Insulation & Thermal Mass: Efficient Buildings in Rio de Janeiro

Proper use of insulation and materials of appropriate thermal mass can improve the energy efficiency of buildings

Thermally insulating buildings at the construction stage can be a cost effective way of reducing cooling loads. This reduces running costs and improves energy efficiency. Photo by Dept of Energy Solar Decathlon licensed under creative commons.

Insulation can be a cost effective way of reducing the heat input to a building, thereby improving thermal comfort. Strategic use of thermal insulation can therefore be a relatively cheap way of improving the energy efficiency of a building.

Thermal insulation is a barrier to heat flow and helps reduce solar gain. Insulation also helps prevent cooled air from warming up.

It has been suggested that in Brazil adding insulation to buildings provides significant cost savings – though this is probably the case for air-conditioned buildings only [1].

Insulation can reduce the cooling load on a building by preventing solar gain. Because less heat passes into the living space, less energy is required to provide thermal comfort for occupants. Insulation can be installed in the roof, ceiling and walls, and can be either bulk insulation or reflective.

Reflective insulation is typically installed under roof sheets, where it reduces the amount of heat that enters the building.

Bulk insulation slows convective and conductive heat transfer and is generally used to keep air within the building cool. Bulk insulation of walls is probably most useful where buildings are air conditioned – in this case, insulation helps keep interior air cool and reduces the demand for cooling energy.

High Thermal Mass Materials

Materials that readily absorb, store and release heat – such as concrete, bricks, stone and masonry – are said to have a high thermal mass. In temperate zones, these materials absorb heat during the day and radiate it away at night. In tropical areas like Rio de Janeiro, such materials heat up during the daytime, but warm night time temperatures prevent heat loss. This has the potential to increase the requirement for cooling energy.

When high thermal mass materials are used, the avoidance of incident solar radiation is extremely important. External shading by means of proper landscaping and building-installed shading is crucial. See the article on shading in this toolkit. In addition, high thermal mass materials should be insulated internally to prevent heat being transferred to living spaces.

Low Thermal Mass Materials

In terms of energy efficiency, lightweight construction materials such as timber may be preferable to concrete and masonry. Lightweight materials are rapidly cooled by breezes but should still be insulated to prevent heat passing into the building [2]. Where possible, these materials should be light coloured to further minimise solar heat gain.

High Thermal Mass with Night Ventilation

Thermal mass within a building can have a useful daytime cooling function. For this to be optimised, night ventilation needs to be properly employed so that the heat absorbed during the day can be removed from the building. For more information on night ventilation, see the natural ventilation section of this toolkit.

Some research suggests that for modern buildings in hot, humid environments, high thermal mass materials are appropriate for spaces used primarily during the day. However, such materials should not be used in bedrooms, since the heat absorbed during the day will be released at night – adversely affecting thermal comfort levels [3]

It is conceivable that a well-designed non air-conditioned building that optimises the use of natural ventilation and night-time flushing could successfully use high thermal mass materials as a daytime heat sink. This could passively improve thermal comfort levels.


  1. T. F. Reichardt, “Technical and Economic Assessment of Medium Sized Solar-Assisted Air-Conditioning in Brazil” Master’s Thesis, PUC Rio de Janeiro, 2010. [Online]. Available:
  2. Cairns Regional Council, “Sustainable Tropical Building Design: Guidelines for Commercial Buildings,” 2011. [Online]. Available:
  3. T. Chenvidyakarn, “Review Article : Passive Design for Thermal Comfort in Hot Humid Climates,” Journal of Architectural/Planning Research and Studies Volume 5. Issue 1., 2007. [Online]. Available: