Cavities & Air Spaces

In principle, use of cavities is similar to use of a insulating material. If an air space is left between two layers making a wall or roof in any building, the air trapped between two layers being poor conductor of heat acts as a barrier to heat transfer.

Heat is transferred across an air space by a combination of conduction, convection and radiation. Heat transfer by conduction is inversely proportional to depth of the air space. Convection is mainly dependant on the height of the air space and its depth. Heat transfer by radiation is relatively independent of both thickness and height, but is greatly dependent on the reflectivity of the internal surfaces. All three mechanisms are dependent on the surface temperatures. The mathematical treatment of air cavity would be similar to that of insulation if natural convection in air is neglected. The thickness of air cavity is a very important design parameter that governs its effectiveness by controlling the heat transfer coefficient as in case of insulation.

It has been found that with gaps broader than 50 mm, movement of trapped air due to temperature gradient starts that in turn increases the coefficient of heat transfer. This increase in heat transfer takes place due to convective heat transfer taking place in addition to conductive heat transfer. Therefore, cavities broader than 50 mm are normally not preferred. However, if more thickness of air cavity is required for getting heavy insulation, by putting partitions in the main broad cavity multiple cavities can be used as an alternative.
Some typical values of thermal resistance for air cavities are given below:

 Air cavity Placement Thickness of air layer (mm) Thermal resistance (m2K/W) Vertical 10- 20 0.14 20- 50 0.17 Horizontal- heat flow from bottom to top 10- 50 0.17 Horizontal- heat flow from top to bottom 10- 50 0.21

If it is possible to ventilate the air gap between the roof and the ceiling, then we could expect a reduction of heat transfer especially by convection. If the ventilation is effective then the air in the void will remain close to the ambient temperature, thus reducing the convective heat transfer to zero. Ventilated air, however, does not reduce the radiative heat transfer from the roof to the ceiling. The radiative component of the heat transfer may be reduced by using low emissivity or high reflective coating (e.g. aluminum foil) on either surface facing the cavity.

In addition to application on walls and roofs, the concept of air cavities also finds very important place in development of insulating windows using double and triple glazing details.

Temperature Profile in a Construction

In a steady-state situation, because there is no heat-storage or heat-production in the construction, the heat flow through every layer in the building construction must be the same. The temperature change across each layer is linear and the rate of change depends on the thermal resistance. When the thermal resistance is small, the temperature difference across the layer is small, but when the thermal resistance is large, the temperature difference across the layer is also large.

The same calculation method is followed as in electrical theory. Ohm’s Law says that the voltage differences over resistances, connected in series, are proportional to the measure of the resistances