‘Why do so many Australians believe earth construction
provides such a comfortable home?’
This question was posed by The Commonwealth Scientific and
Industrial Research Organisation in a media release of 27 April 2000
– ‘Mud walls give poor insulation: CSIRO’. The article describes
recent CSIRO test results which “effectively end the controversy”
over the thermal efficiency of earth walls. The research compared
the overall thermal resistance (R-value) of two earth wall samples
to that of common insulating materials such as glass fibre batts,
with the earth panels achieving a significantly lower R-value.
Anecdotal evidence gathered during twenty years of building and
living in earth-walled structures overwhelmingly suggests that earth
construction can – and does – provide comfortable, energy-efficient
homes. While not disputing the test results, asEg believes that
R-value is just one of many factors contributing to a comfortable
living space.
The thermal mass of earth allows for heat gains during the day to
be stored and slowly released at night. The process whereby a solid
mass wall accumulates and disperses heat is known as the ‘thermal
flywheel’ effect, or thermal lag. For a 300mm thick earth wall,
thermal lag is around 8 – 9 hours, meaning that the heat stored by
late afternoon will be felt in the house throughout the evening.
This is desirable in a temperate climate with a high daily
temperature range, where the time lag ensures a comfortable and
consistent environment inside the house.
In contrast is the low heat storage capacity of insulated
lightweight construction. While an insulator with high R-value will
keep heat from escaping, it cannot store heat. The insulated walls
require continual mechanical intervention (either heating or
cooling) to maintain a comfortable temperature. An open window
ventilating a room, for instance, or frequently opening and closing
doors, will be significant sources of heat loss for the insulated
structure, while heat stored in thermal mass will not be affected by
these variables.
This raises the topic of ‘operational energy’, which describes
the energy required to heat, cool and illuminate a building on a
day-to-day basis (distinct from 'embodied energy', which measures
the energy used to obtain, manufacture and install building
materials and components). A well designed earth-walled house on a
concrete ground slab will potentially demand far less energy through
mechanical heating or cooling, thanks to its advantage of being able
to store warmth each day. The benefits gained will depend largely on
the design: a good one will respond to the site’s demands by
addressing landform, solar aspect and prevailing winds, and will
allow these features to help shape the built environment. Design
features can then make the best use of these influences: glazed
living areas to the north to capture winter’s sunlight (shaded to
exclude summer’s); earth walls which radiate warmth inside; adequate
ventilation throughout the whole building.
As the Building Code of Australia comes to adopt standards for
the mandatory minimum energy performance of houses and other
buildings, it is important for decision-makers and regulators to
address all the relevant attributes of the material before
condemning earth construction solely on the basis of R-value.
Perhaps the answer to the CSIRO’s question lies beyond thermal
comfort alone. The creation of space and mood – for instance – and
the use of light, are quintessential to defining our living spaces.
With this in mind, earth walls will always have a clear and valuable
contribution to offer the ‘comfortable home’.
