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Thermal Properties
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Thermal Performance of High Mass [Concrete] Houses
Research work undertaken by the Cement and Concrete Association of New Zealand (CCANZ) into the benefits of building a house from concrete is now complete and is the culmination of three distinct stages of work that was started in 1997. The work has confirmed that high mass construction is well suited to New Zealand conditions. Housing designs with expansive areas of glazing provide a key ingredient for deriving maximum benefit from thermal mass, i.e. high levels of thermal gain.

The findings of the research can be summarised as follows:

  • The amount of glazing, and its orientation to the sun, has a significant effect on the performance of a home.
  • The concrete building used 15.5% less energy than the identical timber one for similar comfort conditions.
  • The concrete house was more comfortable when a large window was fitted, the timber home overheated significantly.
  • The concrete home was more than 5oC cooler than ambient on a 30oC day, while the temperature inside the timber home approximated the outside temperature.
  • Overnight, the timber home was on average, 1 degree cooler than the concrete one.
  • The minimum temperatures for the concrete and timber buildings were 15.6oC and 12.8oC respectively.
  • The timber home required four times the shading needed by the concrete home (to control overheating).

Concrete has an inherent capacity (related to its mass) to absorb and store thermal energy. This quality is referred to as 'thermal mass'.

Quite simply concrete will absorb thermal energy, store it, and release it when the internal home temperature drops below that of the concrete. This buffering effect means that the intermittent nature of heat sources such as space heaters and the sun becomes less apparent - temperature fluctuations are reduced and a more comfortable home is the result.

In summer, energy from direct sun and from warm circulating air is absorbed by the cooler concrete mass thus reducing the air temperature within the home. As the air temperature decreases in the evening, stored energy within the concrete mass re- radiates ~ providing consistent comfortable temperatures within the home. This cooling effect of thermal mass is especially beneficial in very warm climates.

Eaves should be designed to shade windows from high angled summer sun and there should be sufficient opening windows to allow cross ventilation.

Capturing the free energy of the sun is relatively simple with a concrete home. This energy is most efficiently captured if the sun shines directly onto concrete surfaces, although reflected radiation will also be absorbed by concrete surfaces not directly exposed to sunlight. Convection and conduction also play a part.

Solar gain can be achieved by maximising the glazing that faces north ( ± 20° off north is best) and using low insulation floor coverings such as tiles on a concrete slab. Coloured concrete systems are also ideal. Carpet will insulate the concrete floor slab, which reduces its ability to absorb solar energy. Likewise plasterboard lining on concrete walls will reduce solar gain compared to hardwall plaster. Eaves and verandas should not prevent winter sun penetrating the glazing.

Windows Position
Living areas should be located on the north facing side of the house and least occupied spaces such as garage and laundry should be placed on south facing walls where they act as a buffer between living areas and the coolest southern wall.

Glazing on south facing walls should be minimised. Additional insulation, beyond minimum code requirements, may be desirable in cold climates, on south facing walls.

Regardless of the energy source, it is important that concrete homes are adequately insulated to slow the rate at which stored energy is lost from the home. Most of the principles relating to insulation are the same for concrete homes as they are for other types of home.

The greater the R value of the insulation and the more complete the insulation layer, the better the performance. South facing walls lose thermal energy at a greater rate than other walls and therefore benefit from more efficient insulation. Glass is a poor insulator - heat loss through glass can be minimised by double glazing and by the use of curtains as is the case with any house.

The main difference with a concrete home, however, is that insulation on or near the exterior surface of the wall will generally give better results than interior insulated walls. Insulation on the interior of walls largely isolates the thermal mass thus reducing the thermal mass benefits.

The insulation standard NZS 4218 recognises the beneficial thermal mass effect of concrete in homes and requires concrete homes to have less insulation. The standard allows three alternative methods of determining the insulation requirements.

The first simply prescribes R values for various building elements, the second allows some R values to be reduced provided these are compensated for by higher R values elsewhere in the building. The third method uses sophisticated computer modelling techniques to model thermal performance more accurately for an individual design.

R Values for Typical Concrete Wall Systems

System R value °C/W
Strapped (25mm) & lined 150mm concrete masonry (with reflective foil)
Strapped (25mm) & lined 150mm concrete masonry (pumice aggregate)
Strapped & lined 150mm concrete masonry (with 25mm polystyrene insulation)
200mm cavity insulated concrete masonry block (Partially filled)
250mm cavity insulated concrete masonry block (Partially filled)
150mm concrete masonry block with 50mm expanded polystyrene exterior insulation
Precast panel with polystyrene (50mm polystyrene) cast in
200mm insulated concrete formwork block