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Rockwool Flexi - Flexible Edged Building Slabs

Standard Rockwool Product, generally readily available in most thickness’s.

Product Functions

Rockwool Flexi - is a multi-use, dual purpose thermal and acoustic insulation product with a unique flexible edge along one side. The flexible edge ensures a perfect fit is maintained between the product and its supporting framework for optimum effectiveness.

Flexi slabs ensure essential tight fitting for thermal and acoustic integrity between frames in walls, partitions, floors and roofs, without the need for cutting or waste. Made from renewable volcanic rock, they are extremely fire resistant and significantly reduce the carbon footprint of a building.

Product Description

Rockwool Building Slabs for installation versitility around the house

Rockwool Flexi is a product that consists of a semi-rigid slab with a flexible edge along one side. Rockwool Flexi is stone wool, or rock, insulation product with a flexible edge along one side, to ensure a perfect fit every time, within floor joists, between rafters, metal and timber studs and metal and timber frame buildings.

This ingenious volcanic rock slab with the patented flexi-edge that makes installation so easy.

Unlike rigid insulants, Rockwool Flexi will hold itself within the stud centres required, first time, every time. Timber expands and contracts,

Rockwool Flexi can be fitted between damp timber and will expand into the wider space created when the timber has dried out and shrunk back ... while other rigid or very flimsy insulants may fall out or be displaced, resulting in cold spots and poor acoustics.

Rockwool Insulation needs to deliver good thermal performance and avoid unplanned thermal bridges. It is not enough to have a product with a good thermal performance, if it is difficult to install it without gaps, cracks or other insulation errors in the final construction. Therefore the insulation must be easy to install to achieve an exact fit in the construction.

Avoiding unplanned air gaps is even more relevant, as the insulation efforts on the building envelope are increasing and the relative effects of unwanted air-gaps are consequently becoming bigger. This is particular true for exterior wall insulation systems, where mortar penetrating the insulation layer can add a substantial, unplanned thermal bridge to the construction. For indoor use, lightweight Rockwool insulation products are manufactured with slightly flexible edges. This allows them to knit together at the joints and fit closely into building structures without leaving unplanned gaps.

Product Benefits

Patented ‘flexi’ edge offers accurate fit to all widths

Will not slump, even if studs shrink

Fits all typical metal and timber frame spacing

Excellent thermal, acoustic and fire properties

Non-combustible (Euroclass A1)

Easy to handle and install

Superior fit with no waste

Product Properties and Performance


Sizes: 600mm x 1200mm slabs and 400mm x 1200mm sizes
Thickness’s: 25, 50, 75 and 100mm

Thermal Properties

Rockwool Flexi® has a thermal conductivity of 50 – 120mm, 0.038 W/mK when tested to EN13162.
At 140 mm thickness, it has a thermal conductivity of 0.035W/mK.
Refer to the Rockwool Flexi datasheet for U Values.

Acoustic Properties

Rockwool Flexi works in two distinct ways to reduce noise, either by impeding the transmission of sound through an element of the structure, or by absorption of sound at the surface. Noise absorption is expressed as a factor between 0 and 1.0. The more sound that a surface absorbs, the higher its absorption coefficient. The structure of the fibres in these slabs make them ideal for use as a sound absorber, with characteristically high coefficients over a wide frequency range.

Fire Properties

Rockwool Flexi achieves a reaction to Fire classification of A1, as defined in EN13501-1.
Refer to datasheet for relevant fire rated constructions.
Specifically for details about the Rockwool Flexi 1 Hour fire floor and fire rated internal partition. Refer to the Application Guide for construction information.

Green Guide Rating

Rockwool Flexi corresponds to the BRE Global Green Guide online generic specification ‘Stone wool insulation - density 45 kg/m³ ’ (ref. 815320008) which achieves a summary rating of ‘A+’ within all building types.

Water Repellency

All Rockwool stone wool insulation repels liquid water due to its fibre orientation and the presence of water repellent additives.

Third Party Approvals

Rockwool Flexi has the following certifications: LBCB 022E (Loss Prevention Certification Board) KM 06661 (KiteMark) LUL (London Underground Limited) CE Mark

Ozone Depleting Potential

Relying on entrapped air for its thermal properties, we are proud to say that ROCKWOOL FLEXI® does not contain (and has never contained) gases that have an ozone depleting potential (ODP).

Global Warming Potential

Relying on entrapped air for its thermal properties, we are proud to say that ROCKWOOL FLEXI® does not contain (and has never contained) gases that have a global warming potential (GWP). Therefore, they have a zero GWP for the purposes of rating schemes such as the Code for Sustainable Homes.

Product availability indicators

Rockwool Flexi - Price and Availability Advisory:

Presently there are no availability restrictions.

Excellent = Product Availability Excellent Good = Product Availability Good Fair = Product Availability Fair Poor =  Product Availability Poor Undetermined = Product Availability Undetermined
Availability relies on stock levels at closest depots and upon transport logistics to effect deliveries

Technical Data

Dimensions 400mm & 600mm x 1200mm
Thickness's 50mm - 200mm
Thermal Conductivity Thermal - 50mm - 120mm, 0.038 W/mK // Acoustic - 140mm - 200mm, 0.035 W/mK
Acoustic Properties Achieves Part E (resistance to sound) when installed in accordance to ROCKWOOL guidelines
Fire Properties Euroclass A1
Water Repellency Water Repellent
Vapour Resistivity Negligible
Environmental Zero ODP & GWP
Standards BS EN 13162, ISO 14001: 2004

Product Information and Certification

Links to manufacturer's information. Click to download in a separate browser window.

Rockwool Flexi Slab Datasheet | Rockwool SoundPro Acoustic Insulation Guide | Rockwool meeting Part L Building Regs.


Rockwool Building Slab Range - Certification

Product Certification

The following NBS clauses include Flexi®: p10:140, p10:210, p10:230, p10:240, p10:250, k10:115, k10:125, k10:145, k10:155, k10:165, k10:185, k10:420, k11:215, k11:225, k11:235, k11:245, k20:150, k20:160, m10:290, m13:260

Rockwool Slabs conform to BS EN 13162: 2001: Thermal insulation products for buildings – factory made mineral wool (MW) products specification.

They also satisfy the requirements of BS 5422: Method for specifying thermal insulating materials for pipes, tanks, vessels ductwork and equipment.

  • Rockwool RW Slabs

    Thermal / acoustic slabs suitable for roofs, ceilings, walls, partitions, floors, and can be fitted between timber and metal frames.

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    RW Slab Range is used for thermal, acoustic and fire insulation in general building applications. The product range consists of high quality resin bonded slabs in a variety of thicknesses and densities.
  • Knauf Building Slabs

    Knauf Earthwool Building Slabs are a range of rigid and semi-rigid, resilient, non-combustible rock mineral wool slabs.

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    Earthwool Building Slabs are a range of rigid and semi-rigid, resilient, non-combustible rock mineral wool slabs, available in densities from 33 to 140kg/m3.
  • Acoustic Partition

    Designed for metal and timber stud partitions, APRs are flexible, resilient, unfaced glass mineral wool for standard 600mm centre studs.

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    Knauf Insulation acoustic insulation rolls have been especially developed to utilise the sound absorption properties of glass mineral wool
  • Earthwool Loft Rolls

    Ceiling joist level loft rolls that are flexible, lightweight, low density, resilient and non-combustible glass mineral wool quilts.

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    Knauf Insulation have specially designed our loft insulation products for the thermal insulation of loft and attic spaces, when insulating at ceiling level between or above joists

Installing Rockwool Flexi

Rockwool Flexi® Slabs are light and easy to cut to any shape with a sharp knife.
Ensuring a perfect insulation fit is essential to maintain the thermal and acoustic integrity of the wall.
Typical softwood timber moisture contents can range between 6-30%, dependant on exposure to the elements. As a rule of thumb, timber will expand 1% for every 4% of moisture content.
If insulation is installed when timber has high moisture content, it can result in a potential 6 mm vertical void on each side, when the timber dries out.
Rockwool Flexi® expands into this void, ensuring thermal and acoustic integrity, whilst rigid insulations may leave a 6 mm gap, or even fall out.
Where studs or joists are spaced at 400mm centres, 400mm wide Rockwool Flexi® should be used.
Flexi® is also fast and easy to friction fit between floor joists from below, prior to fixing the plasterboard ceiling.
Once installed Rockwool Flexi® does not require any maintenance.

Acoustic applications – Walls

Rockwool Flexi® works in two distinct ways to reduce noise, either by impeding the transmission of sound through the structure, or by absorbing sound at the surface.

Acoustic and thermal insulation for timber frame party walls to help reduce Party wall bypass Heat loss via “Separating party cavity walls” Building Regulations Approved Documents L1A & L2A of England and Wales’ and Section 6 of Scotland’s Building standards have recognised that where party cavity-walls between connected buildings are untreated, considerable heat can escape through them. Minimising heat loss from party walls.

Rockwool has a range of solutions to help eliminate the heat loss from timber frame party walls.

Extensive site trials have demonstrated that the U-value for a party wall can potentially be reduced to zero if the cavity of a timber frame party wall is fully filled with Rockwool Flexi® insulation and effective edge sealing (such as Rockwool PWCB) is applied around the perimeter edges of the party wall cavity.

Acoustic applications – Separating walls

The following are required ...

  • A minimum of 100mm of existing solid masonry wall, plastered on both faces Independent timber or steel studs.
  • A minimum 10mm gap to be maintained between the frame and the existing wall
  • 50mm of Rockwool Flexi® between studs
  • 2 layers of plasterboard at a minimum of 20kg/m² (approximately equal to 2×15 mm layers)
  • Avoid flanking transmission: seal perimeter edges of new plasterboard with tape or Rockwool Acoustic Sealant
  • If the existing masonry wall is not plastered or is less than 100mm thick then independent panels should be applied to both sides
  • Pre-completion site testing

Download the Rockwool Flexi Datasheet for full details. Click here.

Contact Manufacturer

Rockwool Insulation Limited

Pencoed Bridgend CF35 6NY United Kingdom

Rockwool Customer Service

Telephone: +44 (0) 1656 868400
Web site: Rockwool Home Page

Contact us

Insulation Online Limited

Sales Office only:39 Normandy Road, Worthing, West Sussex, BN14 7DX, UK

Telephone: 0333 8000 828


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