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Building Obsolescence and Revitalisation 555
fittings and furnishings introduced into the building after completion. Sealing a building
to prevent air infiltration (and unnecessary heat loss) makes it necessary to introduce
controlled airflow – either by natural or mechanical means – to create air changes and
hence remove stale air from within the building. Air changes allow the removal of gases
and moisture, particulates and other airbourne contaminants, such as dust, mineral fibres
and allergenic substances. This helps to prevent surface and interstitial condensation and
contributes to a healthy internal environment.
Surface condensation
Surface condensation occurs when air becomes saturated (100% relative humidity), resulting
in water droplets forming on cold impermeable surfaces, such as glass, ceramic tiles
and metal. Left unchecked, this will lead to mould growth, the risk of corrosion, and
damage to textiles and other materials. Improving ventilation when cooking, drying
clothes and bathing – such as opening a window and/or switching on an extract fan – can
help to reduce the relative humidity of the air and hence reduce the risk of surface
condensation.
Interstitial condensation
Interstitial condensation forms within the building fabric, for example, within a wall or a
roof. As the water laden air passes through the permeable fabric (e.g. plaster, blocks and
bricks), it will move from warm air to cooler air. As the air cools, its capacity to hold
moisture is reduced and 100% relative humidity is reached at the dew point. This is where
condensation forms. Interstitial condensation can occur if the building fabric has not been
designed correctly or constructed precisely. Over time the condensation will cause timber
to rot and metal to corrode, resulting in structural damage. Unlike surface condensation
interstitial condensation cannot be seen without opening up the building fabric; thus it is
a hidden problem until such time as the damage becomes evident in some form of visible
damage.
11.5 Revitalising existing buildings
Recent legislation and attention to the environmental impact of buildings has helped to
emphasise the importance of building durability and adaptability. Reuse of our existing
building stock is often desirable for environmental, cultural, economic and social reasons.
Only around 1% of the existing building stock turns over each year, so our attention should
be directed to improving the performance and durability of our existing buildings. This is
particularly relevant to reducing the carbon footprint of existing buildings. Photograph
11.3 shows a domestic house that was upgraded and extended to better suit the needs of a
growing family and also to improve the thermal performance of the fabric. The finished
result is shown in Photograph 11.4, where new cement render has been applied to the front
of the property to tie in the new work and enhance the visual appeal of the property. With
the exception of new double glazed windows, the thermal upgrading (cavity and roof
insulation) is not visible to the occupants. However, the improvement in the thermal insulation
values is reflected in significant reduction in heating costs.