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Floor Insulation: Insulating solid ground floors below ground bearing slab

Sand blinding and hardware

Radon barrier / DPM

Quinn Therm QF PIR Insulation Boards

Quinn Therm edge insulation

Concrete slab on separating layer

Timber flooring on polyethylene slip sheet

  • Sand blinding and hardware

  • Radon barrier / DPM

  • Quinn Therm QF PIR Insulation Boards

  • Quinn Therm edge insulation

  • Concrete slab on separating layer

  • Timber flooring on polyethylene slip sheet

Insulating a solid ground floor below the ground bearing concrete slab can be done using either PIR insulation boards or EPS insulation boards. EPS can be a cost-effective alternative to PIR for floor insulation, and the same thermal performance can be achieved with increased thickness.

Using insulation below the slab helps maintain steady internal temperatures in the building by bringing the ground bearing concrete slab into the insulated envelope of the building making use of the concrete thermal mass.

How to install floor insulation below the concrete slab

1. Once foundations are built to DPC level, use hardcore and sand blinding to achieve a level surface.

2. Lay the appropriate radon barrier / DPM (damp proof membrane). Where radon control measures are required, the specified DPM should be gas resistant. Ensure the DPM is well lapped and folded, and extends to the outer faces of any external walls to prevent any possible moisture ingress from below.

3. Floor insulation boards (Quinn Therm QF PIR insulation boards or Quinn Lite Pac EPS insulation boards) are laid on top of the DPM in a break bond pattern, and are butted tightly together at the joints to ensure a continuous layer across the entire floor. Ensure insulation boards are cut neatly around services to minimise thermal bridging heat loss.

4.
For perimeter edge insulation fit a continuous vertical strip of Quinn Therm QF insulation boards at least 25mm thick around the floor perimeter against the walls to avoid heat loss through thermal bridging. This insulation strip should be the full depth of the concrete slab, and the top edge should be level with the top of the floor screed. Ensure this top edge is protected from any damage until covered by skirting or wall finish.

5. Overlay the insulation boards with a polyethylene slip sheet to act as a vapour control layer to prevent condensation forming below the insulation, and this will also act as a break between the concrete slab and insulation board which prevents the poured concrete from filtering between the joints of the boards.

6. Pour the concrete slab and leave a level finish ensuring your floor insulation boards are fully protected.

Required insulation thickness of Quinn Therm QF:

Perimeter / Area ratio
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Thickness (mm) U-value (W/m2K)
50 0.15 0.21 0.24 - - - - - - -
60 0.14 0.19 0.21 0.23 0.24 0.25 - - - -
70 0.13 0.17 0.19 0.21 0.22 0.23 0.23 0.24 0.24 0.24
75 0.12 0.16 0.18 0.20 0.21 0.21 0.22 0.22 0.23 0.23
80 0.12 0.16 0.18 0.19 0.20 0.21 0.21 0.21 0.22 0.22
90 0.12 0.15 0.16 0.18 0.18 0.19 0.19 0.19 0.20 0.20
100 0.11 0.14 0.15 0.16 0.17 0.17 0.18 0.18 0.18 0.18
110 0.10 0.13 0.14 0.15 0.16 0.16 0.16 0.16 0.17 0.17
120 0.10 0.12 0.13 0.14 0.15 0.15 0.15 0.15 0.15 0.16
125 0.10 0.12 0.13 0.14 0.14 0.14 0.15 0.15 0.15 0.15
140 0.09 0.11 0.12 0.13 0.13 0.13 0.13 0.13 0.14 0.14
150 0.09 0.10 0.11 0.12 0.12 0.12 0.13 0.13 0.13 0.13
160 0.08 0.10 0.11 0.11 0.12 0.12 0.12 0.12 0.12 0.12
180 0.08 0.09 0.10 0.10 0.10 0.11 0.11 0.11 0.11 0.11
200 0.07 0.08 0.09 0.09 0.10 0.10 0.10 0.10 0.10 0.10

U-value results above based upon floor construction shown:

Radon barrier / DPM; Quinn Therm QF; 150mm concrete; timber flooring (floor finish omitted); 150mm depth of Quinn Therm QF vertical edge insulation. Calculations performed to BS EN ISO 6946 and BS EN ISO 13370, taking account of repeating thermal bridges.

Required insulation thickness of Quinn Lite PAC EPS Pearl:

Perimeter / Area ratio

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Thickness (mm)
75 0.14 0.20 0.23 0.25 0.26 0.27 0.28 0.28 0.29 0.29
80 0.14 0.19 0.22 0.24 0.25 0.26 0.27 0.27 0.27 0.28
90 0.13 0.18 0.20 0.22 0.23 0.24 0.24 0.25 0.25 0.26
100 0.13 0.17 0.19 0.20 0.21 0.22 0.23 0.23 0.23 0.24
110 0.12 0.16 0.18 0.19 0.20 0.21 0.21 0.22 0.22 0.22
120 0.12 0.15 0.17 0.18 0.19 0.20 0.20 0.20 0.20 0.20
130 0.11 0.14 0.16 0.17 0.18 0.18 0.19 0.19 0.19 0.19
140 0.11 0.14 0.15 0.16 0.17 0.17 0.18 0.18 0.18 0.18
150 0.10 0.13 0.15 0.15 0.16 0.16 0.17 0.17 0.17 0.17
160 0.10 0.13 0.14 0.15 0.15 0.15 0.1 0.16 0.16 0.16
170 0.10 0.12 0.13 0.14 0.14 0.15 0.15 0.15 0.15 0.15
175 0.10 0.12 0.13 0.14 0.14 0.14 0.15 0.15 0.15 0.15
180 0.09 0.12 0.13 0.13 0.14 0.14 0.14 0.14 0.15 0.15
190 0.09 0.11 0.12 0.13 0.13 0.13 0.14 0.14 0.14 0.14
200 0.09 0.11 0.12 0.12 0.13 0.13 0.13 0.13 0.14 0.13

Need technical support?

For help and advice on any of our products and how they might fit your needs please contact our Technical by Quinn team.

  • Floor Insulation
  • Pitched Roof Insulation
  • Flat Roof Insulation
  • Wall Insulation