Rocky Mountain Institute, Basalt, Colorado Architect - ZGF

Top 5 Steps to Redefining Thermal Comfort

When the story broke that RMI’s 15,610 square foot innovative new office building and convening center in Basalt, Colorado, has no traditional central heating and cooling system in spite of being located in the coldest climate zone in the continental U.S., we definitely raised some eyebrows. How would occupants stay warm with external temperatures that averaged 38 degrees Fahrenheit with only a small, distributed heating system that is the energy-use equivalent of 1.2 average-sized homes? The answer is passive, integrative design.

When RMI needed a new office, we focused on achieving net zero energy through passive, integrated design instead of flashy new technology. There are many exciting aspects to the building, from batteries to reduce peak demand to biophilic design, which can be explored on RMI’s website. However, one of the aspects that is most innovative and replicable for other building design teams is the innovative approach to thermal comfort and passive design. In a previous blog post, we wrote about the growing awareness that the standard approach of over-conditioning buildings with big central cooling systems is expensive and inefficient. Now, we’ve been through our first winter in our own building and have been able to see and experience the results firsthand.

With the Innovation Center, we decided to approach thermal comfort differently. Instead of focusing on making a central system more efficient, we took a step back and focused on making an occupant more comfortable. Any person sitting next to a single pane window in the winter can tell you that it doesn’t matter if the thermostat on the wall says 72 degrees. The focus must shift from the air temperature to the conditions the occupants themselves experience. Five key ideas let us achieve this:


As our previous blog detailed, emerging industry and academic research is showing that there are six variables, beyond temperature, that control a person’s comfort:

People intuitively know these factors affect their comfort, but they have not been translated into metrics building designers can use, until now. A new comfort standard uses a formula to translate these six variables into a single variable, called predictive mean vote (PMV). Through extensive testing, the PMV predicts what percentage of people will be comfortable at a given condition. With this metric, the language changes from abstracted variables like temperature, and shifts to a direct measurement of people’s comfort. PMV provides an extremely powerful tool for the owner and design team, which is further explored in the Thermal Comfort Insight Brief.

This targeted focus on all six variables can be implemented across all building types, and even provide significant energy savings in existing buildings. Occupants in over-conditioned buildings can turn down the heating and cooling and instead use fans or window shades to target all of these comfort variables. By stepping back and examining the best way to meet those six comfort variables through often cheap and easy passive means, building managers can reduce their dependence on expensive and energy intensive central systems.


When the design team focuses on the most efficient way to control comfort variables like radiant temperature and airflow, passive approaches immediately become the cheapest and most efficient option. In the Innovation Center, we first focused on reducing loads and temperature swings with our building envelope and high thermal mass. We used R-13 windows, an R-67 roof, and R-50 walls to maintain a stable indoor temperature and prevent occupants from feeling chilly radiative temperatures during the cold Basalt winter. Our windows have higher insulation levels on the colder north side while letting in more sunlight on the warmer south exposures. To contain all the valuable heat we capture from the sun, we use Passive House standards of air tightness. With these approaches, combined with a suite of other design elements, we were able to eliminate the need for any cooling system, even in our densely occupied convening center, and reduce the heating to a small, distributed radiant system.


Many variables impact a person’s comfort, including clothing, metabolism, activity level, or even proximity to a window, making it impossible for a central system to meet everyone’s needs simultaneously. Instead, technologies are available that allow people to fine-tune their own personal comfort. These approaches can be as simple as a desk fan or window shade and as complex as a personal heating and cooling Hyperchair.

Personal comfort solutions is a rapidly growing field, with even more emerging research in using clothing or wearable devices to actively heat or cool occupants. Many of these technologies are still in development, but will soon be available for the general public. In the meantime, many age-old tools such as fans, insulated blinds, and operable windows can allow anyone to fine-tune his or her personal comfort.


When you expand the design to look at all six variables, the conversation shifts from what mechanical systems can meet the specified air temperature most efficiently to how the building can best meet all six variables. The significant role of mean radiant temperature and airflow requires the architects and contractors to become integrated in those early design conversations. The interplays between the variables controlled by architects, engineers, and contractors push the team to a truly integrated design process.

Early in the Innovation Center’s design process, we found that so many parties having to simultaneously optimize so many different variables changed how we exchanged information and documented our design decisions. Clear, consistent documentation of all variables and the effect of design changes was crucial. To achieve this, the team instituted room data sheets—detailed design specifications—and modeled results for each zone. These are commonly seen on much larger projects, like complex laboratories, but with minimal additional effort the clarity and alignment they provided was invaluable.


Even with such an integrated and motivated team, taking such an untested and unconventional approach to thermal comfort was a significant risk. To mitigate this risk, the team openly discussed possible failure scenarios and created a process for managing these issues. RMI also contractually assumed the risk of people’s comfort using this approach, leaving the design team free to focus on meeting the six variables themselves.

To proactively manage our own risk, we created a contingency plan where the design team created a conceptual design for additional-capacity backup systems. Where it made financial sense, we even designed the existing systems for easy installation or upgrades of the plan B systems in the future. Planning for and enabling these contingency systems did increase initial capital costs. However, this approach enables us to decide if we want to install the additional “conservative” capacity based on the building’s actual performance rather than on a vague factor of safety.

This open acknowledgement and active management of risks was pivotal in aligning the team behind the risk associated with innovative strategies.


We have been occupying our building for three months now and have already learned a great deal about how people experience personal comfort using these new strategies. Using our building as a learning lab, we will continue to tune our building and trial new technologies and approaches as they become available. We hope to share these lessons in a future blog post, so stay tuned! In the meantime, come visit the Innovation Center and see for yourself!

Download the Thermal Comfort Insight Brief

This approach was a key component to RMI’s net-zero strategy at the Innovation Center. This and other trends in innovative, economic approaches to ‘getting to zero’ will be explored at the 2016 Getting to Zero Forum in Denver, Colorado. Register now.

Photo courtesy of Tim Griffith.