Overview
Ground floors fall
predominantly into two categories, ground bearing and suspended.
The type chosen by the designer is largely dependent on the nature
of the building and the site conditions.
Recent changes to the thermal requirements of the Building
Regulations have made it necessary to include insulation in nearly
all ground floors.
The thermal performance of ground floors is determined by a
combination of the thermal resistance of the floor construction and
the insulation provided by the ground. Until the late 1990’s, the
thermal resistance of the uninsulated floor and the ground was very
often sufficient to meet Building Regulation requirements. Recent
changes have required a significant improvement in the thermal
performance of all building elements, but the improvement required
in floors has been the most marked.
To meet these new requirements, floors of large buildings that
previously would not have required insulation, may now require it.
The thickness of insulation required in smaller buildings has also
greatly increased. This has necessarily prompted designers to
reconsider the specification of insulation in ground floors.
Advantages
Both ground bearing and
suspended ground floors can offer excellent thermal performance by
including high levels of insulation without dramatically altering
the building shape or geometry.
Ground bearing floors can include insulation either below or above
the concrete slab, dependent on the choice of the designer. If the
insulation is installed below the slab, this acts as a thermal
store, helping to maintain steady temperatures in the building. If
it is installed above the slab then the building will respond much
more quickly to the heating system.
Suspended floors are usually insulated in such a way that they
offer lower thermal mass than ground bearing slabs and respond
quickly to the heating system. In the case of suspended concrete,
the insulation is installed above the deck, either under a screed
or timber boarding. Suspended timber floors are normally insulated
between the joists.
The specification of floor insulation is of particular importance
if installing under floor heating.
Knauf Insulation Products
- Polyfoam Floorboard is a high
performance, 100% ozone friendly, extruded polystyrene, rigid board
insulation. It is lightweight, yet has excellent structural
strength and long term effectiveness. It is available in three
grades:
Standard– light commercial loading
Extra– commercial, industrial flooring
and cold storage
Super– very high load commercial,
industrial and cold storage floors
- Crown Loft Roll 44 is made from
glass mineral wool and formed into unfaced rolls which are
lightweight, flexible, resilient
and non-combustible
Summary
Knauf Insulation provides products for a range of ground floor
constructions, giving options that will comply with the Building
Regulations.
the building to heat up quickly because the insulation is close to
the inside of the building.
Positioning the insulation below the slab increases the thermal
mass of the building. This results in more even temperatures and a
reduced risk of condensation. This is best suited to buildings that
are continuously heated or benefit from significant solar
gains.
In ground level suspended timber floors above a ventilated airspace
the insulation is normally located between the joists. Timber
floors have little thermal mass and are therefore best suited to
intermittent heating systems.
Building Regulation Requirements
.
Calculation of
U-values
Unlike walls and roofs, the heat loss through a ground floor varies
with its size and shape. The Building Regulations require that when
ground floor U-values are calculated,
BS EN
ISO 13370: 1998
should be used.
The British Standard uses the ratio of the exposed floor perimeter
to the floor area to take account of the variation in heat loss due
to floor size and shape.
The measurement of the perimeter and area should be to the finished
inside surfaces of the perimeter walls that enclose the heated
space. Projecting bays should be included, but unheated spaces such
as porches or garages should be excluded.
In the case of semi-detached and terraced dwellings and blocks of
flats, the floor dimensions can either be taken as those of the
individual dwellings themselves, or of the whole building. When
considering extensions to existing buildings, the floor dimensions
may be taken as those of the complete building including the
extension.
Determining the U-value
The charts below show the thickness of insulation needed to achieve
a U-value of 0.25 W/m2K when insulating a ground
floor.
The charts are based on the ground having a thermal conductivity of
1.5 W/mK. The U-values for Crown Loft Roll assume it is placed
between 50mm wide timber floor joists spaced at 600mm centres.
However, look up tables, particularly for suspended ground floors,
only give a guide to the expected performance. The high number of
variables that have to be taken into account can significantly
affect the U-value for a particular set of conditions. The Knauf
Technical Advisory Centre can carry out the calculations for your
specific requirements.
Condensation
A vapour barrier is not normally required for most ground floor
constructions. However, a vapour barrier should be installed
between the insulation and a chipboard floor, especially if there
is a risk of excessive moisture from the floor slab drying
out.
Thermal Bridging
Thermal bridges are a significant source of heat loss. They may
also cause localised condensation and mould growth. Correct
detailing at the junction of the floor slab and external wall, as
shown in the details
above,
will reduce thermal bridging and thus the risk of
condensation.
Where insulation is placed between timber joists, the joists have
the potential to act as thermal bridges. However, where joists are
at least 150mm deep and the space between them is fully filled with
insulation, the timber does not constitute a thermal bridge.
Air Leakage
With suspended timber ground floors,
care is needed to minimise air leakage from the ventilated sub
floor void into the heated space. As well as gluing the joints
between the floor deck panels, the floor perimeter should be sealed
by applying expanded foam tape under the skirting and a continuous
bead of sealant to the back of the skirting prior to fixing.
British Standards
Designers should consult Agrément Certificate 93/2936 for Polyfoam
Floorboard, or
BS 5669:
Part 2 and
BS 7916 when
selecting chipboard or other floor decking boards.
Guidance on the design and installation of concrete bases and
screeds to receive in-situ flooring is given in
BS 8204: Part 1.
Advice on resilient floor coverings, including recommendations for
suitable bases is given in
BS 8203.
Typical Specification Clauses
1) Ground bearing slab
1a) Insulation under slab
The whole of the ground floor area between brick, block or concrete
subwalls to be insulated with Polyfoam Floorboard
Standard*Extra*/Super* …...mm thick and laid directly over blinded
hardcore. Polyfoam Floorboard, at least 25mm thick, to be cut and
placed vertically against the subwalls to the depth of the concrete
slab. (*delete as required)
The insulation to be laid above*/below* damp proof membrane, which
should lap the perimeter wall dpc. Concrete slab and floor finish
as specified by the designer. (*delete as required)
Alternatively, refer to NBS clause: E20/30 or E20/200
1b) Insulation above slab and below
screed
Polyfoam Floorboard
Standard*Extra*/Super*......mm thick, to be closely butted and
placed over the whole area of the floor. Polyfoam Floorboard, at
least 25mm thick, to be cut and placed to full depth of screed at
the floor perimeter.(*delete as required)
The insulation to be overlaid with 1200 gauge polythene, taken up
and over the perimeter insulation. A 75mm thick sand/cement screed
with wire mesh*/ polypropylene reinforcing fibres* to be laid on
top. Floor finish as specified by the designer. (* delete as
appropriate)
Alternatively, refer to NBS clause: M10/40 or M10/290
1c) Insulation above slab and below
chipboard
The whole area of the concrete floor to
be lined with Polyfoam Floorboard Standard*Extra*/Super*, …...mm
thick. All boards to be close butted.(*delete as required)
The insulation to be (overlaid with a vapour control layer of 1000g
polythene and)* covered with 18mm t&g flooring grade chipboard.
(*delete as required)
Alternatively, refer to NBS clause: K11/25 or K11/225
2) Suspended masonry floor
2a) Insulation below screed
Polyfoam Floorboard Standard*Extra*/Super*....mm thick, to be
closely butted and placed over the whole area of the floor.
Polyfoam Floorboard, at least 25mm thick, to be cut and placed to
full depth of screed at the floor perimeter.(*delete as
required)
The insulation to be overlaid with 1200 gauge polythene, taken up
and over the perimeter insulation. A 75mm thick sand/cement screed
with wire mesh*/ polypropylene reinforcing fibres* to be laid on
top. Floor finish as specified by the designer. (* delete as
appropriate)
Alternatively, refer to NBS clause: M10/40 or M10/290
2b) Insulation below chipboard
The whole area of the concrete floor to
be lined with Polyfoam Floorboard Standard*Extra*/Super* …...mm
thick. All boards to be close butted..(*delete as required)
The insulation to be (overlaid with a vapour control layer of 1000g
polythene and)* covered with 18mm t&g flooring grade chipboard.
(*delete as required)
Alternatively, refer to NBS clause: K11/25 or K11/225
3) Timber suspended floor
Polypropylene netting to be (draped over and between the joists and
stapled to the sides of each joist*/stapled to the underside of the
joists*). (*delete as required)
Crown Loft Roll 44 of ......mm thickness and of width to suit joist
spacings, supported on the netting to fit tightly under the floor.
Crown Loft Roll 44 to be cut and placed to fully fill the gap
between the last joist and the perimeter wall. T&g flooring
grade chipboard to be nailed or screwed to the floor joists, all as
specified by the designer.
Alternatively, refer to NBS clause: P10/240 or P10/250