The Building Time Constant: Your Key to Thermal Resilience
by Enrico Bonilauri - October 19, 2015
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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:
- Heat Storage: How much heat can be stored by the building’s interior mass (like concrete floors or thick plaster walls).
- 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.
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Thanks for this informative post! A few comments on the “dao of tau.” A long time constant (tau) is actually pretty useful for “thermal storage,” so it can help greatly with off-grid/renewable energy schemes. Secondly, the time to bring a building to temperature with its mechanical system is a function of the thermal mass (a factor in the time constant), not the time constant directly (i.e., a low thermal mass building with low conductance would respond more quickly to internal heat gains/losses than a high thermal mass building with higher conductance, but the same time constant.) The time constant is a measure of how quickly the interior of the building responds to a temperature differential between inside and outside. Lastly, thermal mass in a building with wide daily occupancy swings (like a school) can be pretty important to level the internal heat gains.
Thank you Graham, I think the time constant is often overlooked.
One change in PHPP 9 (but I might be wrong) seems to be that you can no longer customize the internal specific capacity, so you are stuck with 60 kKh/m2 – the equivalent of a timber frame house. This has no effect on the overall energy demand, and it plays “safe” on the overheating assessment. However, for our brick houses, the thermal capacity is around 140 kKh/m2!
You may also want to read our article on time constant and blackouts: http://emuarchitects.com/2015/02/16/the-blackout-proof-home/
I would suggest that the thermal mass of the air inside the building might have to be added to the time constant calculation. This is normally discounted because its heat capacity is so low, but it seems to me that three factors unique to passive house design might make it non-discountable:
1) up to 95% heat recovery of ventilation air exchanged,
2) very low levels of infiltration and
3) usually a large volume relative to the enclosing heat loss surface area.
Given the 20degC internal set temperature, the impact of thermal mass for buildings in constant use is entirely discountable. This may explain its apparent omission in PHPPv9. Thermal mass essentially only becomes important in calculating the time taken between comfort and over-heating (20degC and 25degC) and may therefore only appear as a slightly reduced solar gain or internal gain effect.
Buildings with intermittent, but highly predictable, use (like schools) can, as Graham suggests, be “tuned” to ameliorate overheating in the afternoon due to the effects of very high internal gains. Often ventilation is used in mitigation as it is the most rapidly responding of the various factors. In Ireland, we find that MVHR will run in by-pass mode in the afternoon even in winter, to balance out the internal heat gains.
I have not seen real data from PH schools, but some of our modelling suggests by-pass mode will be used every day, it will just kick-in at 9am or 4pm, depending on the external temperature. Needless to say, automatic by-pass is essential in those circumstances.
We find that additional cross ventilation (beyond MVHR in by-pass mode) is required in PHPP for Irish schools in all but the coldest weather. Making sure this is closed off during the morning warm-up period is critical to controlling heat inputs.
We have also modelled the effect of phase-change materials in Passive House buildings and find particular synergies between intermittent occupancy and PCMs inside a PH thermal envelope. These models indicate PCMs to be several times more significant than the thermal mass of concrete, for example, because of their active heating and cooling phases and the small temperature difference between comfort and over heating.
This is an important consideration as we begin to add embodied carbon to the construction budget: heavy masonry, particularly concrete, very quickly looses all of its advantages when a Passive House envelope with a very efficient MVHR system gets lined with even a small amount of phase change plasterboard.
PCMs act like a battery to passively store excess energy and release it when required
Hello Simon,
your point about time constant and heat capacity of the air inside the house is interesting.
If we take the smaller of the two passive houses in Cavriago, the heat stored in the structure of the building (incl. internal partitions, stairs, slabs) is about 23.200 WhK, with a time constant of 209 h.
The net volume of the building is 525 m3, with a stored heat of 173 WhK. If we add this to the total, the resulting time constant is 210 h, which corresponds to +0,5% from the starting value.
To correct my preliminary understanding of PHPP 9, the thermal capacity of the building is taken into account, as it is its time constant.
Thermal mass does not play a significant role in the overall energy performance for heating, where you have a steady state heat transfer. However it makes the environment more stable and resistant to changing temperatures, where the heat transfer is dynamic. This happens in summer, and in case of power outages. If you read the article on passive houses and black-outs, you can see why I think that masonry is not outdated after all.
To be honest with you, I am suspicious about PCM: I have not spent that much time on the subject, but I’d like to know what they actually are (salts?), and what their effect can be on health, end-use recyclability and so on. I am usually this cautious with any new material: our industry has an history for implementing new stuff, and then realizing it causes cancer (ooops!)
If you can send me any material you have on PCMs, I’d be happy to read it: info [at] emuarchitects.com