The Building Time Constant: Your Key to Thermal Resilience

What is the Building’s Time Constant?

When the power goes out, how long does your home stay comfortable? The answer is directly related to a little-known but critical metric: the building time constant. This value measures how slowly a building reacts to changes in temperature, whether from a sudden cold snap or a power outage that disables your heating and cooling system. A building with a high time constant offers greater thermal resilience, maintaining safe indoor temperatures for a much longer period. This capability is known as passive survivability.

How is it Calculated?

The time constant isn’t a measure of a single material but of the building as a whole. It’s calculated by dividing the building’s internal heat capacity by the average heat loss through its thermal envelope.

In simpler terms, this depends on two things:

1. Heat Storage: How much heat can be stored by the building’s interior mass (like concrete floors or thick plaster walls).
2. Heat Loss: How well the building’s insulation and airtightness prevent that stored heat from escaping.

A well-insulated building that also has significant interior mass will have a high time constant. It holds onto heat for a long time, increasing its thermal resilience.

If you’re working on a Passive House project, the time constant is calculated automatically by PHPP (although it’s not an upfront result shown by default by the software).

Why Materials and Design Matter

The design of your building has a dramatic effect on its thermal resilience. Let’s compare two Passive House designs with identical, excellent insulation levels.

A house built with a lightweight timber frame has less mass in direct contact with the interior air. It might have a time constant of around 90 hours. In contrast, a house built with heavy masonry and exterior insulation might have a time constant closer to 200 hours.

Both are highly efficient, but the masonry house has far greater thermal resilience. Its massive walls store more thermal energy, allowing it to “coast” through a power outage or extreme weather event for days, maintaining a stable and comfortable temperature. This showcases how design choices directly influence passive survivability.

A high time constant is ideal for primary residences that are occupied continuously. For a building used intermittently, like a vacation home or school, a lower time constant can be better, allowing the heating system to bring it to a comfortable temperature more quickly. Understanding this concept allows us to design homes that are not only energy-efficient but also safe and resilient in a changing world.

Thermal Mass Alone Won’t Do

It’s important to note that thermal mass alone is not enough to achieve a high time constant. The foundation for strong thermal resilience starts with three critical steps:

• outstanding air sealing
• high-quality insulation without thermal bridging
• high-performance windows

Only when these Passive House envelope requirements are in place does adding thermal capacity—such as increased interior mass—truly enhance a building’s ability to maintain comfortable temperatures during extreme events. In other words, boosting thermal mass improves thermal resilience and passive survivability, but only if the building envelope is already performing at a Passive House level.

Building Time Constant – FAQ

What is the time constant of a building?
The building time constant is a measurement of how long it takes for a building’s interior temperature to change in response to outdoor temperature shifts. A higher time constant means the building maintains a stable temperature for longer, indicating good thermal resilience.

What is passive survivability?
Passive survivability is a building’s ability to maintain safe and habitable conditions for its occupants during an extended power outage or interruption of heating or cooling. A high time constant is a key factor in achieving passive survivability.

How does thermal resilience relate to the time constant?
Thermal resilience is a building’s ability to withstand and adapt to extreme temperature events. The building time constant is a direct measure of this resilience—a longer time constant means the building is more resilient to outdoor temperature swings.