T W E N T Y F I R S T E D I T I O N
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INTERIORS
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adequately drained, and the room above is
adequately ventilated, a permeable ground floor
can allow equilibrium to be reached between
the ground and the air above, reducing the
chances of concentrated damp areas which can
cause the building’s fabric to deteriorate.
Traditional earth floors
Due to their density and low thermal
conductivity, earth floors which are dry are
thermally efficient, capable of absorbing heat
and releasing it gradually over time – the
thermal ‘flywheel’ effect. English Heritage,
in its guidance note Energy Efficiency and
Historic Buildings: Insulating solid ground
floors, observes that ‘the ground itself
maintains a surprisingly stable temperature
of around 10°C’. For these reasons, earth
floors are among the most sustainable forms
of flooring, and there has been a revival of
this type of floor construction for domestic
situations by the environmentally-conscious.
However, from a commercial perspective,
laying an earth floor is extremely labour-
intensive and time- consuming, and has long-
term maintenance implications. Earth floors
do not always tolerate load points and in damp
climates like the UK, managing groundwater
is an issue.
Modern earth floors
Preventing ground water penetration is best
achieved by incorporating some form of
‘capillary break’. This can be an impermeable
damp-proof membrane (DPM) as is common
in the US and would usually include petro-
chemical based insulation. An arguably better
approach is to use foam glass or expanded
clay aggregate insulation as a base. Made from
recycled glass or clay, these are in many ways
more sustainable than an insulation based on
petro-chemicals, despite the energy used in
reprocessing them. (Particular environmental
concerns raised by insulation materials like
polystyrene include the environmental impact
of the gases used to foam them and of the
waste material generated.) Furthermore, due
to the open pore structure of an insulation
layer created with these aggregates, there is far
less capillary attraction to draw moisture into
the base of the walls.
Modern concrete floors
The modern approach of making floors
impermeable with a DPM and concrete is
designed to work with a building of modern
design and construction which incorporates
a damp proof course in the walls. A modern
floor design with a DPM and concrete may
work well in a traditional building too if the
groundwater is minimal, but all too often
the capillary effect of the substrate draws
groundwater to the underside of the floor
slab where it accumulates. However, as the
surrounding footings remain porous, they act
like a wick, drawing the water up. Evaporation
concentrates at the base of the wall, just above
ground level. This can lead to excess moisture
at the junction of the wall and floor, and
mineral salts can be drawn into the structure.
This excess moisture can cause simple
discolouration, failure of decoration or plaster
or degradation of structural timbers or earth
walls. Salt crystallisation in particular can be
highly damaging to some types of masonry.
CONSERVATION OF TRADITIONAL
BREATHABLE FLOORS
William Morris, one of the founders of
the Society for the Protection of Ancient
Buildings, said: ‘We are only trustees for
those that come after us’. It is
important to
remember this when making alterations to
historic buildings. Even if the building is not
listed, is it right to remove flagstones or a lime-
ash floor that have been part of the building
for centuries? Will this damage the character
that an interior acquires with age, or, worse
still, the structural integrity of the building?
Homes and workplaces need to be
functional spaces where people can live and
work comfortably. Increasingly, they also need
to be energy efficient, satisfying the ever-
growing demand for carbon reduction while
retaining their distinctive character. It is a
difficult juggling act but it is very satisfying if
it can be mastered. Inevitably, compromises
will have to be made by owners, conservation
officers or building control inspectors, and
sometimes by all three.
If the building has an original feature
floor or one that is historically significant, is
it practical to retain it? Even if the floor is not
perfect, can its idiosyncrasies be tolerated?
A little undulation here and there or higher
moisture level at times may be minor issues.
Natural oils and waxes were often used
to protect floors and to help keep them
dust-free, while retaining most of their
permeability. We should maintain and repair
our traditional buildings with sympathetic,
like-for-like materials. If this approach
is deviated from, careful thought should
first be given to the likely consequences
and whether they can be justified.
If repairs are necessary, try to identify the
materials that were used originally and seek
to source these from specialist suppliers. This
need not be hugely expensive and can save
money in the longer term as these materials
will be compatible.
If a different floor covering has been
specified, is it possible to lay it and still
retain the original floor intact underneath?
A protective geotextile membrane laid on the
existing floor before a new covering is laid may
be a good option.
THE LIMECRETE OPTION
Where a new floor slab is to be created,
one option is to use a slab based on lime
(‘limecrete’) which is breathable, rather than
one based on cement (concrete), which is less
permeable. A limecrete floor can be designed
to meet modern insulation requirements and
can incorporate under-floor heating (UFH).
It may be possible in some cases to re-lay
the original surface on top of the new slab
if desired, although this can be difficult to
achieve successfully and requires a methodical
approach if the character of the floor is not to
be altered.
More usually the ground is first excavated
and levelled, then a breathable geotextile
membrane is laid and the loose-lay insulation
is added followed by another layer of geotextile
membrane. The limecrete slab is then cast
using a mix containing aggregate and
hydraulic lime (or lime and a pozzolan). This is
usually left for around three weeks before the
UFH pipes are fitted and covered with a lime
mortar mix or flags are laid.
The two main materials currently used
for loose-lay insulation and capillary break
are expanded clay aggregate, and recycled
glass foam granulate. A typical expanded clay
aggregate has a lambda value of 0.1 w/mk.
(Lambda values refer to thermal conductivity,
measured in watts per square metre of surface
area: the lower the value, the better the
thermal efficiency.) Glass foam granulate is a
is a more expensive option but it has a lambda
value of around 0.075–0.08 W m
-1
K
-1
so is
around 20 per cent more thermally efficient,
and it is more structurally stable as it requires
compaction.
It may even be considered appropriate
to opt for a combination of impermeable and
permeable flooring. A hybrid floor is an option
where a modern floor is constructed in the
centre of the room with a trench between the
concrete floor and the surrounding walls (see
diagram). This trench is filled with a build-
up of limecrete to a depth suitable for the
foundations. This provides the capillary break
required, allowing the substrate to breathe
and preventing the footings from drawing
trapped moisture into the walls. This type of
floor is becoming increasingly popular. The
reasons for choosing this option would be to
get the necessary u-values associated with
Building Regulations and underfloor heating
without increasing the risk to the integrity of
the structure.
If you choose to create a modern floor
with a slab cast over conventional insulation
bats, it is important to choose the insulation
carefully. There have been reports of some
types of insulation compressing over a number
of years, allowing the floor to drop and
reducing its insulation value. Make enquiries
on the potential long-term performance of
your chosen materials before fitting.
A limecrete slab being layed over a geotextile
separation layer with clay aggregate below: the
result is a solid floor that ‘breathes’ across its entire
surface. (Photo: David Blair)
