In this era of increased awareness surrounding energy efficiency and environmentalism (due to both the media and our rapidly rising power bills), we are looking for solutions that address these issues. For new homes and renovations, the most significant savings can be gained through using solar passive design principles, including orientation, thermal mass, insulation, and ventilation. These principles effectively use basic design conventions and knowledge of the local climate to ensure the home’s temperature spends as much time in the thermal comfort zone (18-24°C, 64-75°F) as possible. This reduces the requirement for artificial heating and cooling, including the use of energy-intensive air conditioning systems.

Detailed lifecycle cost analysis has shown that a house’s operational energy use accounts for over 90% of its overall energy consumption and greenhouse gas

emissions. The easiest way to reduce energy consumption is to design a thermally efficient house to reduce heating, ventilation and air conditioning (HVAC) requirements.

Researching Thermal Comfort

Over the last 10 years, Think Brick Australia and the University of Newcastle have been involved in a research project to examine the energy efficiency of various building systems and the effectiveness of thermal mass. This research has produced many significant findings that can be easily incorporated into home designs to help reduce our carbon footprint.

Some of the initial research involved the instrumentation and observation of full-scale housing modules (6 x 6 m) built with different construction materials and subjected to real-world climatic conditions. The systems investigated included cavity brick, insulated cavity brick, brick veneer and insulated lightweight walling systems with the modules allowed to free float.

Findings from this research showed a distinct relationship between thermal mass and thermal comfort. The best example of this was on January 1, 2006, when the modules were subjected to heat wave conditions. The lightweight module experienced a peak temperature of about 32°C and spent nearly eight hours above 30°C. In contrast, the insulated brick module did not even reach 30°C, peaking at only 29°C for the day—despite the maximum external temperature exceeding 46°C.

As shown in Figure 1, occupants of the lightweight home would experience much greater temperature fluctuations, not to mention significantly higher maximum temperatures during the summer. On the other hand, the internal temperature of the insulated cavity brick module was more moderated, with daily temperature swings about half that of the lightweight construction.

Interestingly, these results also show that the maximum temperature reached during the day occurred much earlier in the case of the lightweight construction, with the higher thermal mass construction inducing more “thermal lag.” This is clearly illustrated on the heat wave day in Figure 1. The external temperature peaked at 5:15 p.m., and the lightweight house peaked shortly after 5:30 p.m., indicating little (if any) thermal lag. The brick building peaked at about 10 p.m., indicating significant thermal lag.

Analysis of heat flux/flow measurements showed that heat readily passed through the lightweight building envelope into the living space. In the case of the cavity brick envelopes, much of the heat was absorbed by the brickwork layers during the day and then released back into the atmosphere in the evening when the outside temperature had cooled, reducing the amount of heat that enters the inside of the house.

Figure 2 illustrates how the high thermal mass walling systems attenuate the heat flow. Between 700 and 900 W/m2 of heat impinges on the external wall surface; the majority of this heat is reflected. Only 200 W/m2 passes into the external leaf, and only 50 W/m2 passes through the cavity. The inner leaf then absorbs more energy, allowing only 5-6 W/m2 to reach the living space. All the energy absorbed by brickwork induces the thermal lag. Some of this energy is later released back into the cooler atmosphere, reducing the heat that enters the living space.

Testing Theories

In Australia and many other countries, building regulations that govern the design of energy-efficient structures revolve around minimum R-values that must be achieved by walling systems. The R-value is a steady-state measure of the thermal resistance of a material. Based on these requirements and the specifications that currently exist, it would be expected that wall constructions with higher R-values would be more energy efficient compared to those with lower R-values.

In this instance, when we compare two systems with similar R-values (insulated cavity brick R=1.30 and insulated lightweight R=1.51), the performance is

markedly dissimilar. It seems clear that increasing thermal mass is the dominant factor in achieving improved thermal efficiency.

We looked at the energy consumption of the modules when maintained in the thermal comfort zone using reverse-cycle air conditioning. Figure 3 shows data for spring conditions consisting of cool nights and warm days, with moderate solar ingress conditions ideal for passive contributions (e.g., closing the curtains at night). Similar to the free-floating results, the internal temperature of the lightweight module tended to mimic the external temperature and exhibited limited thermal lag. This module was always the first to require artificial intervention to maintain the internal temperature with heating and cooling when the external temperature tended toward the daily extremes. The insulated brick veneer module behaved similarly, but did not require as much intervention as the lightweight module.

During the same period, the higher thermal mass brick modules were better able to regulate their internal temperatures, requiring limited cooling and no heating, with the insulated cavity brick module again performing the best. Figure 4 shows the total energy consumption for the month of October. These findings again show that no correlation appears to exist between R-value and thermal efficiency. In particular, the performance of the cavity brick and insulated cavity brick modules was similar, despite a difference in R-value of almost a factor of three.

To further test the theory that thermal mass improves thermal efficiency, brick internal partition walls were added to each housing module. This produced the energy savings outlined in Table 1 and showed that additional thermal mass results in further energy saving. However, the effect is magnified when the internal leaf is not masonry, as those with internal masonry walls are already benefitting from the thermal mass.

When comparing insulated reverse brick veneer to insulated lightweight, the latter was found to offer better thermal efficiency. The benefits of internal vs. external thermal mass were not clear; however, insulated cavity brick was the best-performing system in all scenarios.

As part of the thermal mass design, the modules were built in a concrete slab on ground formation, providing more thermal mass. This design was shown to be able to act as part of the solar passive design, absorbing heat from solar radiation in winter and releasing it into the living space in the cooler evening. In the summer, there was less heat to absorb due to the high solar angle, resulting in less heat to release at night.

The addition of carpet and underlay reduced the positive effect of the thermal mass floor in the case of the lightweight modules and increased the internal temperature by 2-3°C. In the case of the cavity brick modules, there was no discernible effect.

Embracing Thermal Mass

When designing energy-efficient homes, thermal mass should be taken into consideration. Building with insulated cavity brick walls has proven to be the most thermally efficient design. Combining this with other solar passive design principles and practices will help reduce reliance on artificial heating and cooling, consequently reducing energy bills.

Further research is being undertaken to verify the applicability of a new factor called the T-value, a measure of dynamic thermal response that more accurately represents the thermal efficiency of walling systems. The aim is for it to replace the R-value in the specification and design of energy efficient homes.

For additional information, visit www.thinkbrick.com.au.