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Plastering/Rendering the Insulation Board

Marmox Multiboard is a thermal insulation board

They can be used both internally AND externally as base boards for plaster or render coatings. Being thinner than insulating plasterboard, Marmox Multiboards provide space-efficient alternative offering thermal insulation and waterproofing.


1. As an external base for render

ALL Marmox Multiboards comprise a core of waterproof polystyrene providing high levels of thermal insulation and a skin of a frost-resistant polymer-concrete. The skin is designed to offer the optimum level of porosity to allow cement-based, lime-based and silicone-based render systems to bond to it.

External plastering  

Because they are unaffected by variations in temperature, being submerged in water or frost, they can be used externally on walls to provide a layer of waterproof, insulating cladding which can be rendered upon.

General Advice – applying a simple sand/cement render

The board has a textured concrete surface and as such can be rendered as any concrete surface can be.  Typically, a conventional mesh is still required to be placed over the whole surface area of the boards. The mesh is generally pressed into the base layer of render however different render suppliers will have their own application instructions which must be followed.  The method to be followed is as though rendering onto a concrete surface.  Marmox Multiboards can be fixed directly onto solid masonry walls, concrete, window reveals etc. They are fixed to these masonry surfaces with a continuous bed of cement-based tile adhesive. For a detailed method, see the attachment IB4 in the attached specifications

Marmox Multiboards can also be fixed onto battens. If fixing onto timber battens, you must be confident that there will be no movement in your timbers as that could result in hairline cracks between the boards. Using any small render boards such as Marmox boards (which have proportionally more board edges and therefore joins than a large sheet does) are not ideal on timber framed structures. This is because of potential movement in the frame which could result in cracks in the render developing between each board.

Typical application on timber battens…

  • Fix to battening every 30cm.
  • A breather membrane should be included on the masonry if placing the boards on battens.
  • The thinnest Marmox Multiboards which can be used for this application is 12.5mm - they should be fixed so that there is absolutely no movement whatsoever in the boards.
  • Boards should be laid with staggered joints and fixed using screws and washers every 30mm along each timber. All vertical and horizontal edges of the board must be supported.
  • Place a dab of Marmox MSP360, the waterproof sealant over each screw hole prior to inserting the screw.
  • A 5mm gap between vertically and horizontally laid boards should be left and filled with Marmox MSP-360.
  • To stop curing issues over the board joints, seal the boards together with Marmox MSP360.
  • A mesh must cover the complete surface of the boards prior to applying the render, not just the joints. Around corners of window and door frames, additional mesh should be placed at 45O.
  • The boards can now be rendered according to the manufacturers’ instructions.

Marmox Multiboard as an external render base board is non-breathable so will stop water vapour generated within the building escaping through the normal path (through the walls). This could result in both surface and interstitial condensation within the structure therefore measures must be taken to improve the ventilation.

2. As a base for plastering

Marmox Multiboards are CE marked as rigid insulation panels. They can provide a base for plastering on both internal walls and ceilings. As an alternative to plasterboard, they provide thermal insulation and a create a waterproof barrier.

The concrete skin of the board comprises just two millimetres of the total thickness of the board meaning that the rest is all insulation product. This allows thinner boards to be used because there is no need for a layer of plasterboard. If the wall if flat, the boards can be fixed directly onto the existing wall without having to use battens which again reduces the space needed. The insulation within the board is XPS which is completely waterproof. XPS is not affected by moisture so it is as effective in wet conditions as it is in the dry. In addition to providing thermal insulation, it provides a vapour control layer when the boards are sealed together which also reduces air leakage and interstitial condensation. 

Improving the U Values

Marmox Multiboard is an all-in-one solution to Internal Wall Insulation. U value requirements for walls vary from 0.17 to 0.35 Watts/m2K depending on which part of the UK or Ireland you are in and whether it is a new build, an extension or a refurbishment. With non-insulated walls (solid wall construction), to achieve current new build levels of insulation with any internal wall insulation is extremely difficult but using Marmox Multiboard will significantly reduce the U value. For example, a typical solid wall will have a U value of 2.1 Watts/m2K but with a 40mm thick Marmox Multiboard this heat loss is reduced by 70% to 0.60 Watts/m2K.

Depending on the condition of the wall, Multiboard can be fixed directly to the brick or blocks mechanically or using a cement-based tile adhesive. Plasterboard adhesive is not suitable.

Marmox Multiboard (10mm and thicker) is CE marked meaning that it meaning that it meets the criteria proving that it significantly reduces heat loss through walls. The most popular thicknesses for internal wall insulation is 20mm and 30mm, which does not take up too much extra room space. Around door and window reveals thinner board such as 10mm or even 6mm can be used to reduce the risk of thermal bridging and condensation.

General Advice – applying a plaster coating

Marmox Multiboards ‘pre-primed’ so as soon as they are installed, they are ready to be plastered.  The boards can be coated with either a traditional base and top coat or just a skim coat of finishing plaster.  The boards can be fixed directly onto solid masonry walls, concrete, window reveals or onto battens.  Detailed methods are shown in the attached specifications PDF below.


  • If fixing onto battens, fix the boards (minimum thickness 12.5mm) with 300mm centres.
  • Seal the edges together using a bead of Marmox Multibond.
  • Cover the edges prior to plastering with reinforcing tape/scrim tape. 

For further information click on links below