How can natural ventilative cooling provide thermal comfort and reduce energy use in commercial buildings?

If no ventilation strategy is in place, a commercial building can experience overheating, especially in spaces where there is increased internal gains caused by machinery, occupants, electronics or a lack of solar shading during hot days. Natural ventilative cooling works to improve thermal comfort in commercial buildings with the introduction of outdoor air at the right time and rate. This cools indoor spaces, reduces heat gains and provides building occupants with a comfortable indoor environment for work, study or leisure.

Mechanical cooling like air conditioning is often used in buildings to manage an increase in internal heat gains. But with a passive cooling strategy like natural ventilative cooling, energy usage can be reduced by as much as 50% (CE classifications according to EN Standard).

How does overheating occur in a building?

Overheating refers to a rise in indoor temperature that causes building occupants to become uncomfortable. This can happen due to several factors: the number of people occupying a space, electronics, lighting, outdoor climate, thermal mass, solar gains through glazed facades.

How can natural ventilative cooling be used to manage indoor temperature and provide thermal comfort in a commercial building?

If hot air is unable to be removed, a building becomes uncomfortable to spend time in. Humidity, carbon dioxide levels, pollutants and elevated indoor temperatures can contribute to higher absence rates and lowered productivity. But with the right ventilation strategy there can be a 10% increase in productivity (Olesen, 2010). Reports also show a reduction in employee absenteeism by up to 3.2% (Sterling) when a comfortable indoor environment is provided for building occupants.

Ventilative cooling utilizes outdoor air at its actual temperature to improve the thermal comfort of building occupants, using little or no energy compared to mechanical cooling. Mechanical, hybrid and natural ventilation can all be used for ventilative cooling purposes. When it comes to natural ventilation, there are 4 subdivisions to consider:

• Single-sided ventilation – relies on openings located at same or different heights in the same façade or roof of a building 
• Stack ventilation (thermal buoyancy) – relies on openings located at different heights in the façades or roof of a building or room
• Cross-ventilation – relies on openings located on different façades of a building or room
• Stack & cross ventilation – relies on openings located at different heights and on different facades of building or room

This is considered a simpler configuration of natural ventilation as all openings are located on the same façade, at the same or different heights. But it is also important to consider that this form of natural ventilation is the least efficient in terms of air renewal and difficult to predict due to the influence of weather. Several factors will need to be considered in the planning stage of a natural ventilation strategy. These include wind direction, building location and the type of opening being used.

Cross-ventilation

Cross-ventilation is a specific configuration of a natural ventilation strategy where openings in the same room or space are located on different facades or roof surfaces of a commercial building. This type of natural ventilation relies on the use of wind forces to create an airflow across the space. Cross ventilation can achieve high air flow rates with increasing outdoor wind speeds.

Stack effect

To enable stack effect, openings should be located at different heights, with façade windows supplying air and rooflight and roof windows acting as air outlets. This type of natural ventilation relies on the temperature difference between indoors and outdoors to create a temperature gradient inside a building, and on the stack height (vertical difference in height between supply and extract). Thanks to thermal buoyancy (warm air rises), cold air entering the building through the lower openings is drawn upwards as it gets warmer and leaves the building at high velocity through the upper openings.

• Solar gains – a building being heated from exposure to the sun
• Thermal mass – buildings with a heavy structure (for example, based on concrete) store heat during the day and release it to the space, which makes it slower to cool down
• Materials – structural fabric of a building influence how heat is transmitted and stored
• External air temperature – warm air being brought into a space
• Internal heat gains – activities such as cooking, machinery and heat emitted by people

How does night cooling work?

Night cooling or night flushing takes advantage of lower night temperatures to rapidly replace hot and stale indoor air with cooler air from outside. Outside air cools down a building’s thermal mass which acts as a heat sink during the day. During the night, heat is released from the building structure, which is to be removed by night cooling, finally cooling the building down. Windows and rooflights allow large volumes of cool air to flow into a building, especially if stack and/or cross-ventilation can be used.

4 additional benefits of night cooling:

• A passive solution possible in most buildings
• Reduce energy reliance
• Lower carbon emissions with less dependency on mechanical systems
• Cost-efficient

How efficient this ventilation method is will depend on the geometry and thermal properties of the building, as well as outdoor temperatures and wind speed. Natural ventilative cooling can reduce and sometimes replace the use of mechanical cooling (air-conditioning).

Natural ventilation is being introduced at the planning stage of building design. Solar shading can be added to rooflights and roof windows to provide additional thermal comfort, by limiting the amount of solar energy that can enter the building. Shading and the calculated positioning of roof windows and rooflights allow building occupants to better manage thermal comfort and adapt to changes in temperature.

Rooflights can also be specified with electrochromic glazing. This smart glass works to control glare, connect people with the outdoors—and in the case of the new ACTIS HQ, the European leader in reflective insulation—reduce a reliance on air conditioning by 628 hours in a single year.

Hessenwald School in Germany is an example of energy efficient, contemporary architecture. At the centre of the school is a three-storey atrium with venting skylights/rooflights. Overheating in summer months is prevented by use of thermal mass in exposed ceilings, and by automated night cooling through panels on the façade and venting modular skylights in the atrium. The result is what head of the school, Markus Bürger, describes as ‘a climate in which one feels comfortable, one shaped by great architecture, lighting conditions and a pleasant temperature.’

Passive cooling systems enable a good thermal environment for building occupants and reduce the need for high energy usage more commonly found in mechanical cooling systems such as air conditioning.

By 2050, the global cooling demand in commercial buildings will rise by 275% (O' & Donovan, 2020). This will result in an increase in demand for air conditioning and result in higher energy usage and a rise in carbon dioxide levels. Natural ventilative cooling is a reliable way to reduce air conditioning demand without compromising on occupancy comfort.

How to ensure good thermal comfort for commercial building occupants with natural ventilative cooling?

When incorporating a natural ventilative cooling strategy into commercial building plans, several factors will need to be considered in relation to thermal comfort. These include the local climate – outdoor temperatures during the day and during the night, number of sunny days per year, wind speed and more. Building usage, materials, window location and potential for solar shading also play a role when incorporating an effective natural ventilation strategy into commercial building design plans.

The combination of these key considerations coupled with informed design planning and product selection will positively impact the successful implementation of ventilative cooling in a building.

Find out more about our rooflight solutions that offer ventilation to provide occupants of commercial buildings with good thermal comfort.

FAIA, 2019. Sustainable Workplaces for Human Health and Productivity, Global, FAIA.

O' Donovan, A. et al. 2021. Passive control strategies for cooling a non-residential nearly zeroenergy office: Simulated comfort resilience now and in the future, Cork: Elsevier.

Olesen, B,W 2010. Productivity and Indoor Air Quality, Technical University of Denmark.

Sterling, E & A The Impact of Different Ventilation Levels and Flourescent Lighting Types on Building Illness: An Experimental Study, Canadian Journal of Public Health.