SA Affordable Housing May - June 2019 // Issue: 76 | Page 22

FEATURES
Thermal insulation material cannot be installed in a cavity after a house has been built and therefore must be installed during the
construction phase.
The thermal performance of all components and systems
(except windows and doors) are expressed in terms of
R-values. For windows and doors performance is expressed
in terms of U-values. Insulation materials are rated for their
performance in restricting heat transfer, which is expressed
as the R-value, also known as thermal resistance. The
R-value is a guide to performance as an insulator – the
higher the R-value, the better the insulation (such as
resistance to heat flow) it provides.
ROOF INSULATION
The highest total thermal resistance for a roof assembly in
order to comply with SANS 10400-XA is 3.7m²K/W. The
minimum added R-value for thermal insulation in this roof
assembly is 3.37m²K/W.
Nowhere does the standard refer to a one size fits all
thickness. Products have different thermal resistance
properties and must be tested in accordance with SANS
8301 ‘Thermal insulation -- Determination of steady-state
thermal resistance and related properties – Heat flow meter
apparatus’, to determine the thermal resistance (R-Value).
Developers and contractors are the biggest culprits as
they do not install insulation in roofs or, if they do, it is way
below the requirements. Because the insulation is in the
roof, no-one checks whether it is there or not and this is a
very big challenge as there is no enforcement and
ultimately it is the client or consumer who loses out on
energy efficiency.
WHAT IS A CAVITY WALL?
A cavity wall consists of two ‘skins’ separated by a hollow
space (cavity). The advantage is that a cavity wall gives
better ‘thermal insulation’ than a solid wall. It is as a result
of the space provided between the two leaves of cavity
walls which is full of air and reduces heat transmission into
the building from outside.
This practice has already been followed in Cape province
as cavity walls are also better for damp prevention than
solid walls. The introduction of cavity walls nationally, is
to satisfy regulatory requirements for building
energy efficiency.
Since 2011 specifiers, contractors, developers and new
build clients are all exposed to the regulatory requirement
for buildings to be efficient in their energy demand and
usage. However, there is a case to be made for insulating
the wall cavity at the time of construction. It makes no
economic sense to build a cavity wall without adding
thermal insulation as thermal insulation material cannot be
installed in a cavity after a house has been built.
WHY INSULATE CAVITY WALLS?
As houses become smaller, the ratio of roof to wall area
decreases, making the walls more relevant in terms of heat
flow in and out. Also, when extra floors are added, the wall
area can far exceed the roof area and uninsulated walls
rising above shading trees are more susceptible to direct
heat impingement, particularly on the north, east and
west sides.
Cavity wall insulation increases the thermal mass effect
of the bricks and implies that the inner masonry leaf is at
internal, rather than external, temperature. When heating or
cooling the home, the insulated walls help to maintain the
desired internal temperature for longer and reduce the time
and cost of achieving interior comfort.
Good thermal insulation such as Expanded Polystyrene
(EPS), Extruded Polystyrene (XPS) and Polyisocyanurate
(PIR) insulation boards can also prevent moisture
transferring through the inner leaf. As the walls are warmer,
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MAY - JUNE 2019
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