Building characteristics
Whatever the climate, a building, such as a house, can be thought of as a mixture
of mass materials (e.g. brick wall, mud brick vault, concrete slab) and insulation
materials (e.g. fibreglass batts in the wall, thatched roof, expanded polystyrene
sandwich cladding panel). How a building performs depends upon the mixture of these materials, but it can be simply thought of in the following way. For the cave
dweller, who lived in a mass building with no insulation, the internal temperature
would settle at the annual average temperature. ForNew Zealand that was 13.1C
in 2005, the fourth highest such annual average on record1 and in the UK it is
8.5–11C2. This explains the old adage that a house with thick stone walls always
felt warm in winter and cool in summer, since that was what it was relative to the
outside temperature, even if the actual indoor air temperature around 10C or
even 13C did not represent comfort. Thus, having a lot of mass in a building
means the internal temperature will tend to be stable. The presence of insulation in
combination with mass will tend to raise the stable internal temperature above the
average annual temperature. The experience with Hockerton houses in the UK,
which have no space heating system apart from the gains from solar energy, the
occupants and the equipment inside, suggests that a 7–8C temperature rise above
the annual average can be achieved with a very high mass construction with
300mm of insulation to walls, roof and floor, and with the best available offthe-
shelf windows. The latter consisted of plantation-grown softwood frames with
triple-glazed units, along with krypton gas filling and low-emissivity coatings on
two of the glass layers (Vale and Vale 2000: pp. 187–194).
Conversely, the temperature inside a house that has minimal mass and insulation
will follow the outside temperature unless energy is put into the house in the form
of sunlight, or from the people and equipment housed in it. Temperatures over a
day are lowest in the night and highest around midday. In a lightweight house,
insulation will lift the internal temperature above the outside temperature.
However, the temperature in the house will still go up and down, following the
track of the outside temperature but a number of degrees above it. The level of
insulation will determine how much the temperature inside is lifted above that
outside. For an unheated house in New Zealand with 150mm of insulation in
walls and floor, and 200mm in the roof, the temperature was lifted about 7C
above that outside when the outside temperature was at its lowest. This meant the
minimum indoor temperature recorded in a bedroom was 14C (Vale and Vale
2001). The windows in this instance were double-glazed with one low-emissivity
coating in aluminium frames with no thermal break.
Although it is true to say that in New Zealand the majority of houses are of the
lightweight model, in many countries houses are a mixture of mass and lightweight
materials, often having masonry walls (mass), concrete slab ground floor
(mass), timber joisted upper floor (lightweight) and a timber frame roof (lightweight),
and hence their characteristic performance, if they are unheated, will
also be somewhere between the two extremes. It is important to have a basic
understanding of how buildings might behave. This is because it may fall to the
user to attempt to correct any shortcomings in the original design at points of major refurbishment in the building lifetime. During any refurbishment it is
unlikely that mass will be added to the building but it is often possible to add
insulation. However, before discussing this subject further, it is also necessary to
consider the behaviour of small buildings in hot climates and the behaviour of
large buildings.
In a hot climate, whether hot wet or hot dry, the aim is usually to keep the
building cooler than outside, although in some desert climates where the nights
are cold it is also desirable at times to try to raise the inside temperature above
that outside. From this it can be seen that in the hot dry desert climate with a
large swing in temperature between night and day, the very high mass building
is a good solution as it will maintain the annual average temperature. For
example, the mean annual temperature in Egypt is 20–25C3, which would
provide a good comfort temperature in a building. The classic high mass
building for this climate was made of mud brick, had few openings to keep
out the sun and formed part of a cluster of buildings to keep as much exterior
surface as possible shaded from exposure to direct sunlight. In a hot, humid
climate the temperature swing day and night and summer to winter is often less,
so there is less need of the tempering effect of mass, and the traditional building
was often lightweight, and open as much as possible to any cooling breezes to
help keep the occupants comfortable. In all hot climates the roof is an important
element in keeping out the sun. Often, ventilation paths would be open
under the roof in order to keep air flowing over its underside, with the aim of
channelling away any heat coming through. Roofs would also be insulated
against heat gain. In all warm climates a light-coloured roof is also an advantage
to reduce the solar gain into the building.
A small building is dominated by the performance of the surface, walls, roof and
floor, as the volume of space enclosed is relatively small. However, a large
building has less surface area for the volume enclosed, so its thermal performance
tends to be dominated by what happens in it – the gains from people and activities
– rather than by the skin, although large areas of glass cladding exposed to the
sun will have an effect on internal performance. Large buildings, with one
exception, also differ because they tend to be used during the working day and
so there is no necessity to maintain comfortable conditions during the night, the
time of lowest external temperatures in climates that need heating. The exception
is the apartment block, which, especially in Asia, is becoming the norm. The
improved performance of such buildings lies more with the designers and constructors
as the improvements to the life cycle impact that can be made by the
users are limited. Because this chapter is about the effect of the building user, the
remainder of the discussion will be centred on the home and the small-scale
building.
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