What's the "time constant" of a building?

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Among the many parameters than play a role in the thermal behavior of a building, whether or not it is built to be a Passive House, the time constant is one of the least known ones.

Nontheless, this parameter influences the way a building responses to changes in internal and external conditions, and therefore it is has a very important role in the correct operation of the building and its heating/cooling system.

The time constant is the result of the internal heat capacity of the building, divided by the average transmittance of the structures that build up its thermal envelope. In other words, on one side this value depends on the amount of heat stored inside the building, and on the other, on how much insulated it is.

This value has nothing to do with buffer tanks and/or electric batteries - a misunderstanding we run frequently into. If a building has a high time constant, it does not mean it can go off the grid - it has nothing to do with it.

The mass of the building that is “useful” in terms of time constant is the one in direct touch with the interior environment, up to 10 cm (about 4”), or up to (and not including) the first insulation layer from the inside (according to ISO 13786).

The time constant is calculated for the building as a whole, not for individual components (walls, roof etc.), or materials. The very same sequence the materials are layered in the assembly of a structure, has a strong effect on the time constant of the building. For example, in a masonry structure, having the insulation layer on the inside or on the outside of the bricks is going to dramatically change the time constant and the way that the building is going to react to changing thermal conditions.

Emu Architetti - Refurbishment of a masonry house

Let’s take as an example one of the two passive houses of Cavriago: the external walls are built in heavy masonry, with the insulation on the outside:

Total heat capacity of the building: 21295,6 Wh/K;

Average transmittance of the envelope: 0,185 W/m2K;

Time constant: 190,5 h.

In case we had decided to build the very same building, using a timber assembly (either timber frame or solid wood panels with insulation), the result would have been very different:

Total heat capacity of the building: 10325,1 Wh/K;

Average transmittance of the envelope: 0,185 W/m2K;

Time constant: 92,4 h.

Which value is better? It depends on the use of the building!

The interior of the Berthoud passive house

The higher the time constant, the longer it is going to take to alter the internal conditions, regardless whether this change is driven by a sudden weather evento or by the building system (1).

A high time constant is advisable, for example, in buildings that are used continuously throughout the year, with a constant indoor temperature: it’s the case of the primary residency home. Such a building is also going to be more resilient in case of extreme whether events.

For a vacation home, on the other hand, the situation is quite the opposite, because it is used only a few days a week. The same happens for schools, for example: in these case, a high time constant may be counterproductive.

The time constant is a parameter that has a direct effect on comfort inside the building, and its influence can either be positive or negative, depending on the use of the building.

As far as the total energy demand of the building, the effect of the time constant is limited. However, this parameter is part of the overall energy design, because it dictates the response time of the building to the change of external conditions, and to the operation of the heating/cooling system.

The decision regarding the type of structure (masonry, wood or other), needs to be taken with consideration of the type of activities that are going to take place in the building: they dictate whether the optimal time constant is either “low” or “high”.

Note (1): although the time constant has an effect on how the building responses to the operation of its heating/cooling system, the amount of hours of its value (see example above) does not represent the time that the building requires to adapt to a new setting in temperature. In other words, if a building has a time constant of 200 hours, it does not mean that it is going to take 200 hours to heat it or cool it!