National Laboratory, 2012.

Lower outdoor PM2.5 can explain only part of the substantially lower indoor PM2.5 levels measured in HENGH (mean of 8 μg/m3) compared to CNHS (mean of 13 μg/m3). The ratio of median indoor to median outdoor concentration was approximately 0.5 for HENGH and approximately 0.8 in the CNHS. Other possible explanations include the possibility of lower particle emission rates inside the home, the benefits of higher-performance air filters in HENGH homes, and a potential benefit of filtration by the building shell associated with the exhaust ventilation systems, as reported by Singer et al. (2017).

CO2 concentrations averaged 620 ppm and were highest overnight in bedrooms (about 100 ppm higher than the mean). Only one home had a bedroom concentration above the 1,100 ppm level suggested by ASHRAE 62.1 guidelines as a basis for odor control. Indoor CO2 concentrations measured in the main living space for HENGH were not substantially different from CNHS.


The field study found that most homes met most ventilation requirements, and the central ventilation fans on average moved 50% more airflow than the minimum specified in the California building code. Air pollutant concentrations were similar to or lower than those reported in the previous CNHS study conducted in 2007–08. Notably, the mean formaldehyde level was 45% lower. Measured concentrations were below health guidelines for most pollutants, indicating that IAQ is acceptable in new California homes when whole-dwelling mechanical ventilation is used. However, the whole-dwelling mechanical ventilation fans were operating in only one quarter of the homes when first visited, and the control switches in many homes did not have informative labels as required by the standards. Corrective action needs to be taken to improve labeling and controls for ventilation systems.

Iain Walker, Rengie Chan, and Brett Singer are research scientists studying indoor environmental quality in the Residential Building Systems Group at Lawrence Berkeley National Laboratory.

The work described in this article was supported primarily by the California Energy Commission through Contract PIR-14-007. Additional support was provided by DOE under Contract DE-AC02-05CH11231. SoCalGas provided direct financial support to the GTI to purchase equipment and to conduct field data collection. Staff support was contributed by PG&E, which funded Misti Bruceri & Associates to provide a staff person to support the study, and by SoCalGas, which allocated engineering and technical staff to contribute to the field work in SoCalGas service territory under GTI direction. SoCalGas and PG&E also supported the project by allocating gas service technicians to conduct gas appliance safety inspections in study homes.

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