How to improve indoor air quality with natural, mechanical or hybrid ventilation

How does natural ventilation improve indoor air quality? 

Natural ventilation replaces stale air with fresh air and can improve indoor air quality by lowering carbon dioxide levels in a commercial building. Using openable roof windows or rooflights and façade windows can also reduce reliance on mechanical cooling such as air conditioning to create healthier environments for occupants.

What are the benefits of good indoor air quality and what impact does it have on a building and its occupants? 

If there are indoor pollutants and low ventilation rates in a confined space, this negatively impacts people’s ability to focus and can lead to an increase in fatigue. But studies reveal that with the right ventilation strategy, performance can increase by as much as 15% when building occupants work in an indoor environment with fresh air (Olesen, 2020).

Comfort venting modular skylights

The importance of indoor air quality

In educational settings such as schools and universities, the introduction of ventilation has significantly improved attention span, study focus and academic performance. Research revealed that computerised tasks performed by over 200 students resulted in faster and more accurate responses in relation to memory, vigilance, and word recognition when higher ventilation rates were provided (Bakó-Birób, 2011). The introduction of fresh air coupled with a reduction in carbon dioxide levels creates a pleasant and enjoyable environment in any commercial building.

World-leading research highlights a need for fresh indoor air

It is not only learning environments that are benefitting from the introduction of ventilation. World-renowned Professor at Queensland University of Technology Lidia Morawska considers ventilation critical to improving the general health and well-being of people. The implementation of ventilation into building design reduces exposure to unwanted toxins, chemicals, and fumes.

Dome rooflights with comfort ventilation

Fresh air improves health and lowers building operational costs. A recent survey by the VELUX Group focused on the emergence of an ‘indoor generation’ and the impact this has on people’s health, focus and productivity. Results revealed that 78% of people are unaware that indoor air is often more polluted than outdoor air. Poorly ventilated spaces negatively impact the health of building occupants, resulting in a rise in recorded sick days, as well as the potential transmission of viruses (Dijken & Boerstra, 2021).

Common ways viruses are transmitted indoors:

• Crowded places with people in proximity
• Close-contact settings with face-to-face conversations
• Confined and enclosed spaces with a lack of ventilation

Incorporating a proper ventilation strategy ensures that fresh air is provided to building occupants in all seasons at the lowest cost. For example, a hybrid ventilation strategy is the most energy and cost-efficient way to ventilate. In winter, the use of a mechanical ventilation system with heat recovery is prioritized to reduce heat losses. In summer, natural ventilation achieves high air renewal while saving on fan electricity.

Should I choose mechanical, hybrid or natural ventilation?

Although natural and mechanical ventilation both have specific benefits, hybrid ventilation – based on seasonal needs – remains the most efficient and cost-effective way of achieving good indoor air quality in buildings. Hybrid ventilation maintains controlled ventilation that recovers heat from renewed air during the heating season (via mechanical ventilation) and therefore limits heating needs, but also ventilates efficiently outside of the heating season via natural ventilation if openings such as roof windows or rooflights are controlled by a building management system (BMS).

What are the main differences between natural, mechanical and hybrid ventilation?

There are key differences between natural and mechanical ventilation, but both provide a building with fresh air. A combination of each ventilation strategy is often preferred to best serve a commercial building with the provision of fresh indoor air. The three types of ventilation are natural, mechanical and hybrid. Here we look at how they each function within a commercial building.

What is mechanical ventilation?

Mechanical ventilation operates independently from weather conditions such as high or low winds. This makes it easier to maintain a consistent temperature, as well as control the direction of air flow throughout complex building spaces. 

Mechanical ventilation can include a heat recovery system which retrieves part of the heat from extracted air before releasing it outside of the building. Supply air from the mechanical ventilation inlets is typically 16-19 degrees, even on a cold day. Mechanical ventilation uses either low pressure to force the extraction of indoor air, or high pressure to force air in.  

Low pressure – removes indoor air (extraction) 
High pressure – introduces outdoor air (inlet)

A table showing the strengths and weaknesses of natural and mechanical ventilation

What is natural ventilation?

Passive ventilation is also known as natural ventilation and utilizes weather factors such as wind pressure and thermal buoyancy to exchange air from outside to the inside of an indoor space through openings such as windows, roof windows and rooflights. Night cooling (night purging) relates to operating the openings in the evening when a building is not in use to generate high airflows and remove stale and warm air from a building. 

By making use of the wind and thermal buoyancy, natural ventilation can achieve much higher air change rates than mechanical ventilation under the right conditions, which makes it a good solution to get rid of indoor pollutants quickly, for example, after a meeting with many people gathered in a small space.

In warmer seasons, natural ventilation can provide the necessary fresh air without significant heat losses when outdoor temperatures are close to indoor temperatures. This means mechanical ventilation is not necessary. In these specific conditions, natural ventilation becomes more energy-efficient as there is no need for an electricity-powered fan.

However, it takes energy to heat air. This means the heat recovery quality of mechanical ventilation is more energy-efficient than natural ventilation in winter, as the heat from the extract air is “recovered,” despite electricity powered ventilation fans, and the fact that mechanical ventilation runs when windows and rooflights are not open. Natural ventilation leads to heat loss in winter and should be limited to occasional airing.

What is hybrid ventilation?

Hybrid ventilation combines the qualities of natural ventilation with a mechanical system and complements seasonal changes in weather. Mechanical ventilation provides a continuous air supply which can be combined with natural ventilation’s ability to quickly remove stale air via occasional airing. A typical hybrid ventilation strategy is to use mechanical ventilation with heat recovery in winter to reduce heat losses, and prioritize natural ventilation in warmer periods to save cooling energy, or quickly remove high levels of indoor pollutants.

Rooflights open for natural ventilation

Is a hybrid system better in a commercial building?

When considering a year cycle, relying on hybrid ventilation (mechanical ventilation in cold periods and natural ventilation in warm periods) is more energy-efficient than using mechanical ventilation all year long. This remains true in warm, moderate, and cold climates.  

Hybrid ventilation combines the best of both worlds: good winter energy performance and mechanical ventilation with heat recovery, and good summer performance with natural ventilation. The purpose and structure of a building will dictate what ventilation components to include and how the potential for hybrid ventilation can be harnessed.  

A ventilation strategy will need to consider the design of a commercial or industrial building, internal thermal loads, the positioning of rooflights and windows, and the impact solar gains have on temperature. Through the understanding of structural design and building usage, ventilation can be introduced in the most cost-effective manner. This allows building owners and architects to make more informed decisions when choosing a natural, mechanical or hybrid ventilation system to improve indoor air quality. 

Important factors to consider include: 

• Thermal comfort
• Ease of operation
• Safety of building occupants
• Maintenance staff with access to roof windows, skylights or rooflights
• Fire and regulatory standards
• Use and shape of a building

Healthy, comfortable and productive indoor environments

Monitoring carbon dioxide levels is a key consideration to ensure the proper ventilation system is selected and make sure an indoor climate remains healthy year-round for occupants. But mechanical ventilation in cold months and natural ventilation in warm months is more economical than using mechanical year-round. Natural ventilative cooling can also reduce the need for air conditioning in commercial buildings. In most commercial buildings ventilation systems are controlled by sensors and can incorporate a building management system (BMS) which helps to accommodate hybrid ventilation. 

Find out more about our daylight solutions that offer ventilation to provide occupants of commercial buildings with a healthier indoor climate.

Sources

Bakó-Birób, Z., 2011. Ventilation rates in schools and pupils' performance, Reading: The University of Reading.

Olesen, B., 2020. Productivity and Indoor Air Quality, Copenhagen: Technical University of Denmark.

Dijken, F. V. & Boerstra, A., 2021. Implications of COVID-19 pandemic for application of natural ventilation, Brussels: REHVA Journal.