Natural vs. Mechanical Ventilation: Comparing CO₂ Profiles, Ventilation in Bedrooms

From ASHRAE Journal Newsletter, August 10, 2021

Bedroom ventilation that provides good indoor air quality (IAQ) could contribute to a better night’s sleep.

“Sleep is not only rest. It is vital for our health and well-being,” said Chandra Sekhar, Ph.D., Fellow ASHRAE. “Sleep is an essential part of our life, and research in this area is warranted to gain a deeper understanding of the role of IEQ in bedrooms on sleep quality.”

Sekhar and other researchers including Pawel Wargocki, Ph.D., Member ASHRAE; Mariya Bivolarova, Ph.D.; and Mizuho Akimoto, Student Member ASHRAE; wrote a recently published Science and Technology for the Built Environment article that presents a detailed characterization of bedroom carbon dioxide profiles and ventilation during the heating season in Denmark.

 Sekhar discussed the research’s findings and how it can help further the industry on studying IAQ and sleep with ASHRAE Journal.  

What is the significance of this research?

Bedroom ventilation is essential for achieving adequate dilution of indoor contaminants and providing good IAQ. Based on the present—albeit limited evidence—bedroom ventilation and IAQ have been found to affect sleep quality and even next-day performance.¹

This research gains better insights and understanding of the local bed micro-environment ventilation characteristics, which are influenced by the design and operation of bedroom ventilation systems—whether natural ventilation (NV) or mechanical ventilation (MV). It is the bed micro-environment that matters when it comes to exposure levels associated with a sleeping person.

How can this research further the industry's knowledge on this topic?

Both the NV house and the MV apartment [in the study] are quite representative of residential dwellings in a cold climate. Therefore, our findings are thought to be generalizable and can be applied in bedrooms with NV and trickle vents and bedrooms with balanced MV in residential buildings.

The following are some simple takeaways:

• A bedroom with a balanced MV is better able to ensure adequate ventilation in keeping breathing zone CO₂ levels around 1,000 ppm. A NV bedroom with trickle vents and its door kept ajar and augmented with extraction fans is a much improved operational strategy. But it would still not be able to maintain breathing zone CO₂ levels around the 1,000 ppm level.
• It is important that CO₂ measurements are obtained in and around the breathing zone of a sleeping person, as this provides a more realistic observation of local or bed micro-environment ventilation characteristics and exposure levels during sleep.
• The position of the bed in the bedroom is immaterial if sensors are in and around the breathing zone of a person lying on the bed. The idea then is to study the local bed micro-environment, rather than the total bedroom volume.
• The focus of bedroom ventilation design and operation should be on providing adequate quantity of outdoor air and channelling it through the bed micro-environment, i.e., the breathing zone of a sleeping person.

What lessons, facts and/or guidance can an engineer working in the field take away from this research?

Our research involved two different types of bedrooms: one in an apartment with balanced MV and another in a semi-detached house with NV having trickle vents on windows and doors. In the house, NV was the basic mode of ventilation in the bedroom, which could be augmented with mechanical extraction obtained by operation of a bathroom exhaust fan and a kitchen hood exhaust fan.

Our findings show a clear benefit in keeping the NV bedroom door open in achieving better ventilation. Further enhancement of ventilation in the bedroom could be achieved when the open bedroom door configuration is assisted with mechanical extraction, such as bathroom and kitchen hood exhaust fans.

Also, our results show the trickle vents offer an enhanced ventilation in the NV bedroom when coupled with some level of extraction. But when the bedroom door is closed, the effect was relatively small.

In addition, we found the location of the sensors in both the NV and MV bedrooms was shown to be important in estimating local ventilation rates close to a sleeping person. The impact in the measured CO₂ levels in the immediate vicinity of the breathing zone of a sleeping person (<0.5 m) is small, but placing them further away may provide inaccurate estimation of the actual breathing zone air change rates.

The position of the bed in the bedroom does not appear to have any significant impact on the absolute CO₂ levels measured if the measurements are made only in the immediate vicinity of the breathing zone.

In typical MV bedrooms [similar to the one studied: 3.2 m × 2.7 m (volume: 23 m³) {10.5 ft x 8.9 ft (volume: 812 ft³)}], an air change rate (ACR) of about 0.6 h^-1 is envisaged to be adequate to keep the average CO₂ concentration close to the breathing zone of a sleeping person to about 1,000 ppm.

However, in typical NV bedrooms equipped with trickle vents [similar to the one studied: 4.5 m x 2.7 m (38 m³) {14.8 ft x 8.9 ft (volume: 1342 ft³)}], an ajar bedroom door coupled with bathroom and kitchen hood exhaust fans that results in a practically achievable ACR of 0.4 h^-1, would still not always be able to keep the breathing zone CO₂ level to about 1,000 ppm.

The thermal comfort parameters during all the experimental conditions in both the NV and MV bedrooms were well within recommended levels.

Why is it important to explore both mechanically and naturally ventilated homes?

The basic research question in our study pertains to ventilation in and around the breathing zone. The breathing zone is the bed micro-environment of a sleeping person that is influenced by airflow profiles in the bedroom caused by air distribution.

The air distribution in the bedroom (total) and the bed micro-environment (local) is a function of the operational “push-pull” ventilation strategy that would eventually govern not only “how much” outdoor air moves through the bedroom volume but “how” that movement occurs.

In a MV home, the forced convection fan system would ensure the supply of design outdoor air quantity into the bedroom, and the location of supply air outlets could—to a large extent—impact both the total and local air distribution.

In an NV home, there is little control over the amount of outdoor air and the total and local air distribution in the bedroom. Hence, the research design included both NV and MV bedrooms with a view of first understanding the ventilation characteristics through a performance comparison framework followed by simple practical interventions to enhance bedroom ventilation, especially in NV settings.

Why is it important to explore this topic now?

We spend about 20 years in bedrooms over our lifetime, and sleep is essential for health as documented by the newest research. Sleep is not only rest; it is vital for our health and well-being. Sleep is an essential part of our life, and research in this area is warranted to gain a deeper understanding of the role of IEQ in bedrooms on sleep quality.

Most of the indoor environmental quality (IEQ) research is focused on other environments—mainly classrooms, offices and other parts of dwellings. Bedrooms were studied in a limited number of studies and did not entail observing the potential effects on sleep quality but just on measuring ventilation rates.²

To date, studies have typically been cross-sectional in design and have reported bedroom environmental conditions as normally used by occupants without much of a comparison with any intervention that could potentially alter the bedroom ventilation characteristics.² It can be intuitively seen that the distribution of measured CO₂ concentrations in bedrooms could be attributed to different airflow distribution characteristics caused by different ventilation principles, such as NV or MV.

Factors (such as forced convection and directional airflow) coupled with some effects of the thermal plume of a sleeping person would govern the breathing zone air distribution profiles in a MV bedroom. On the other hand, natural wind-driven forces at the window openings, coupled with cross ventilation across a NV bedroom and/or mechanical extraction, would influence the breathing zone air distribution profiles in a NV bedroom.

This lack of information is observed as a critical gap. Our research attempts to fill this gap by presenting the findings of field measurements in naturally and mechanically ventilated bedrooms. The local air distribution patterns in the immediate breathing zone of a sleeping person was studied in detail to gain a better understanding of the effects of the ventilation system on bedroom ACR and, consequently, on the CO₂ exposure levels of the sleeping person.