Thermal physics and indoor climate of a building

Thermal physics and indoor climate of a building

15.08.2018
In order to understand the thermal resistance of buildings and designing of building envelopes, one must know something about construction physics, namely, constructional thermal physics. Thermal physics describes various thermal effects and explains movement of substance particles.

The thermal resistance of a building is largely determined by the thermal conductivity of a building envelope. When assessing thermal conductivity, the main criteria are thermal comfort, i.e. the temperature of the internal surface of a building envelope must not be too low, and the payback period of construction and exploitation costs, which should be as short as possible.

Thermal conductivity U shows how much heat is transferred through a wall of one square metre under the condition of one degree difference of inside and outside temperatures in an hour W/(m2K).
U=1/R or thermal conductivity of a building envelope is the inverse of thermal resistance. Thermal resistance R is a sum of the resistance of internal and external surfaces and resistances of individual material layers of the building envelope. This allows for calculating thermal conductivity for each envelope at particular outside and inside temperatures.

The lower the thermal conductivity of a building envelope, the higher the thermal resistance of the house. However, thermal conductivity is not the only criterion and every building should be assessed as a whole where the primary objective should be creating a good indoor climate.

To achieve a good indoor climate in a building, the room temperature and the temperature of the interior surface of external structures are important because they influence the level of human comfort. Moreover, air vapour condensation or mould may appear on the interior surface of a structure if the temperature of the interior surface is too low, and this will worsen the indoor climate significantly and cause a risk of health damage.

An important aspect of a building envelope is its thermal capacity. Thermal capacity is the amount of heat that is necessary to increase the temperature of a particular quantity of substance (kg, m3) by one degree.
The higher the thermal capacity of a building envelope, the more stable the indoor climate.

Stove heating or some heating solutions that are cyclical suit well for buildings with high thermal capacity and inertia. Namely, in such a case, thermal energy is stored in walls that will gradually release heat back to the room after heating.
A wall structure with good thermal capacity behaves in the same way in summer because it smoothes out larger fluctuations in the outside temperature. Namely, the room temperature of a building does not increase so quickly in very warm weather and the indoor climate is much more stable. This thermal inertia is the reason why buildings with high thermal capacity do not usually require a cooling system.

Although stone buildings are usually considered to have good thermal capacity, log houses actually have superb thermal inertia.

When designing building envelopes, and to achieve good indoor climate, indoor air moisture must be taken into account. Namely, relative air moisture that is a ratio of the amount of air vapour in the air and the absolute maximum air vapour at the same physical conditions expressed in percents. The relative air moisture is usually 40−60% to provide a good indoor climate, but it is often much lower inside a building in the winter period.

A dew point can be calculated based on the room temperature and relative air moisture that shows at which temperature air vapour condensates. For example, if room temperature is +22 degrees and relative air moisture is 45%, the dew point is +9 degrees. If there is a surface in the room with a temperature +9 degrees or lower, condensation water will start forming on this surface. Usually, such surfaces are windows.

The air tightness of envelopes plays an important role in the thermal tightness of building envelopes because it helps to reduce the thermal loss of a building. The air tightness of building envelopes is examined based on standard EVS EN 13829:2000, which gives criteria related to the ventilation solution of a building. For example, in the case of natural ventilation, the allowed air leakage of an envelope is up to 3 l/h and up to 1 l/h in the case of forced ventilation.

From a practical point of view, risks are higher when multi-layer building envelopes compared to one-layer envelopes (for example, full log houses) are built. The reason is that although modern construction and thermal physics enable very precise calculating of the thermal conductivity of envelopes by layers and achieve good results, the errors and deficiencies that may occur during the installation of different wall layers decrease the actual results to some extent.

To sum it up, an indoor climate of each house depends on the sum of all the above factors where each component is important and significant. A good indoor climate is necessary for people that occupy the building as well as for the building itself, so that it can function long-term.

Reference:
Presentation on thermal tightness of building envelopes
Mart Jõgioja, energy auditor
OÜ JÕGIOJA Ehitusfüüsika KB

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