While cavity brick walls have high thermal mass, without insulation they are usually too cold in winter, and often too hot in summer if exposed to prolonged heat wave conditions. That’s because the bricks heat up in summer and radiate heat late into the evening, while in winter they stay cold and absorb heat from the house.
They do that because thermal mass stores and re-radiates heat while insulation stops heat flowing into or out of the building. A high thermal mass material is not generally a good thermal insulator and it certainly isn’t a substitute for insulation.
Insulation is essential to protect the occupants from external temperature extremes that are exacerbated by the external brick skin.
If the cavity is insulated, the internal thermal mass (ie. the internal brick skin) is protected from external temperature changes, and becomes highly effective at regulating temperatures within the home, as long as the home is oriented well.
While reverse brick veneer is much more thermally efficient than brick veneer because the thermal mass is on the inside, good insulation is still important to maintain thermal comfort.
Correct use of thermal mass can delay heat flow through the building envelope by as much as 10 to 12 hours producing a warmer house at night in winter and a cooler house during the day in summer.
However, a high mass building needs to gain or lose a large amount of energy to change its internal temperature, whereas a lightweight building requires only a small energy gain or loss. This is the reason why thermal mass is primarily beneficial where there is a big difference between day and night outdoor temperatures, typically exceeding 10˚C.
Remember that some thermal mass materials, such as concrete and brick, when used in the quantities needed, have high embodied energy. The Your Home Technical Guide suggests that in this instance it’s important to consider the lifetime energy impact of thermal mass materials.
“Will the savings in heating and cooling energy be greater than the embodied energy content over the life of the building?” it asks. “Poor design of thermal mass may result in increased heating and cooling energy use on top of the embodied energy content.”