Center for the Built Environment

Creating a comfortable thermal environment is the necessary measure to safeguard human sleep quality but it requires a substantial amount of energy. Personal comfort systems have the potential to improve sleep while significantly reducing energy consumption compared to typical air conditioning systems. Despite some studies reporting favorable outcomes when using personalized approaches to cooling or heating in bedrooms, a comprehensive summary of the impact of personal comfort systems on sleep is lacking. This systematic review of 25 sleep studies estimates the effect of personal comfort systems on sleep quality, sleep stages and sleeping thermal comfort. Configuration of personal comfort systems and sample characteristics are summarized and compared. Calculated effect sizes show that using personal comfort systems are generally effective in improving sleep quality and sleeping thermal comfort. However, there are potential negative effects of personal heating on slow wave sleep and personal cooling on pre-sleep thermal comfort. Relatively mild contact temperatures for both heating and cooling and steady air velocities below 0.9 m/s in warm environments are preferred. The head and feet are the most frequently targeted body parts for cooling and heating, respectively. Most subjects were aged between 17 and 40 years, suggesting a lack of data for both children and the elderly. Four future research directions are proposed: personal cooling at pre-bed phase, human subject tests with radiant systems, dynamic control strategies, and application of machine learning approaches.

Correlational analysis, such as linear regression, does not imply causation. This paper introduces and applies a causal inference framework and a specific method, regression discontinuity, to thermal comfort field studies. The method utilizes policy thresholds in China, where the winter district heating policy is based on geographical location relative to the Huai River. The approximate latitude of the Huai River can be considered as a natural, geographical threshold, where cities near the threshold are quite similar, except for the availability of district heating in cities north of the threshold, creating a situation similar to a natural experiment. Using the regression discontinuity method, we quantify the causal effects of the experiment treatment (district heating) on the physical indoor environments and subjective responses of building occupants. We found that mean indoor operative temperatures were 4.3 °C higher, and mean thermal sensation votes were 0.6 warmer due to the district heating. In contrast, using conventional correlational analysis, we demonstrate that the correlation between indoor operative temperature and thermal sensation votes does not accurately reflect the causal relationship between the two. We also show that the indoor operative temperature could be either positively or negatively correlated with occupants’ thermal satisfaction. However, we cannot conclude that increasing the indoor operative temperature in these circumstances will necessarily lead to higher or lower thermal satisfaction. This highlights the importance of causal inference methods in thermal comfort field studies and other observational studies in building science, where the regression discontinuity method might apply.

Causal thinking emphasizes the understanding of asymmetric causal relationships between variables, requiring us to specify which variable is the cause (independent variable) and which is the effect (dependent variable). Reversing the causal relationship direction can lead to profoundly different assumptions and interpretations. We demonstrate this by comparing two linear regression approaches used in thermal comfort research: Approach (a), which regresses thermal sensation votes (y-axis) on indoor temperature (x-axis); Approach (b), which does the reverse, regressing indoor temperature (y-axis) on thermal sensation votes (x-axis). From a correlational perspective, they may appear interchangeable, but causal thinking reveals substantial and practical differences between them. Approach (a) represents occupants’ thermal sensations as responses to indoor temperature. In contrast, Approach (b), rooted in adaptive comfort theory, suggests that thermal sensations can trigger behavioral changes, which in turn alter indoor temperature. Using the same data, we found that two approaches lead to different neutral temperatures and comfort zones. Approach (b) leads to what we call a ‘preferred zone’, which is 10 °C narrower than the conventionally derived comfort zone using Approach (a). We hypothesize that the ‘preferred zone’ might be interpreted as thermal conditions that occupants are likely to choose when they have significant control over their personal and environmental thermal settings. This finding has important implications for occupant comfort and building energy efficiency. We highlight the importance of integrating causal thinking into correlation-based statistical methods, which have been prevalent in building science research, especially given the increasing volume of data in the built environment.

Desk fans allow individual thermal adjustment in shared spaces which increases occupants’ thermal satisfaction. When associated with the increase of room conditioning system setpoint temperature, they can also reduce energy use. In comparison to other Personal Comfort Systems (PCS), low-power desk fans can be very efficient for cooling. Nevertheless, previous studies identify some barriers to their implementation and show no clear guidelines on how to overcome them. Therefore, this study presents the results of a field implementation of desk fans in an open office in Brazil. The intervention consisted of providing one desk fan for each occupant and progressively increasing the setpoint temperature. Indoor thermal conditions were recorded simultaneously with occupants’ thermal perception using sensors and surveys. Results show fans increased thermal satisfaction by 20 %. And, when fans were available, the preferred indoor air temperature increased by 1 °C. However, many constraints affect the results. Based on this experience, we propose guidelines for future implementation. We emphasize the need to understand the HVAC system, engage building operators, and apply gradual temperature modification. As occupants’ expectations had a great impact on the potential temperature extension, we suggest a way to limit temperature extension in future implementations.

We conducted tracer gas experiments in three connected nursery rooms to track simulated exhaled air trajectories and mixing. We emulated exhaled air with a pulse release of ethanol and measured its concentration with 2s time resolution with spatially distributed metal oxide sensors in both the upper and occupied levels of the room. We found that the overhead cooling supply air enhanced vertical air mixing within the rooms. When a room had higher supply airflow than the others, the tracer gas mixed quickly in that room because the supply air dominated the room mixing, thereby isolating it from air mixing with the other rooms. When the forced air system was not operating, the tracer gas resided longer in the upper room before descending. The tracer method also depicts air trajectories from different release locations by detecting the release point and affecting nearby sensors. These experiments support the potential of this method to be used in the field for understanding air trajectories in rooms with upper-room GUV.

We evaluate the heat transfer from radiant ceilings that have suspended acoustical panels present for noise reduction. An upward-directed ceiling fan is added to offset the reduction of heat exchange due to the acoustical panels. We systematically simulate the indoor thermal environment and the changes to heat transfer coefficients caused by the interaction between radiant ceiling panels, acoustical panels, and ceiling fan under four influencing factors: (1) coverage ratio of acoustical panels, (2) fan rotational speed, (3) radiation panel temperature and (4) room height. The simulation method is validated with experimental data. Numerical results show that the augmented air speed increases convective and total heat transfer for radiant panel. Simulated temperature non-uniformity, air and operative temperature in the occupied part of the room is reduced with increased fan speed, and with decreased acoustical panel coverage ratio. The PMV increased with increased acoustical panel coverage ratio and radiant surface temperature, and also with reduced fan speed. When using fans, the radiant surface temperature can be raised 2 ℃ while maintaining equivalent thermal comfort, allowing higher water supply temperatures. The radiation heat transfer coefficient of the bare ceiling is decreased 25% by adding 63% acoustical panel coverage. The total heat transfer coefficient of radiant ceiling increases with fan speed up to 106.2% over a no-fan base case, and decreases with increased acoustical panel coverage ratio. The study indicates that an upward-directed ceiling fan is a worthwhile method to enable raised radiant surface temperatures, save cooling energy, and reduce surface condensation risk.

Small conference rooms are often used for either face-to-face communication or for virtual meetings involving an electroacoustical link between a talker and a listener. The intelligibility of speech in such environments depends on a number of factors, one of which is the nature of the reverberant sound within the space. Treating such a room with sound-absorbing materials helps reduce the so-called “cognitive load” for people who are spaced some distance away from a talker or who are listening to monaural speech reproduced by a loudspeaker. This study describes an acoustical retrofit of a small conference room to attain the reverberation time criterion found in LEED version 4.1 ID+C. Several mathematical models were used to predict the reverberation time before and after adding soundabsorbing treatment. In addition, measurements were conducted to quantify the before and after room reverberation characteristics. We found that speech was always intelligible both before and after the retrofit; however, one’s cognitive load is noticeably reduced when listening to speech after installation of the treatment.

The purpose of this paper is to evaluate the performance of a novel office temperature control system. To make occupants more comfortable with less energy, we have been developing a new system that uses an inexpensive infrared camera to evaluate occupants’ thermal sensation and optimize room temperature. The system (1) detects the positions of a person’s face, nose, and hands in a thermal image taken by an infrared camera and measures temperatures in those areas; (2) predicts thermal sensation using measured skin temperatures; and (3) adjusts an HVAC set-point temperature based on the predicted sensation to optimize occupant thermal comfort. We compared the comfort and energy performance of the new system to conventional control using a fixed setpoint of 72.0 °F (22.2 °C) in a small conference room. The results indicate that the conventional control often overcooled the occupants, whereas our system reduced cooling energy consumption and made the occupants more thermally neutral and comfortable than the conventional control.