Using thermal inertia for better comfort and heat savings

18 September 2007 - Comments (1)

Construction

Thermal inertia, that we have defined in a previous article, represents the capacity of a material to store heat. A construction with high thermal can provide better comfort for less money comparing to one with low thermal inertia. This article explain how to take advantage of thermal inertia in construction in order to increase indoor comfort and cut heating and cooling expenses.

Two sides of thermal inertia in one house

Most of construction materials have some sort of thermal inertia. Heavy materials such as clay blocks, bricks, stones or concrete, have high thermal inertia but are not very good for thermal insulation. On the contrary, thermal insulation materials, are good for thermal insulation but do not have much thermal inertia.

Thermal inertia for heat storage

In a well designed house, thermal inertia of heavy construction materials can be used for heat storage. It means that the material will accumulate excess of heat during the day and will restitute it in the air when temperature drops during the night. This can level out indoor temperature variations, provide better comfort in summer and reduce the need of heating. This is the aspect we detail in this article.

Thermal inertia for thermal phase lag

Even tough, thermal insulation materials do not have much thermal inertia, their thermal inertia plays a crucial role for comfort in summer by providing thermal phase lag. We have already explained that aspect of thermal inertia in a previous article.

Where is thermal inertia located

Depending what materials are used to build, a house can have more or less thermal inertia. Wooden or prefabricated houses have no thermal inertia (if none is added). Stone houses have a lot. Traditional clay blocks construction are in between. Thermal inertia is not related to how well a house is insulated. The best houses being the ones that combine high thermal inertia with excellent thermal insulation.

Natural thermal inertia in the construction

For solid houses, made of clay blocks or concrete, the wall system usually provides some sort of thermal inertia. To keep the thermal inertia of the walls usable, thermal insulation must be placed on the outdoor side of the wall. If thermal insulation is placed on the indoor side, it will prevent any exchange of heat between the indoor air and the wall.

One very interesting material for construction is the large clay block such as Wienerberger POROTHERM. It is very interesting because it combines good thermal insulation together with high thermal inertia. With one material, a massive wall made of large clay blocks, provides all advantages of a well insulated wall and all advantages of a wall with high thermal inertia that we describe below.

Exemple of thermal inertia in the walls using large clay blocks in Amadeo (Photo Beodom)

Additional thermal mass

Thermal inertia can also be added in some of the interior walls or in the interior floor. Thermal inertia will be obtained by using heavy and dense material such as clay, stones, concrete or bricks. When adding heavy materials in the construction for the sole purpose of gaining thermal inertia, it is called a thermal mass. That thermal mass should be well exposed to capture heat from the sun and store it.

The goal of adding a thermal mass in one house is to capture radiant heat from the sun during the cold months and to capture excess of indoor heat during the hot months.

Good materials for thermal inertia must have low thermal effusivity and diffusivity and high thermal inertia. Various metals, such as steel or aluminium, have high thermal inertia but high thermal effusivity and diffusivity as well. That means they can store lot of heat but they will release it very quickly as well. To be efficient, thermal mass must store heat during the day and release it slowly over the night.

Benefits of having thermal inertia

Thermal inertia to level out indoor temperature variation

During the heating season, building components with high thermal inertia that are in contact with the indoor air will slowly absorb indoor heat until they get to room temperature. They will store that heat and release it slowly as soon as the indoor temperature drops below their surface temperature.

The same mecanism will also apply in case of sudden spike of indoor temperature (cooking, more people...). That excess of heat will be absorbed and released later when temperature drops.

This aspect of thermal inertia levels out indoor temperature variations and provide a more comfortable indoor climate. The walls feel warm and temperature is more uniform in space and time.

Thermal inertia for better comfort in summer

A stone house might not be so comfortable in winter but in summer, it stays fresh all day long. That is because, stones, while being a poor thermal insulation, have high thermal inertia; it absorb so much heat that outdoor temperature peak can never make it indoor.

In modern constructions, the goal is to find a compromise between good thermal insulation and high thermal inertia. Materials with high thermal inertia will prevent heat to enter indoor by storing it during the day and releasing it during the night when temperature cools down.

If enough heat can be released during the night, so that temperature of the material doesn't rise too much over many hot days, freshness can be maintained all summer. This is the secret of the old stone houses.

Thermal inertia to save on heating expenses

During the heating season, it can be frequent to have good sunny days. The radiant heat from the sun, even if the outdoor temperature stays fresh, is a very good source of free energy. Thermal inertia can be used to capture that free solar energy and store it. It can then be reused later during the day when there is no more sun. This is one of the principles of passive houses and it can lead to big saving on heating cost.

This principle can be applied using a well exposed thermal mass in the living area of one house. It can also work with well exposed wall made of construction material with high thermal inertia.

Storing free energy during the day (solar energy, outdoor warmth) and releasing it during the night is also key to shorten the heating season. When days start to be warmer but nights are still fresh, materials with strong thermal inertia will store heat available during the day and slowly release it during the night.

Conclusion

A house with no thermal inertia will be quick to warm up but will cool down quickly as well. A constant source of heat is needed to maintain comfort and indoor temperature variations are greater. There will be no way to store excess of heat for reuse. In summer, such house will not offer much protection against outdoor heat and will quickly become uncomfortable.

Understanding thermal inertia is a key factor to design better houses. High thermal inertia combined with good thermal insulation provides fantastic comfort all year round and cut heating cost. It will also remove the need of air conditionning in summer which will result in better indoor climate and more saving. Large clay blocks, such as Wienerberger POROTHERM, are the perfect building material to implement these principles.