COVID-19: How Passive House = Passive Health

Researched and written by Kearns Mancini Architects Inc. 

Across the globe wildlife is reclaiming its natural habitats; dolphins in the Italian canals, foxes in Toronto, deer in the streets of Japan and penguins in the suburbs of Cape Town. We, the humans of the world, would also like to return to our cities. But how do we do that safely amidst a pandemic?

The way our cities are designed are expressions of the people who inhabit them, and throughout history pandemics have had an impact on how those expressions are implemented. Paving streets with flagstones became popular because there was a desire to seal in poisonous gases from the earth that were thought to carry disease. London’s impressive sewer system and New York’s Central Park came to be because of cholera. Tuberculosis, in part, drove the cleanliness of modernist interiors. Now, in 2020, the world is learning how to mitigate the effects of COVID-19, the new form of Corona virus causing respiratory illness. We’re beginning to question what our buildings are doing to help or are they in fact aggravating the problem?


In the last 300 years there have been approximately three influenza pandemics in each century so we can safely assume they will continue to occur. In an increasingly urbanised world, frequently confronted by such challenges, it is important that cities are resilient enough to respond efficiently, especially when humans cause the disease to spread. Like most other viruses, it is commonly understood that COVID-19 is transmitted by means of respiratory droplets that become aerosols, tiny particles generated by coughing, sneezing, shouting, and to a lesser extent, singing, talking and breathing.

COVID-19: How Passive House = Passive Health - Figure-1-Transmission
Figure 1: Current understanding and mitigation strategies. Direct (solid line) & Indirect (dashed line) exposure mechanisms of COVID-19 droplets with control strategies in pink.
Adapted by KMAI from WHO reported exposure mechanisms of covid-19. Francesco Franchimon

Aerosols are miniscule particles that overcome gravity and travel easily through the air. Aerosols evaporate to become a solid particulate known as the “droplet nuclei”. Having very little mass these nuclei become airborne and can travel great distances. Furthermore the nuclei crystallize, maintaining  the stability of the virus and thereby allowing it to remain viable. The image above shows the three main ways in which these infectious particles (pathogens) can then be transmitted. It is important to understand transmission because the risk of infection is increased indoors.

People and Buildings

Buildings can either support or suppress our immune systems. Beyond the effects of COVID-19, recognizing that buildings have a constant effect on our physiological health is important because ordinarily a form of Corona virus accounts for 15% to 30% of common cold flu’s. Moreover, almost all global pandemics (Spanish Influenza, Ebola, SARS and MERS) have come from viruses, the leading cause of respiratory tract illnesses.

Under normal circumstances North Americans typically spend 89% of their time indoors. During this pandemic however, that number is considerably higher as we are forced to rely on buildings that enable us to recover, self-isolate or practice physical distancing. Considering our daily intake of fresh air falls between 11 000 to 15 000 litres having clean indoor air to breathe, especially when isolating inside, is of paramount importance.

Sick building syndrome (SBS), a term coined by the World Health Organization (WHO) in 1986, is a condition commonly affecting office workers where poor indoor air quality (IAQ) was found to cause respiratory problems, amongst other symptoms. During a pandemic where our respiratory systems are already under attack, we should not have buildings that make it worse. Due to its’ geography, Canada may be particularly susceptible to rapid virus spread.

COVID-19: How Passive House = Passive Health - Figure-2-Sick-Building-Syndrome_scaled
Illustration of sick building syndrome by KMAI

Infectious diseases display distinct seasonal patterns in temperate climates like that of Canada. The Canadian population mostly inhabits a narrow east-west (30-50oN) corridor where COVID-19 has had significant community spread in cities and regions at consistently similar weather patterns; most notably low temperatures and low humidity.

COVID-19: How Passive House = Passive Health - Figure-3-COVID-spread-map_scaled
Source: Climate Change Institute. University of Maine.

It has been verified that cold, dry air contributes to the spread of respiratory illnesses. It is interesting to note that the virus did not spread aggressively to countries immediately south of China but locations where significant community spread did occur, had a fairly dry indoor humidity level in the weeks prior to the outbreak. This can be attributed to the fact that the dryer the air, the more droplets evaporate, the longer the virus survives, the further it travels and the more people get infected.

In warmer climates, air conditioning used for cooling may exacerbate conditions as it tends to lower the relative humidity of the air. Overall, the effect can be countered by raising the moisture level inside our buildings. This is known as increasing the percentage of Relative Humidity (RH).

Relative Humidity and Buildings

Relative” means “compared to“. RH is the amount of water vapour compared to a specific air temperature. It is expressed as a percentage and the higher the percentage, the more moisture the air holds. Air, regardless of its temperature, has to maintain enough water vapour to be safe to breathe.

COVID-19: How Passive House = Passive Health - Figure-4-Relative-Humidity
Illustration of Relative Humidity by KMAI

Traditionally we heat our buildings during winter by taking cold outside air, with a relatively low moisture content, and heating it to a comfortable temperature indoors. Heating the relatively dry air without adding any moisture deprives the air we breathe of the water vapour our body needs to function properly. The RH inside a space affects the rate of evaporation of moisture from our skin and causes sensations such as dryness of mouth, nose and eyes (where the mucous membranes are found that transport COVID-19). This comparatively moisture-free air then provides a clear path for airborne pathogens to enter our bodies.

COVID-19: How Passive House = Passive Health - Figure-5-Relative-Humidity-and-Dry-Air
Illustration showing how RH drops when relatively dry winter air is heated in buildings without adding moisture, by KMAI

In 1987 the WHO published results on the possible health risks associated with cold temperatures in a study titled Health impact of low indoor temperatures. In 2007 lab experiments confirmed that cold temperatures and low RH are favourable to the spread of influenza viruses, but concluded that surveillance data was necessary. Since then a growing body of research has proved that low RH leads to increased spread of influenza viruses.

What is unique about pandemic viruses is that they replicate deep inside our lungs. When our cells detect this, they trigger a strong immune response which leads to an influx of fluids into our lungs and it restricts the capacity of airspace we have to breathe. Inside the body mucus needs to be moved to promote clearance of pathogens that enter the respiratory system. Research has shown that lower temperatures are associated with a reduction of mucus movement, disrupting the body’s mechanical defence and immunity towards infection. The low humidity makes the mucus difficult to move.

The effect of low RH on human health is two-fold; not only does it negatively affect the immune system, but low relative humidity has found to aid in the survival and transmission of viruses. Influenza viruses are five times more infectious at a lower relative humidity because the droplet crystallizes and the stability of the virus is maintained which makes propagation in nasal mucosa easier. In addition, dry air encourages evaporation and reduces the size of aerosol particles, making them easier to transmit because they can travel farther in dry air.

COVID-19: How Passive House = Passive Health - Figure-6-Droplet-Transmission-under-different-RH_scaledpng
Iwasaki. Seasonality of Respiratory Viral Infections.

Mitigating transmission in buildings becomes particularly important because usually low outdoor temperatures result in higher indoor densities. Research specific to Canada, which evaluated data from Toronto over a five-year period, found that a RH range of 50% to 60% is where the least amount of transmission occurs. Canadian building operators are advised to maintain humidity levels between 35% and 55% but most fall short of this threshold, especially in colder and dryer climates where it is difficult and expensive to sustain those levels.

There are two reasons for this. Firstly, modern construction focuses on keeping building materials dry, not on the transition of heat/cold through the material. As warm air holds more moisture than cold air, this transition from warm to cold causes water vapour to get trapped inside the material, condense into liquid water and subsequently cause mold growth. Secondly, our current building stock is fairly old and built to satisfy minimum construction codes, that never paid much attention to energy use in building operation.

So how can we effectively manage the moisture levels in our buildings? Clearly it is imperative to get the envelope right to allow for the optimal RH range to be met while also ensuring condensation and mold does not occur.  In the interest of health we need a science-based response to building, so we look to the world’s most rigorous building standard for answers.

COVID-19: How Passive House = Passive Health - Figure-7-Water-vapour-condensing
Illustration of water vapour condensing into liquid as it moves from hot to cold through the building envelope and causing mold, by KMAI


Passive House (PH) is an established, reliable methodology originally inspired in Canada in 1977 and later refined in Germany in 1991. It developed as a comfort and health standard with super energy efficiency as a residual result. Today, it is considered a precise, science-based energy performance standard that uses functional requirements to deliver incredibly high levels of comfort, health, energy efficiency, durability, and resiliency. This meticulous approach to design and building, simultaneously minimizes operating energy by 60-80%, with space heating and cooling demand reductions of roughly 90%, whilst maintaining impressive indoor thermal comfort levels. The standard has a fabric first approach with critical emphasis on air tightness (that reduces energy leaks), a heavily insulated building (minimizing heat transmission/losses), a mechanical ventilation system with energy recovery ventilators (producing the highest level of indoor air quality), and heat recovery (allowing for heating and cooling systems to be used when if and when needed).

PH design and construction is built on 5 principles that minimizes heat gain or loss and is shown in the figure below.

COVID-19: How Passive House = Passive Health - Figure-8-5-Principles-of-PH_SCALED
Illustration of the 5 principles of PH, by KMAI

 One of the many benefits of PH is that a powerful ventilation strategy gives the building operator enhanced control over the indoor air quality. Managing the level of RH needed for healthy environments becomes easy. However, simply increasing the RH of an indoor space does not solve the problem because too much moisture in a building causes fabric degradation that also leads to various human health problems. Special attention needs to be paid to a safe range of RH, avoiding building degradation and ensuring occupant health.

Providing for a tighter range of humidity (40-60%) can mitigate pathogens without causing other adverse effects. This is shown below as the “optimum zone”.

COVID-19: How Passive House = Passive Health - Figure-9-Optimum-Zone
Source: ASHRAE HVAC Systems and Equipment Handbook, Chapter 22. 2016.

Most governments currently set indoor air quality standards with regards to temperature, fresh air introduction and limiting pollutants, but no specific standards have been set for RH. Mandating a minimum indoor humidity level in buildings should be considered as it can help save lives.

The spread of infection can be accelerated or controlled by heating, ventilation, and air conditioning (HVAC) systems. In fighting COVID-19, public health doctors have continually emphasised the importance of introducing fresh air in the building.20;25 Balancing heat and moisture inside a building requires a proper ventilation strategy as well as a controlled, tight envelope – like a balloon. PH buildings are designed according to building physics where a constant balance exists between the envelope, HVAC system, and air tightness.  All potential design conditions are tested and qualified to satisfy these interior/exterior conditions so that the HVAC system can properly maintain constant fresh air and adequate levels of RH.

The American Society for Heating Refrigeration and Air Conditioning Engineers (ASHRAE) has recommended 4 ways a proper ventilation strategy can help mitigate the pandemic. 1) Set RH and temperature targets, 2) Filter the outdoor air coming into the building, 3) Dilute the air inside the building by bringing in more outdoor air, and 4) Purge the air in the building. In ASHRAE’s recent position document on infectious aerosols, they focus on three ways to manage airborne pathogens with a mechanical system: trap it (filtration), kill it (disinfection), or flush it (ventilation).

They recommend improving the central air filtration system by using MERV-13 filters but state that flushing the pathogen by increasing the amount of outdoor air whilst increasing the air change rate, is the most proven approach to reducing unwanted airborne particles. To ensure the success of these two approaches, ASHRAE further recommends running the mechanical systems for longer periods of time, 24/7 if possible. More stringently however, the Federation of European Heating Ventilation and Air Conditioning Associations (REHVA) recently called for engineers to stop re-circulating air in buildings in areas with a COVID-19 outbreak.

These steps are not common in traditional building operations because they place a massive demand on energy use and consequently, costs. This is why following the PH standard is extremely beneficial; it targets 3 of ASHRAE’s recommendations by default. Not only does PH require MERV-13 filters as the standard in their ventilation systems, no air is re-circulated in a PH building and the ventilation system is mandated to run 24/7. Moreover, the dramatic increase in heating demand caused by constantly bringing colder fresh air into the building is countered by the high efficiency heat/energy recovery system mandated. In conjunction with the ventilation system the airtight, insulated envelope removes infiltration and exfiltration, thereby eliminating the need for any make-up air units. This maintains superior air quality resulting in a consistently healthy environment.


We should be cognisant of the fact that weather alone, such as an increase of temperature and humidity during summer months, will not necessarily lead to lower case counts without the continuation of extensive public health interventions. As builders, architects, operators, and owners we have the responsibility to advise and help improve public health. Cities are made of people and any effort to design resilient, sustainable cities must therefore take into account the social contexts of urbanism. Safety is a basic human right and buildings should ensure safety, not hinder it. We should realize the massive health benefits that can be unlocked by building better.

Passive House presents a wonderful opportunity to build better, create healthy environments and future-proof our buildings simultaneously. This standard has the ability to respond to COVID-19 while not ignoring climate change as the systems it employs allow for a total building design where environments are healthier, energy is saved, and emissions are avoided. Passive House is the easiest, most defined way to achieve Net Zero Carbon and Net Zero Energy.

***It is important to keep up to date with new information on Covid-19. The way in which this research was presented was accurate as of June 2020.

SBS – Sick Building Syndrome
IAQ – Indoor Air Quality
RH – Relative Humidity
PH – Passive House
HVAC – Heating Ventilation and Cooling
ASHRAE – American Society for Heating Refrigeration and Air Conditioning Engineers
REHVA – Federation of European HeatingVentilation and Air Conditioning Associations
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