Enrico Bonilauri

In our articles dedicated to Passive Houses and deep energy retrofits, we often refer to the “health and comfort” of building occupants. We often describe a “healthy and comfortable building” as one that contributes to a feeling of wellbeing.  The task of designing for optimal health and comfort is really the task of quantifying levels of discomfort and removing any factors which adversely affect human health or cause any sensations of discomfort.

With thermal neutrality as a goal, an ideal building would be one that you wouldn’t notice or feel, one that would allow the body to exist in a healthy state and at a comfortable temperature, or even one which would improve your sense of wellbeing upon entering. You might think that this would be a simple task, to avoid harmful design.

However, the prevalence of building materials that contain toxins, combined with standard construction methods that allow dust and pollen to circulate, leading to what is sometimes referred to as Sick Building Syndrome — when a building is the cause of adverse health and discomfort in its occupants.

In this article, we’re going to focus on “hygrothermal comfort,” which is related to temperature and moisture conditions, and how to define the absence of discomfort in a building. We’ll follow up soon with another article dedicated to health and indoor air quality.

What influences thermal comfort?

First of all, the climate in which a person lives has a direct influence on comfort: the sensation of ‘warm’ or ‘cold’ is obviously related to the current weather conditions. On top of that, scientific studies have shown that what a person describes as an optimal comfort level is not constant throughout the year but instead, this level moves up and down on a seasonal basis.

A person may define the very same conditions as ‘too warm’, ‘too cold’ or ‘comfortable’, depending on the time of year. This is proven to be linked to the weather conditions that that person has experienced in the 3-4 weeks previous to the test. This is called ‘personal comfort history.’

The second group of factors that influence comfort is related to the individual person: we are all different from one another, after all. Scientific studies have proven how optimal comfort conditions are influenced by demographic factors as well. In Italy, for example, a commonly referenced stereotype of German people is that they are known to start bathing on the Riviera around May, a time of the year when no Italian would even dare to touch the seawater because they would find it far too cold.

Apart from the obvious generalities of any stereotype, this particular one speaks to the effect of culture, demographics, and varying personal comfort histories in what they would define as “comfortable” swimming conditions.

Another group of somewhat self-explanatory factors bound to the comfort conditions of each individual are clothing and physical activity.

Everything mentioned so far deals with people and their relationship with climate. To address the topic of thermal comfort inside buildings, we need to add two more groups of factors. Physical factors inside a building that directly influence the comfort level of its occupants include the radiant temperature of interior surfaces (walls, floors, ceilings, openings, etc.), as well as the temperature and speed of indoor air, and its relative humidity.

This is the reason why thermal comfort inside a building highly depends on the quality of its thermal envelope, on the absence of thermal bridges, and on its airtightness. These aspects of the construction influence not only the energy performance, but also the satisfaction of the occupant.

However, it is important to remember that the energy performance of a building alone is not enough to determine its quality and comfort.

The last group of factors is made up of those psychological aspects of how one perceives comfort and controls one’s environment. Is the person fully or partially in control of the thermal environment? Is the person able to open a window or to take off one item of clothing in order to adjust their level of comfort? Often, the ability to make changes allows a person to feel more at ease.

This might be a good time to correct the common misconception that windows in a Passive House are inoperable, which is of course ridiculous and false. Unfortunately, many professionals in the HVAC industry, who stand to lose business to the growing passive design movement, are touting this myth to reinforce “business as usual” practices and scare clients away from a decreased dependency on active systems. This is a protection of their own industry, as opposed to a larger concern for clients’ best interests.

How do you design for comfort?

The shortlist we’ve explained above only begins to touch on how complex the topic is. There’s no one-size-fits-all solution. However, by understanding the outcome of scientific studies on thermal comfort, we can improve our design approach and find practical solutions.

There are two main schools of thought in the scientific community. The core of the debate revolves around the difference between summer and winter comfort, and the different approaches to estimating optimal thermal conditions (and then design buildings accordingly).

We’re going to write more on the topic in our future articles with specific reference to the Pianura Padana of Italy, the geographic area where most of our work is located and a climatic zone that requires a mixed design approach.

The one unifying conclusion of all studies related to the subject of post-occupancy evaluations is:

Given consistent environmental conditions (i.e. identical temperatures, air flows, light access, and air quality, to name a few), there will always be a certain percentage of people, however small, that will remain dissatisfied.

One person’s “comfortable” is not another’s. There is no great surprise there. We can, however, use scientific models to see patterns in discomfort and relate them to specific causes in the design of a building.

One comfort model, called the “static comfort model“, was developed by Fanger. It allows an estimate of the reaction that people would have given comfort conditions, based on their clothing and physical activity. The outcome is the PMV (predicted mean vote) as well as the PPD (predicted percentage dissatisfied). Fanger’s model is based on lab experiments, where people have little or no connection with the outside environment. For this reason, this model can be valid to estimate winter comfort conditions.

An alternative to the static model, the “adaptive comfort model,” was developed by Humphrey, and is based on tests on people in real buildings. This research is based on the possibility of people intervening on their environment and adapting themselves to the conditions. The result is a bioclimatic approach, the “free running building,” which is particularly fit to achieve summer comfort conditions.

As mentioned, we are going to write further on this very important topic, especially on passive comfort strategies. We will also be following up this article with a similar overview of “health” with regards to building occupancy.