The Unseen Pillar of Comfort: Mean Radiant Temperature
Dry Bulb Temperature is Only a Small Part of the Story
The conventional approach to residential and commercial comfort has long been tethered to a single, often misleading metric: air temperature. HVAC systems are primarily designed and controlled to achieve a specific dry-bulb temperature, a standard practice deeply ingrained in industry codes and consumer expectations. However, this singular focus overlooks the most significant factor in how a human being experiences their thermal environment: the exchange of radiant energy with the surrounding surfaces. The inadequacy of air temperature as a sole measure of comfort is a core paradox in building science. A person can be in a room where the thermostat reads a comfortable 72°F (22°C) but still feel uncomfortably cold if they are seated near a large, poorly insulated window or a cold exterior wall. Conversely, a room with cooler air can feel pleasantly warm if the surrounding surfaces are heated, as is the case with radiant systems. This common experience is not a subjective quirk but a direct consequence of a flawed foundational premise. The human body transfers between 45% and 60% of its heat through radiation, while convection, the transfer of heat through air movement, accounts for only 15% to 30%. By concentrating almost exclusively on air temperature, the industry is addressing the least impactful thermal transfer mechanism for the human body, leading to pervasive dissatisfaction even in supposedly "well-conditioned" spaces.
Mean Radiant Temperature
The concept of Mean Radiant Temperature (MRT) was developed to quantify this critical, often-ignored, aspect of thermal comfort. ASHRAE formally defines MRT as "the uniform temperature of an imaginary enclosure in which the radiant heat transfer from the human body is equal to the radiant heat transfer in the actual non-uniform enclosure". In simpler terms, MRT is the area-weighted average temperature of all the objects and surfaces that surround a person. This metric accounts for the collective radiant heat exchange between an individual and their entire environment, including walls, windows, floors, ceilings, furniture, and even other occupants. Unlike a standard thermostat that measures the temperature of the air, MRT considers the radiant heat exchange to and from nearby surfaces, which are constantly absorbing or emitting infrared radiation. The profound difference between the two is that MRT represents the holistic thermal effect of the environment, not just the temperature of the air moving within it. It provides a far more accurate representation of what a person actually feels.
The significance of MRT is cemented by its central role in the most widely accepted thermal comfort models. The Predicted Mean Vote (PMV) and Predicted Percentage of Dissatisfied (PPD) indices, developed by Professor Ole Fanger, provide a scientifically robust framework for predicting human thermal sensation. The PMV model quantifies thermal comfort based on six key parameters: air temperature, relative humidity, air velocity, mean radiant temperature, clothing insulation (Clo), and metabolic rate (Met). The inclusion of MRT as one of these six primary factors, with no special weighting given to air temperature, validates its scientific importance and underscores the inadequacy of any comfort model that fails to account for it. By using this framework, design professionals can move beyond anecdotal comfort complaints and use a scientifically-backed tool to create environments that are genuinely comfortable for the majority of occupants. The PMV scale, ranging from -3 (very cold) to +3 (very hot), correlates directly with the PPD, which estimates the percentage of people who will be dissatisfied with a given environment. For example, even in a perfectly neutral environment (PMV = 0), approximately 5% of people will still be dissatisfied due to individual physiological and psychological differences. The objective of HVAC design, therefore, is not to achieve absolute perfection, but to maintain a PMV between -0.5 and +0.5, which corresponds to a PPD of less than 10%, ensuring comfort for the vast majority.
Despite its critical importance, the accurate and widespread measurement of MRT remains a challenge. The most precise, but also most complex, method is to calculate it from the measured temperatures of all surrounding surfaces and their respective "view factors," or how much of a person's body "sees" each surface. As the number and complexity of surfaces in a room increase, this method becomes time-consuming and difficult to implement. For this reason, a more practical, albeit less precise, method has been widely adopted: the use of a black globe thermometer. This device consists of a hollow copper sphere, typically 15 centimeters (6 inches) in diameter, painted with a black, high-emissivity coating, with a thermometer or sensor at its center. The temperature measured by the globe, known as the globe temperature (t_g), is a result of the balance between heat absorbed through radiation and heat lost through convection. By also measuring air temperature and velocity, one can calculate the MRT using a formula. While the black globe thermometer is affordable and relatively easy to use, it is not a perfect proxy. Its spherical shape may not accurately represent the radiant exchange of a standing person, and the measurement can be skewed by air movement. The underlying problem, as noted in recent research, is the lack of a "feasible, robust, and ergonomic" device for real-time MRT measurement in the built environment. This paradox—that the most important factor for comfort is the hardest to measure easily—highlights a critical gap in building technology and sets the stage for a new, holistic approach to building performance.
The Architect of Thermal Comfort: The Contributions of Robert Bean
Robert Bean is a retired engineering technology professional and a prominent figure in the field of building science, known for his work in indoor environmental quality and high-performance building systems. As an ASHRAE Fellow and a Distinguished Lecturer, Bean is a vocal proponent of a human-centric approach to building design, which he encapsulates in his mantra: "design for people—good buildings follow". His philosophy is a direct challenge to the conventional industry focus on energy efficiency as the primary goal of design. He argues that this approach is flawed because the sole reason society uses energy in buildings is to provide comfort for occupants. Despite this fundamental purpose, the industry's singular focus on enclosures and equipment has led to widespread dissatisfaction, a fact borne out by thousands of post-occupancy surveys that reveal a persistent gap between design intent and lived experience.
Bean's work seeks to reorient the industry by placing human physiology and psychology at the heart of the design process. He notes that a significant portion of the industry lacks a foundational understanding of thermal comfort, with his surveys revealing that only 1.5% of practitioners are competent in applying thermal comfort principles to architectural and mechanical designs. This "illiteracy," as he describes it, is the root cause of much of the dissatisfaction with indoor environments. For Bean, energy efficiency is not a goal in itself but the inevitable and "natural outcome" of engineering an indoor environment based on human factors. This reframing of the design priority from equipment efficiency to occupant well-being represents a profound paradigm shift.
A key part of Bean's legacy is his expertise in radiant heating and cooling systems. These systems, which circulate heated or cooled fluid through pipes embedded in floors, walls, or ceilings, directly manipulate the surface temperatures that define the Mean Radiant Temperature of a space. By providing a comfortable MRT, radiant systems allow a home to maintain a lower air temperature for comfort, leading to improved energy efficiency. For example, a room with a warm radiant floor will feel comfortable even if the air temperature is slightly cooler than a room heated by forced air. This is because the radiant system addresses the dominant mode of human heat loss, providing a stable and uniform thermal environment. This technological approach is in perfect alignment with Bean's philosophy of designing for people. He is an acknowledged expert in the design and history of these systems, having co-authored papers on the topic and developed a 12-step design process for embedded pipe radiant systems.
Bean's philosophical critique extends to what he calls the "3 little pigs" that hinder sustainable progress in the building industry: combustion, customization, and complexity. His argument against combustion is rooted in the concept of exergy efficiency. He posits that it is fundamentally inefficient to use high-temperature energy sources, such as natural gas combustion (which can reach over 3,400°F), to heat a home to a mere 70-something degrees. The vast temperature difference between the source and the use-case represents a massive loss of exergy, or useful energy. Bean advocates for using lower-temperature sources, such as solar thermal energy or heat pumps, which operate at temperatures closer to the target environment and thus have a much higher exergy efficiency. This argument suggests that high-temperature fuels should be reserved for high-temperature industrial processes, with the "waste" heat from those processes "cascading" down to less demanding uses, such as space heating. Bean's second critique, complexity, targets systems that are so convoluted that "the consumer has to actually learn the designer's profession" to use them, which inevitably leads to poor user adoption and performance. These philosophical tenets provide a powerful intellectual foundation for a new, more effective way of thinking about building design.