CELENIT L3AB/A2 Insulation Board Application Guide


Adam Brown

FdSc Sustainable Construction BSc, Building Surveying & the Environment

About the author

Adam has 10+ years of expertise in building conservation & breathable materials.

CELENIT L3AB/A2 is a 2000mm x 600mm (1.2m2) panel composite wood wool thermal and acoustic board which can be directly rendered/plastered onto. Available in 35mm, 50mm and 75mm boards from stock and 100mm, 125mm, 150mm and 175mm boards are available upon request. Please note that minimum order quantities may apply.

Featuring two 5mm layers of mineralised wood, similar to CELENIT N, the CELENIT L3AB/A2 board has an internal layer of high-density mineral wool. The faces of the board, wood wool, is created using wood, Portland cement and marble dust, comprising of 65% pine wood fibres and 35% mineral binders. The use of mineral binders mineralises the fibres, retaining the mechanical properties of the wood whilst making the fibres inert to prevent biological deterioration and increase fire resistance. The raw materials are blended and bound under pressure to create a compact, resilient, dimensionally stable and robust panel for internal and external applications.

Celenit texture.

Cutting, Storage & Use

A handsaw or circular saw can be used to cut the CELENIT L3AB/A2 boards. Store in covered and dry conditions and always keep dry.

For internal use, it is advisable to acclimate the boards to room temperature by keeping them in the room for a few days before fixing them. When fitting the boards, fit or lay closely, using staggered joints similar to a brick bond and appropriate washers to avoid damage or stressing of the boards.


The surface to which the CELENIT L3AB/A2 boards are fixed must be flat and able to support the load. Therefore, if fixing directly to uneven masonry, it is advisable to render or plaster the wall flat beforehand. The boards require 8 fixings per m2, 10 per board.

For plastic fixings, these must penetrate the support by a minimum of 50mm. For metal fixings, these must penetrate the support by a minimum of 30mm. If using metal fixings, only stainless-steel fixings should be used.

Rendering or Plastering the Boards

Ensure the CELENIT L3AB/A2 boards are fixed flat to the surface using staggered joints similar to a brick bond before rendering or plastering. Before application, it is essential that the panels are dry and that environmental conditions are appropriate. Do not use in temperatures below 5°C or above 25-30°C to manage the potential of shrinkage or cracking. Further, protect the fresh render from freezing conditions and avoid rapid drying due to high temperatures, direct sunlight or drying winds. The render coats must cure fully before the next coat is applied.
We always advise following best practices for curing and protecting fresh lime renders and plasters using hessian sheeting, which acts as a barrier between the freshly applied lime and the environment. We have a range of hessian sheeting available as full rolls or per metre. For more guidance on curing and protection, see our helpful online guide Curing Lime Renders and Mortars.

External Rendering

After the boards have been fixed, apply the render as soon as possible. For sheltered or protected locations, we advise applying the render at a depth of 8 – 10mm for the base coat, with an external plasterer’s mesh embedded into this coat and followed by a 7mm top coat. For an exposed location, we advise applying the render at a depth of 8 – 10mm for the base coat, with an external plasterer’s mesh embedded into this coat, followed by a 7–8mm float coat and a 5–7mm top coat.

We recommend pairing CELENIT L3AB/A2 boards with Cornerstone Newbuild Render, a factory-blended render containing hydraulic lime, HL5, kiln-dried sand and specially selected additives. Cornerstone Newbuild offers the main benefits of lime, including vapour permeability and high flexibility, but with increased durability compared to standard lime mortars. If using Cornerstone Newbuild, we recommend using the Newbuild Medium for all coats.

Alternatively, the Cornerstone Promix Render system can be used. Promix is a factory-blended render containing natural hydraulic lime, NHL, kiln-dried sand and specially selected additives. If using the Promix system, we recommend Promix Medium for the base and float coat, followed by Promix Medium or Promix Fine for the finish.

Learn more about Cornerstone Promix

If a site mix render is preferred, then either Saint-Astier Tradiblanc or Saint-Astier NHL3.5 and suitable sands can be used.

Internal Plastering

After the boards have been fixed, apply the plaster as soon as possible. We advise applying the plaster at a depth of 8–10mm for the base coat, with an internal plasterer’s mesh embedded into this coat and followed by a 2mm skim top coat.

We recommend pairing CELENIT L3AB/A2 boards with CLM35 Premixed Lime Plaster or Cornerstone Promix Medium for the basecoat and CLM66 Premium Lime Plaster for the skim coat.

If a site mix plaster is preferred, then Cornish Lime Mature Putty or Saint-Astier NHL2 or 3.5 and suitable sands can be used. For additional information or guidance, please get in touch.


The CELENIT L3AB/A2 boards are vapour open and allow for moisture movement, significantly improving thermal performance when paired with a lime render or plaster. Therefore, we advise using a vapour permeable paint as a protective layer over the render or plaster. We recommend Beeck External Mineral and Beeck Internal Mineral paint.

Find out more about Beeck Mineral Paints

Beeck Mineral Paint bonds to the render through a process known as silicification, making Beeck Mineral Paints remarkably long-lasting and durable. Upon application, these paints absorb into and bond with the mineral background. The result is a highly stable, breathable coating that allows moisture to pass through.

Beeck Mineral Paints natural colours close up.

A lighter-coloured paint with a high light reflectance value (LRV) is advisable with any external render finish. LVR measures the visible and usable light that absorbs or reflects from a painted surface. High LRV paint reduces the amount of UV absorbed into the render, keeping the render cooler and reducing the potential for thermal movement and cracking. In addition, UV absorption over time will degrade the paint’s pigment, making the colour of the paint duller over time. Using a higher LRV will result in a longer-lasting colour.

The information provided by Cornish Lime is for informational purposes only and does not amount to a specification. Every project is unique, so please consult a professional before undertaking a project. Use of this site and reliance on any information on the site is solely at your own risk.

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