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Floor Insulation: Insulating suspended timber ground floors between timber joists

Sand blinding and hardware

Concrete or gravel cover

Joists and support battens

Quinn Therm QF Boards

Timber flooring on polyethylene slip sheet

Internal finish

  • Sand blinding and hardware

  • Concrete or gravel cover

  • Joists and support battens

  • Quinn Therm QF Boards

  • Timber flooring on polyethylene slip sheet

  • Internal finish

The simplest way of insulating suspended timber ground floors is to fit insulation boards between the joists and support battens. This floor insulation method is commonly used for upgrading the thermal performance of existing timber floors as it has no impact on floor levels.

For new build, where a radon barrier is required it should be installed within the concrete or gravel cover of the sub-floor and continued through the entire thickness of the wall.

Ventilation of the sub-floor void is vital for avoiding problems with damp. When installing insulation boards ensure ventilation paths are maintained.

How to install insulation between timber joists

1. Measure the distance between joists and cut insulation boards to fit tight in between

2. Install supports between the joists to support the floor insulation boards. Supports should be either:

  • purpose-made galvanised saddle clips at 400mm centres
  • galvanised nails partially driven into the joists at 400mm centres
  • or preservative treated timber battens nailed to the joists.

The insulation supports should be placed at the appropriate height to suit the thickness of insulation board and nails should remain 40 mm proud of the joist. The depth of the joist above the support should be equal to the thickness of insulation board to ensure a level finish.

If services are to be run between the insulation boards and the flooring finish the supports can be set lower to allow for this.

3. Set the precisely cut Quinn Therm PIR insulation boards between the joists and press down until they sit on the battens, saddle clips or nails. All board joints should be tightly butted.

4.  Pack precision cut perimeter insulation boards around the room between the joists and the walls to prevent heat loss through thermal bridging. This perimeter insulation can be supported by blocks nailed to the underside of the joists. Wall insulation must extend 150mm below the upper edge of floor insulation to further reduce thermal bridging.

5. Lay a polyethylene separating layer across the top of the floor insulation to isolate it from the floor finish to protect against spills and reduce air leakage.

Recommended suspended timber ground floor insulation products

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 0.18 0.25 - - - - - - - -
60 0.17 0.23 - - - - - - - -
70 0.16 0.22 - - - - - - - -
75 0.16 0.21 0.25 - - - - - - -
80 0.16 0.20 0.23 0.25 - - - - - -
90 0.15 0.19 0.22 0.23 0.24 0.25 0.25 - - -
100 0.14 0.18 0.20 0.22 0.22 0.23 0.25 0.24 0.24 0.25
110 0.14 0.17 0.19 0.20 0.21 0.22 0.22 0.22 0.23 0.23
120 0.13 0.16 0.18 0.19 0.20 0.20 0.21 0.21 0.21 0.21
125 0.13 0.16 0.18 0.19 0.19 0.20 0.20 0.20 0.21 0.21
140 0.12 0.15 0.16 0.17 0.18 0.18 0.18 0.19 0.19 0.19
150 0.12 0.14 0.16 0.16 0.17 0.17 0.18 0.18 0.18 0.18

U-value results above based upon floor construction shown:

Timber joists (150mm x 47mm @ 400mm centres); Quinn Therm QF installed between joists; timber flooring (floor finish omitted). Calculations performed to BS EN ISO 6946 and BS EN ISO 13370, taking account of repeating thermal bridges

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