THREE WAYS TO INSULATE CONCRETE SLAB EDGES
BUILD
RIGHT
BY ALIDE ELKINK,
FREELANCE TECHNICAL
WRITER, WELLINGTON
Concrete slab edge
insulation
ALTHOUGH THE BUILDING CODE MAY NOT REQUIRE CONCRETE FLOOR
SLABS TO BE INSULATED, BRANZ CONSIDERS THEY SHOULD BE. WE
LOOK AT THE VARIOUS WAYS TO DO THIS.
140 mm insulated
wall framing
bottom plate on DPC
framing (insulation
omitted for clarity)
timber thermal break
H3.2 140 × 45 mm timber insert
cladding
reinforced concrete floor slab
ground level
polystyrene insulation
DPM on sand blinding
and hardfill
120 mm
165 mm
Figure 1
Floor slab insulation – single storey.
Clause H1 requirements
WHEN A CONCRETE FLOOR SLAB is insulated,
insulating slab edges is far more effective for
the insulation is typically laid under the entire slab.
preventing heat loss than insulating the underside
Building Code clause H1 Energy efficiency requires
However, modelling has shown that approximate-
of the slab.
a minimum R-value of R1.3 for floors, which is
ly 80% of the heat lost from a concrete floor slab
achieved by an uninsulated concrete slab on
is at the edge and that the thermal resistance
Slab edge heat loss ratio
round. Under-slab insulation is only required by
does not significantly improve by fully extending
The amount of heat loss from the slab edges
clause H1 when the floor is heated and an R-value
the insulation under the slab or by increasing
depends on the ratio of area (A) to perimeter
of 1.9 is needed.
the thickness of the insulation under the slab
length (P) of the slab. The higher the A/P ratio,
The BRANZ House insulation guide provides
(see Build 109, pages 28–29). This suggests that
the greater the thermal resistance of the slab.
R-values for uninsulated concrete floor slabs with
24 — Build 134 — February/March 2013
different A/P ratios. For example, a floor slab with
a foundation width of 150 mm or more and an A/P
wall plate
ratio of 2.5 (for example, a 10 × 10 m slab where
A/P (100/40) = 2.5), achieves the minimum floor
concrete floor slab
R-value of R1.3 required by clause H1.
DPM under
slab insulation
and footing
Insulation is better
Regardless of the Building Code requirements,
installing perimeter under-slab and edge slab
insulation in all concrete floors is good practice.
ground
The two most commonly used options for slab
edge insulation are:
●●
a thermal break between the slab and the
perimeter footing (see Figure 1)
●●
insulation to exterior
face of footing
insulation to the exterior face of the footing
sand blinding and hardfill
(see Figure 2).
Both options have some disadvantages, and
continuity of insulation is essential to avoid
thermal, or cold, bridging.
Adding thermal break between slab
and footing
protective layer
over insulation
Figure 2
under-slab insulation
Insulation to exterior face of footing.
A thermal break between slab and footing can
be achieved by inserting timber or polystyrene
between the slab and the perimeter footing.
Although polystyrene is a better insulating material than timber, it is vulnerable to point
timber thermal break
DPM
loads on the floor unless it is protected.
slot cut into
timber and
notched over
reinforcing
– fill with
expanding
foam
For all construction, the floor slab reinforcing
must be tied to the footing reinforcing.
Beating the thermal bridges
ground
The gaps where the thermal break is notched
over the reinforcing provide a pathway or thermal
bridge where heat loss can occur. To reduce the
concrete floor slab
effect of thermal bridging, cut slots into the insulation to fit over the reinforcing and fill the slots
with expanding foam (see Figure 3).
Heat loss may also occur between the wall
under-slab
insulation on DPM
plate and the edge of the floor (see Figure 4). In
single-storey construction with 90 mm framing, a
165 mm wide footing and a 45 mm thick thermal
reinforced foundation footing
slab reinforcing
tied to footing
reinforcing
break (see Figure 4a), there will be a 30 mm wide
thermal bridge between the wall plate and the
sand blinding and hardfill edge of the floor.
Similarly, if 140 mm framing is used in twostorey construction with a 200 mm thick
Figure 3
Thermal break slotted and notched over reinforcing.
footing (see Figure 4b), there will be a 15 mm
Build 134 — February/March 2013 — 25
90 mm
wall plate
on DPC
30 mm thermal bridge
15 mm thermal bridge
140 mm wall
plate on DPC
140 mm wall
plate on DPC
heat flow out
20 mm overlap
timber thermal break
heat flow out
ground
footing
120 mm
45 mm
155 mm
4(a) Single storey (with thermal bridge)
45 mm
200 mm
165 mm
Figure 4
concrete floor slab
under-slab
insulation on DPM
4(b) Two storey (with thermal bridge)
120 mm
45 mm
sand blinding
and hardfill
165 mm
4(c) Single storey (no thermal bridge)
Heat loss between wall plate and edge of floor.
thermal bridge between wall plate and the
edge of the floor.
To protect the polystyrene from UV exposure
factured from EPS polystyrene, which means
and physical damage, it should be covered
the R-value may be compromised by the water
The thermal bridges can be eliminated
with an acrylic or cement-based plaster finish,
uptake in the polystyrene. The external face of
by using 140 mm framing for single-storey
low-absorbency ceramic tiles or extruded uPVC
the footings must also be protected from UV
construction (see Figure 4c) or 190 mm framing
sheet. (Note that fibre-cement sheet should
exposure and physical damage.
for two-storey construction.
not be used in situations where it is continually
Getting the required edge distances for the
wet.) If necessary, additional protection should
bottom plate fixings can be difficult with EPS
Insulating the footing’s exterior face
be provided by fitting a purpose-made durable
foundation walls.
Alternatively, insulating the exterior face of
Z flashing under the cladding and over the
the footing eliminates the problem of thermal
insulation.
bridging. However, polystyrene is vulnerable to
Secondary slab
A third option to address the issue of slab edge
insulation, which is common overseas but seldom
polystyrene (EPS) can also absorb some water,
Insulating concrete formwork or
polystyrene
reducing its insulation R-value.
Another option for slab edge insulation is to
structural slab, then place a 75 mm thick topping
construct footings using insulating concrete
slab over the insulation. This readily achieves a
formwork or polystyrene blocks.
complete thermal break between topping slab
UV exposure and physical damage, and expanded
Extruded polystyrene (XPS) should always be
used for below-ground insulation. It’s denser than
EPS and therefore more water resistant and has a
better level of thermal performance.
26 — Build 134 — February/March 2013
The principal disadvantage of polystyrene
block footings is that the blocks are manu­
used in New Zealand, is to lay insulation over the
and structural slab.