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A thermal mass research project at Lincoln University reveals that concrete walls provide both comfort and cost benefits to homeowners. Although concrete is currently the preferred material for floors, but not for walls, in New Zealand, the research bodes well for the concrete industry.

Previous computer simulation has been criticized for not showing the true benefits of concrete walls. In this unique experimental project, however, measurements are being made in side-by-side, nearly identical buildings, one with concrete walls and the other with timber walls. The walls' thermal mass thus differs.

The main criteria used to evaluate the effect of wall thermal mass on the thermal performance of the test buildings are:

  • Heating energy, heater capacity (affects heating cost)
  • Overheating frequency (a measure of discomfort)
  • Heating load profile (may affect heating energy cost for an electrically heated house)

The concrete walls provided modest savings in heating energy use. The concrete-walled building needed less ventilation because the concrete walls absorbed more energy and cooled the building more effectively than the timber-framed walls. The greater heat flow from the concrete walls during the night led to the energy savings.

The poorer performance of the concrete-walled building on overcast days has some implications for designers. Building sites with poor solar access may not be well suited to heavy-mass homes. The design of windows (in conjunction with thermal mass and insulation) should ensure adequate solar gain using thermal mass.

The peak heating loads of the two buildings were essentially equal. This is an important result because it indicates that a bigger heater is not needed if thermal mass is added to the walls of a house.

On sunny days, the concrete-walled building was cooler than the timber-walled building, overheated less frequently, and was significantly more comfortable than the timber-walled building.

The timber-walled test building was sometimes so warm that it was barely habitable. Reducing the solar and/or increasing the ventilation rate would have reduced overheating, but would also have increased its heating energy requirements.

The results support the notion that exterior insulated heavy mass walls require less insulation than a timber-framed building for equivalent energy performance.

Insulating the timber-walled test building to a higher level would have reduced energy use but worsened the overheating problem.

The results show that the energy savings from wall thermal mass depend on climate. Wall thermal mass should work better, in terms of percentage energy savings, in warmer climates.

Designers need to consider the combined effect of the main factors affecting the thermal performance of a heavy-mass home: thermal mass, insulation, and windows. If concrete walls are to increase their share of the above-grade housing wall market, designers need information and design tools so that they can take this "systems approach" to thermal design. This research project is one step toward providing the information and tools.

This article, written by Dr. Larry Bellamy and Don Mackenzie of the Natural Resources Engineering Department at Lincoln University, is reprinted from the December 1999 issue of Concrete, the Journal of the Cement & Concrete Association of New Zealand.