Suspended Timber Floor - Technical Detail


Suspended Timber Floor

Knauf Insulation Suspended Ground Floor Insulation Glass Mineral Wool

Suspended ground floor INSULATION

In a suspended timber ground floor, insulation is placed between the joists and supported on netting (e.g. polypropylene) or timber battens.

In this application, an important consideration is the thermal and acoustic performance of the insulation.

Our Mineral Wool insulation solutions for suspended timber ground floors ensure that the product used fills all gaps between joists. This is to prevent air movement which can lead to unwanted heat loss.

While Knauf Insulation offer various insulation products into suspended ground floor applications, we would emphasise that site specific conditions should always be considered on a case-by-case basis and any generic product recommendations must always be subject to an onsite evaluation.


Knauf Insulation Solution Product Construction Code U-value Range (High) U-value Range (Low)
Suspended timber floor OmniFit® Roll 40 GF06 0.30 0.10
Suspended timber floor

Rocksilk® Flexible Slab


0.29 0.10

Product Benefits:

  • Friction fitting between timber joists closes joints, preventing air movement and infiltration
  • Flexible products which accommodates movements in floor ensuring all joints remain closed
  • Non-combustible with a Euroclass A1 reaction to fire classification
  • A+ Generic BRE Green Guide Rating


OmniFit® Rolls  are made from Glass Mineral Wool and formed into rolls which are lightweight, flexible, resilient and non-combustible.

Rocksilk® Flexible Slab is a multi-use, flexible, Rock Mineral Wool slab designed for friction- fitting in a range of acoustic, thermal and fire resistant applications.

Typical construction

For a suspended and ventilated timber ground floor, the insulation is placed between the joists and supported on polypropylene netting.

The netting should be positioned to support the insulation so that there is no gap between the insulation and the underside of the floor deck.

The floor joists running parallel with masonry walls should be spaced at least 35mm away from the wall to allow insulation to be placed next to the wall.

The wall insulation should start a minimum of 200mm below the top of the floor insulation to minimise thermal bridging.


If the insulation is the full depth of the floor joists, staple the support netting to the underside of the first joist and unroll the netting, stapling to the underside (or side) of each joist as the netting is unrolled.

Where the joist is deeper than the floor insulation, mark the depth of the insulation on the side of the joists. Staple the support netting along this line and pull taut to the adjacent joist and staple again. Pull the netting over the top of the joist and staple to the depth of the floor insulation. Repeat the process until there is netting support to the whole floor.

Install OmniFit® Rolls ensuring there are no air gaps between the insulation and the underside of the floor deck. Fix the chipboard floor deck in the usual way, using waterproof PVA glue at the joints, and allow a minimum 10mm gap at the room perimeter. If in doubt refer to the chipboard manufacturer's instructions.

When fixing the skirting board, apply a self- adhesive foam strip to the underside of the skirting and two beads of sealant to the back surface. Apply pressure to ensure the foam strip is compressed immediately before fixing the skirting in place.


Thermal performance

Omnifit® Roll 40 has a thermal conductivity of 0.040 W/mK.

OmniFit® Slab 35 has a thermal conductivity of 0.035 W/mK.

Rocksilk® Flexible Slab has a thermal conductivity of 0.037 W/mK, except 140mm thickness which has a thermal conductivity of 0.035 W/mK.

Fire performance

OmniFit® Roll 40, OmniFit® Slab 35 and Rocksilk® Flexible Slab are classified as Euroclass A1 to BS EN 13501-1, which means that they are non-combustible and will not contribute to the development of a fire or flame spread.



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 appropriate)

OmniFit® Roll 40*/ Rocksilk® Flexible Slab* of thickness and of width to suit joist spacings, supported on the netting and to be in contact with the underside of the floor deck. OmniFit® Roll 40*/ Rocksilk® Flexible Slab* to be cut and placed to fully fill in the gap between the last joist and the perimeter wall. Flooring grade t and g chipboard to be fixed to the floor joists, all as specified by the designer.

Alternatively, consult the National Building Specifications, Standard version clause/ clauses…P10/250……………

Knauf Insulation specification clauses can be downloaded here.

U-values of suspended timber ground floors insulated between joists

    U-values (W/m2K)
Thickness (mm) Product Ratio of permineter (m) to area (m2)
    0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
300 OmniFit® Roll 40 0.10 0.12 0.12 0.13 0.13 0.13 0.14 0.14
250 OmniFit® Roll 40 0.11 0.13 0.14 0.14 0.15 0.15 0.15 0.15
200 OmniFit® Roll 40 0.12 0.15 0.16 0.17 0.17 0.18 0.18 0.18
150 OmniFit® Roll 40 0.14 0.17 0.19 0.20 0.21 0.22 0.22 0.23
100 OmniFit® Roll 40 0.16 0.21 0.24 0.26 0.27 0.28 0.29 0.30
250 Rocksilk® Flexible Slab 0.10 0.12 0.13 0.14 0.14 0.14 0.14 0.15
200 Rocksilk® Flexible Slab 0.11 0.14 0.15 0.16 0.16 0.16 0.17 0.18
140 Rocksilk® Flexible Slab 0.14 0.17


0.20 0.21 0.21 0.22 0.22
100 Rocksilk® Flexible Slab 0.16 0.21 0.23 0.25 0.26 0.27 0.28 0.29


Contact our Technical Support Team