Cork Insulating Plaster for London's Period Homes
A breathable, wet-applied alternative to a wood fibre frame, suited to irregular walls and rooms with cornice or joinery worth keeping.
Not every solid wall can take a 140mm timber frame. Where a room has retained cornice, out-of-plumb masonry, or simply not enough depth to give up, a cork and lime insulating plaster is the other legitimate route to a warmer wall without sealing it.
Overview
Cork and lime insulating plaster does for a wall what the wood fibre route does in board form: it insulates without sealing the masonry behind it. Rather than fixing a rigid panel to the wall, it is trowelled on wet, in the same family of materials and for the same reason as a lime plaster finish, just formulated to add real thermal resistance rather than only a decorative coat.
It exists as a separate route because a rigid board is not always the right answer. A wall that is out of plumb, a room with a cornice worth keeping, or simply not enough depth for a full frame, all point toward a wet-applied plaster instead of a board.
Why a Plaster Route Instead of a Frame
The failure mode behind a sealed internal insulation system is the same one covered in full on our wood fibre insulation page. In short:
The mechanism
- Insulating the inside face makes the masonry behind it colder
- A vapour barrier meant to protect it always has a gap somewhere in a Victorian terrace
- Moisture concentrates at that gap and cannot evaporate, because the assembly no longer lets it
A cork and lime plaster avoids this by staying vapour-open throughout, in exactly the way a wood fibre frame does. The full mechanism, including the specific risk to embedded joist ends, is set out on the wood fibre insulation page.
The Material
A cork and lime thermo-plaster such as Diathonite Evolution runs at a thermal conductivity of 0.045 W/m·K, giving roughly 0.222 m²K/W of thermal resistance per 10mm applied. Its vapour resistance factor is µ 4, which is very open, and its porosity is around 71 per cent. Those three numbers together are why it can insulate without trapping moisture, and they are the numbers to ask for when a system is described only as “breathable”.
It is applied directly onto masonry once the existing plaster has been hacked off, in the same base-coat-and-finish sequence as any lime plaster, just formulated with a lightweight insulating aggregate in the base coat rather than a standard sand and lime mix.
The Build-Up We Specify
On the jobs where a plaster route is the right answer rather than a frame, the build-up is 40mm of NHL thermal plaster as the insulating base coat, a 6mm Baumit RK70N coat reinforced with Startex mesh, and a 3mm RK70N skim. Calculated against a typical solid brick wall, that reaches a U-value of 0.78 W/m²K.
Every layer and product in that build-up, alongside the deeper wood fibre frame alternative at 0.18 W/m²K, is set out in full on the wood fibre insulation page and on build-ups.
Where a Plaster Route Wins
Choosing a plaster over a frame
- Follows an irregular wall, since it is trowelled on wet rather than fixed as a rigid panel needing a flat plane
- Suits a room with retained cornice or joinery, where a full frame would usually mean losing or re-running the moulding
- Fixes directly to existing masonry once the old plaster is hacked off, with no new frame to build
- The right answer where the depth for a wood fibre frame is not available or not wanted
What the Numbers Say
0.78 W/m²K is a genuine improvement on a bare, uninsulated solid wall. It is not in the same category as a 140mm wood fibre frame at 0.18, and treating the two as equivalent is selling something. Where the depth exists, the frame is the better answer. Where it does not, or where a retained cornice rules it out, a cork and lime plaster is a specified route in its own right rather than a compromise.
As with every figure on this site, that number is calculated for a defined build-up and a defined base. The actual performance on a given wall still depends on what is already there, and how continuously the plaster runs across the wall rather than stopping short at a reveal or a skirting.
Cost and Scope
Every wall is priced on survey. The variables are the condition and regularity of the substrate, the area involved, and how much reveal and junction work the room needs.
| Related service | Typical context |
|---|---|
| Wood fibre insulation | The deeper, higher-performance route where the depth is available |
| Lime plastering | The finish coat that follows on any wall, insulated or not |
| Wall remediation | Where damp is present before insulation is considered |
| Refurbishment mission plan | A strategy for the building, whether or not you use us |
Next step
Get a quote, or call +44 7468 865983.
Common Questions
What is cork insulating plaster made of?
A lightweight insulating aggregate, commonly cork or a cork and diatomite blend, bound in a natural hydraulic lime matrix and trowelled directly onto masonry once the existing plaster is hacked off. A cork and lime thermo-plaster such as Diathonite Evolution runs at a thermal conductivity of 0.045 W/m·K and a porosity of around 71 per cent.
How does cork insulating plaster compare to a wood fibre frame?
It reaches a lower U-value for the depth involved. Our specified cork-lime plaster route is calculated at 0.78 W/m²K over about 49mm, against 0.18 W/m²K for a full 140mm wood fibre frame. Where the depth exists, the frame is the better answer; where it does not, or a retained cornice rules it out, the plaster is a legitimate route rather than a compromise.
Can cork insulating plaster go over an irregular wall?
Yes, that is one of its main advantages over a rigid insulation board. Because it is trowelled on wet rather than fixed as a panel, it follows the wall as it actually is rather than needing a flat plane to fix to.
Does cork insulating plaster suit a room with retained cornice or joinery?
Usually, yes. A 40 to 49mm plaster build-up is far less likely to require losing or re-running a cornice than a full wood fibre frame, which is one of the main reasons to specify a plaster route over a frame in a room with detailing worth keeping.
Further Reading
- Wood fibre insulation, the deeper alternative and the full condensation mechanism
- Build-ups, every specified assembly with named products and calculated U-values
- Why Breathable Walls Crack
- Wall remediation, if damp is present before insulation is considered
- All technical white papers
Guidance on insulating traditional buildings is published by Historic England and the Sustainable Traditional Buildings Alliance. Moisture management in buildings is covered by BS 5250. Product names are the trade marks of their respective manufacturers.