St Mary the Virgin, Ashford, Kent (Grade I listed) following a major re-ordering
programme including a new
limestone floor with underfloor heating installed in
2010/11 using a dry system developed in co-operation with
Jupiter Heating Systems
Ltd (Photo: Robert Greshoff)
Places of worship come in a wide variety
of shapes and sizes and have evolved
to suit local needs. Generally, however,
they adhere to relatively standard forms of
construction, subject to some regional variation,
and while the following article is based on the
installation of underfloor heating in a typical
Anglican parish church, this guidance is
generally applicable to other places of worship.
Church heating (where it exists at all)
usually comes in two forms. The most common
type of system uses cast iron radiators or large
pipes, often located against the external walls
but less frequently against internal partitions.
Pipes (either elderly cast iron or modern ‘finned’
pipes which release heat more efficiently) are
also commonly found beneath cast iron grilles
which run down the aisles. These are usually
fed by gas boilers in a semi-underground
chamber. Occasionally this is supplemented
by the other widely used system, electric
radiant heating coils fixed beneath the pews
or on the walls. These are often used as a
primary heat source in smaller churches.
Old churches are seldom insulated and
often have large expanses of single glazing.
Some heat radiates from the appliances but
reaches only a small proportion of the floor area,
while the rest circulates through convection
currents, and most of the benefit is lost as it
cools in the upper voids of the building. Much
of the perceived warmth tends to come from
the body heat of the people in the congregation,
who are usually dressed in outdoor clothing.
The 21st-century parish church has often
lost its nearby meeting room and in order to
best provide for the needs of its community,
the church building must be adapted to suit
new functions, such as a meeting space,
concert or arts venue, as well as providing
facilities for youth groups. One option is to
replace the pews in all or part of the building
with more flexible seating. Although this
solution is unthinkable in some fine historic
interiors, in many lesser buildings it provides
a once-in-a-lifetime chance to resolve a
number of building problems in one go.
|Excavation to substrate level
|Foamed glass on geotextile membrane
Underfloor heating is usually considered
in churches where there is a need for an
uninterrupted expanse of flooring and a
desire to reduce energy consumption. With
a low surface temperature of around 29°C,
the primary advantage of an underfloor
heating system in a church is that heat is
provided evenly across a broad area, enabling
an ambient temperature of around 18°C to
be maintained up to two metres above the
floor and emanating from directly beneath
the congregation. This means that the
visitors readily perceive warmth on entering
the building and can shed their coats.
Conventional radiators also have the
disadvantage of drawing moisture through
the fabric due to the increased evaporation
that occurs from the masonry to which the
radiator is fixed. If the underfloor heating
can be used alone in a particular area, this
problem is avoided. However, the time taken
for the floor to reach optimum temperature
on a cold Sunday morning will never be as
rapid as the appliances in a conventional
central heating system, so a few radiators are
sometimes retained to provide local pockets
of warmth while the building warms up.
Considerable fuel savings can be achieved
by installing a dual-fuel system where a
ground source heat pump (GSHP) can run the
underfloor heating, and gas, where available, can
heat the radiators. GSHP is a viable source of
energy even at sites with sensitive archaeology
as it is now possible to extract heat from ground
below the archaeological threshold by using
radial boreholes, avoiding long trenches for
pipes in the churchyard. The possibility of GSHP
as a heat source should be considered early on,
however, as the water temperature is likely to be
lower and the spacing of the underfloor heating
pipes needs to be designed with this in mind.
Underfloor heating can also work with
solar thermal energy as a source, provided
there is space for a large thermal store of
warm water, but there are almost always
difficulties (ethical and aesthetic rather than
practical) associated with installing solar
panels on the roof of a historic church.
Insulation can also be installed under the
floor at the same time as underfloor heating
and, in a historic building, this may be the
only place where it can be introduced.
There are many underfloor heating systems on
the market and each needs to be considered
with the substrate that goes underneath it
and the desired floor finish. The common
types of system are described below.
Conventional screed Underfloor heating
may be incorporated in a conventional screed,
on either suspended beam and block or a solid
concrete slab. Properly designed, this provides
a firm and level base for a new floor finish
and can take high imposed loads. However,
generally the heating is slow to respond and
the structure as a whole is deep and more
expensive to construct, and installation is very permanent. However, the screed provides
considerable thermal mass, which means
the water flow temperature can be relatively
low and during the heating season the
heating does not have to be on continually.
Where lime concrete or ‘limecrete’ is used,
the system can have the advantage of being
breathable and flexible and with pre-heating
of the materials it can be installed at any time
of year. However, the drying time is lengthy,
extending the period the building cannot be
used and increasing the contractor’s cost.
Both types of screed also involve
putting a large quantity of wet material into
a building that may be historically very dry.
In historic buildings such a sudden increase
in humidity can harm ancient fabric.
Thin screed Underfloor heating can be also
be incorporated in a thin (15-25mm) anhydrite
screed and stainless steel matrix, laid on the
same sort of substrate used for a conventional
screed. Because an anhydrite (calcium sulphate)
binder is used instead of cement, the overall
construction thickness is reduced, requiring less
water to be introduced and reducing its drying
time. Being thinner, the slab also heats up
more quickly, although many pipes are needed
as they are smaller than in other systems.
With either screed form it is
usually necessary to allow for carefully
positioned movement joints and these
may be visible in the floor surface.
Insulated tray (dry systems) Some systems
use a pre-formed insulated tray with recesses
to take the hot water pipes. This system has
been widely used with timber flooring, which
enables the screed to be omitted. However,
one manufacturer has introduced a system of
tongue and groove glued terracotta tiles which
take the place of the screed and enables stone
or clay tiles to be laid on top. This can form part
of a completely dry system as described below.
If timber boards are used, it is essential that
they are engineered to be suitable for underfloor
heating and that a probe is incorporated into the
construction that will cut the heat source when
the underside of the boards reaches 27 degrees,
otherwise the boards will cup and distort.
DRY SYSTEM INSULATION
||Insulated heating trays being laid
||Tongue and groove terracotta tiles being
The layered, dry system which has recently
come into use is as follows, starting
from the bottom of the excavation:
1. Old pew platforms, terracotta aisle tiles and
substrate are removed. Any memorial slabs
are carefully recorded and set aside for later
reinstatement. The substrate is probed with
iron rods to locate any voids (alternatively,
if there is time, a ground-penetrating radar
survey can be conducted and the results
interpreted). The lime concrete ‘crust’
commonly found below pew platforms is
then excavated to around 350mm below
finished floor level, which is often the
depth of the void found beneath the pew
platforms. Where sensitive material is
found, excavation must be carried out with
the utmost care, by hand.
2. Geotextile membrane is then laid
on the substrate. The membrane is a
woven artificial fabric which separates
different types of aggregate material
while allowing moisture to pass through.
150mm of recycled foamed glass is then
laid and gently compacted. A polythene
damp-proof membrane is laid on top,
followed by 30mm of crushed slate,
which must be carefully levelled.
3. Next, 20mm tongue and groove gypsum
boards are laid on the slate, with
audiovisual cabling ductwork laid on top
of them and the interstices filled with a
high-density rigid extruded polystyrene
4. 30mm expanded polystyrene moulded
heating pipe tray with aluminium diffusers
and hot water pipes are then installed and
overlaid with a separating membrane.
5. Finally, a proprietary system of glued
tongue and groove terracotta tiles, 20mm
thick, are laid and then overlaid with the
chosen floor finish. If desired, an audio
induction loop can be laid in the bedding of
the floor finish.
ADVANTAGES OF THE DRY SYSTEM
The dry system has many advantages but
its success relies on a proper understanding
of the site and careful detailing. In a
medieval church being adapted in line with
typical current patterns of use there will
be certain performance criteria which the
dry system can usually meet without risk
to the floor finish. The key performance
criteria for dry systems are set out below.
Loading The new floor should be able
to support mobile and stationary loads.
Open spaces should enable high-level
lighting and redecoration to be carried out
from a cherry picker. Generally, a mass of
about 500kg per wheel, moving on relatively
soft tyres on 25mm temporary plywood
sheeting can be safely supported.
Electrical services A new power and
audiovisual system is often required as part
of the re-ordering. Ductwork and access panels can readily be included in the dry
system but detailing is crucial to allow for
proper ventilation where required plus
considerable capacity for future-proofing
and maintenance. Access panels should
allow sufficient depth for the long plugs and
terminals needed for audiovisual equipment.
The new floor should also be able to
incorporate a hearing loop if required.
A type of loop known as a ‘phased array’ has
been found to work well even though the
dry system incorporates aluminium cored
pipework and radiant heating plates.
Drainage and ventilation The advantage
given by the absence of wet trades should
not be negated by lack of control of any
groundwater either below the floor or in the
churchyard outside, which is often at a higher
level than the floor. The new floor will not
be fully breathable and so the junction with
walls and stone columns must allow for the
dissipation of moisture (see diagram overleaf).
All external drainage systems should also be
checked, cleared and, where necessary repaired
or renewed. Where the ground level is higher
than the interior floor level, consideration
should also be given to the introduction of
French drains – a system of drained trenches
which are constructed at the base of the exterior
wall to a depth below the interior floor level, and
backfilled with a free-draining aggregate. The
aim is to reduce the pressure of ground water
by ensuring that the perimeter is well drained.
Protecting and recording historic fabric The creation of a new floor will, inevitably
mean disturbance of historic fabric but, on
completion of the works, this material will at
least not be encased in concrete. Depending
upon the known history of the church, the
implementation of a dry system will mean either
a full archaeological investigation or a watching
brief following trial pits and a desk study.
The dry system means that the depth
of excavation can be kept to an absolute
minimum but even with the most detailed
research beforehand there are almost always going to be unexpected features below
ground, from vaults and early foundations
to Victorian gas lighting and heating
systems and, of course, human remains.
Features that are to be retained, such as
fragile brick vaults with shallow structural
arches, are often far enough below the floor
system to allow them to be spanned dry with
pre-cast concrete lintels but sometimes they
are too high and localised concrete caps
may be required, all subject to agreement
with the county archaeologist and designed
by an experienced structural engineer.
The health hazards associated with
old burial vaults and ruptured lead
coffins must be taken very seriously.
Minimum disruption The nature of the
system and the speed of construction means
that a re-ordering programme can be carried
out with the minimum of disruption to church
activities. With the replacement of the floors
to nave and aisles in a large parish church,
for example, once the church committee
volunteers have taken out the pews and loose
furniture, the stripping out can commence
immediately after the Christmas break and
the church can be ready for use by mid June.
|Typical new stone floor/existing wall junction detail incorporating perimeter duct for power/audiovisual cabling (Image: Lee-Evans Partnership)
DISADVANTAGES OF THE DRY SYSTEM
Few disadvantages have been identified
with the dry system to date but the
primary ones are explored below.
Floor levels To date, the system has
relied on being laid to total flatness where
large areas are being installed because of the
practical difficulties of laying the crushed slate
substrate to very shallow falls. Small areas of
ramp are perfectly feasible, however. While
locally formed ramps can readily be included,
a suitable finished floor level must therefore be
agreed which addresses adjacent floor levels in
the optimum manner. Existing church floors
are seldom level and in a large parish church
there may be a difference of up to 100mm
between one end of the nave and the other.
Future floor fixings Allowance must
be made for any future holes in the floor
(for doorstops, staff or handrail sockets for
example) by putting solid timber blocks in
the underfloor heating layers and recording
their positions accurately. Although there
are specialist tools which can cut the upper
layers without disturbing the pipes below,
this is not without risk and the consequences
of a hole in a pipe would be very serious.
|Typical manifold installation
‘Tick-over’ temperature The thermal
response time is very rapid compared to other
types of underfloor heating but as there is
little dense material to provide any kind of
thermal mass, the flow temperature of the
water should be higher than that required
for a conventional screed. During the cooler
months the heating system needs to be
kept ‘ticking over’ continuously at about 12
degrees so that the heating can quickly be
brought up to the desired temperature.
Housing the manifold(s) A home will
need to be found for the manifold(s) – the
multiple pipe union(s) where single feed and
return pipes are divided into several circuits of
piped warm water. Manifolds must be located
in easily accessible and well-ventilated places.
On balance, dry systems offer perhaps
the safest and most practical method of
introducing underfloor heating into a church.
When combined with the introduction
of more comfortable and flexible seating,
underfloor heating has obvious attractions for
the congregation, and can help to ensure the
viability of an underused church. However, not
all older churches will be able to benefit from
this approach, particularly where their existing
flooring, fittings and finishes are deemed too
significant to change. The effects of fluctuations
in temperature on ancient and fragile fabric also
need to be taken into account. Although more
easily renewed than a screed, the introduction
of a dry system of underfloor heating is not
reversible and the loss of any historic fabric
always requires careful consideration. In
principle and in practice, each case is unique.
British Standard BS EN 12058: 2004, Natural
stone products. Slabs for floors and stairs.
Requirements, BSI, 2005
British Standard BS EN 1264-5:2008, Water based
surface embedded heating and cooling systems.
Heating and cooling surfaces embedded in
floors, ceilings and walls. Determination of the
thermal output, BSI, 2009
Jupiter Heating Systems Ltd
Stone Federation Great Britain
Underfloor Heating Manufacturers’ Association