When builders and homeowners decide to ‘go green’, the first conversation is usually about insulation batts—swapping pink fibreglass for natural hemp or wood fibre slabs.
While hemp and wood fibre batts are an excellent sustainable choice, they are ultimately just stuffing for a cavity. For those seeking high performance, deep retrofits, or a ‘forever home’ finish, there is a superior method: Sprayed Hemp (Hempcrete) or preformed Hempcrete solid boards.
When you combine this monolithic spray technology with a cork aggregate upgrade and finish it with Clayworks clay plaster, you create a wall system that not only insulates but also regulates humidity, protects timber, and purifies the air.
Here is your complete guide to this advanced build system.
The Core Structure (Why Spray?)
Unlike installing rigid boards, which must be cut, taped, and squeezed into place, sprayed hemp is projected as a wet slurry of hemp shiv (the woody core) and lime binder.
1. The ‘Monolithic’ Advantage
The greatest weakness of any board insulation is the joint. Every time you butt two boards together, you create a potential air gap or ‘thermal bridge’ where heat escapes. For many finishes, there is also a risk of cracking even when a mesh is used.
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The Spray Solution: Sprayed hemp hits the wall and forms a single, continuous layer. It wraps around studs, pipes, and wiring, filling every microscopic void. There are no joints, no seams, and no draughts.
Thermal Mass (The Battery Effect)
Lightweight insulation boards keep heat in, but they cannot store it as well. We have seen this with solid wall cork insulation.
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The Spray Solution: Hempcrete is dense. It acts like a thermal battery, absorbing heat during the day and slowly releasing it back into the home at night. This ‘phase shift’ flattens temperature spikes, keeping the house cool in summer and warm in winter without constantly running the central heating.
3. Active Timber Protection
If you wrap an old timber frame in plastic and glass wool, you risk trapping moisture and rotting the wood.
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The Spray Solution: The lime binder in hempcrete is highly alkaline (high pH). When sprayed directly onto timber, it creates an antiseptic environment that naturally repels mould, rot, and insects—essentially ‘petrifying’ the frame and extending its lifespan indefinitely.
The Upgrade (Adding Cork)
For projects requiring higher thermal performance or faster turnaround times, we are able to mix granulated cork into the hemp-lime slurry. This creates a ‘hybrid mix’ with distinct advantages.
1. A Warmer Wall (Better U-Value)
Standard hempcrete has a thermal conductivity ($\lambda$) of roughly 0.070 W/mK. Expanded cork is significantly more insulating at 0.040 W/mK.
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The Benefit: By replacing a percentage of the hemp shiv with cork, you lower the density and improve the insulation value. This is crucial for thin walls where you need to hit strict Part L energy targets without losing valuable floor space.
2. Faster Drying Times
One of the drawbacks of pure sprayed hemp is the long drying time (often months), as hemp absorbs water avidly.
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The Benefit: Cork is closed-cell and hydrophobic (water-repellent). A hemp-cork blend requires less water to lubricate the spray lines and holds less water in the wall. This allows the structure to dry out significantly faster, letting you get to the plastering stage sooner.
The Skin (Clayworks Finishes)
Once your hemp-cork wall is dry, you cannot simply paint it with standard emulsion or cover it with gypsum plasterboard. Doing so would seal the wall, trapping moisture and causing rot. The wall must ‘breathe’.
While lime plaster is the traditional choice, unfired clay plaster (specifically from British manufacturer Clayworks) is widely considered the gold standard for interiors.
1. The ‘Breathability’ Super-Team
The primary reason to specify Clayworks on hemp is moisture buffering.
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Hempcrete holds moisture deep within the wall structure.
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Clay Plaster is hydrophilic (water-loving). It absorbs humidity from the air significantly faster than lime.
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The Result: The clay pulls excess moisture from the room instantly, passes it to the hemp ‘battery’, and releases it when the room is dry. This creates a stable, healthy indoor climate that feels noticeably different to walk into—cool in summer, warm in winter, and never stuffy.
2. The Aesthetic: Can it be Polished?
You cannot polish the raw hemp insulation itself (it is too fibrous). However, you can achieve a polished finish with the clay topcoat.
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Smooth Finish: Clayworks offers fine mixes that can be hard-troweled to look like polished stone or concrete. This provides a sleek, modern aesthetic without the carbon footprint of cement.
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Demi-Rustic: For hemp walls, the ‘Demi-Rustic’ finish is often recommended. It contains slightly larger aggregates which help hide the natural undulations of the sprayed hemp, providing a rich, textured, organic look.
3. Durability and Colour
Unlike paint, Clayworks plasters are pigmented with natural ochres. The colour is ‘through-bodied’—the plaster is the colour.
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Repair: If you chip a painted wall, you see the white plaster underneath. If you chip a clay wall, you see more clay. Repairs are simple: just wet the scratch with a sponge and trowel it smooth.
Installation Summary
This is a specialist system, not a standard DIY job. Here is how the layers come together on site.
| Layer | Material | Function |
| 1. The Core | Sprayed Hemp-Cork Hybrid | The insulation, thermal mass, and draft-proofing. Sprayed directly onto the timber frame or masonry. |
| 2. The Prep | Levelling Coat | The sprayed wall is shaved flat (‘screeded’). A coarse base coat of lime or clay is applied to fill voids and flatten the surface. |
| 3. The Strength | Mesh Reinforcement | A fibreglass mesh is troweled into the wet base coat. This prevents hairline cracks as the different materials settle. |
| 4. The Finish | Clayworks Topcoat | Applied at 2–3mm thickness. Troweled smooth or textured. No painting required. |
If your goal is a quick, cheap fix for a loft, hemp or wood fiber batts are the right choice.
However, if you are building a wall system and want a home that regulates its own humidity, protects its structure, and holds a steady temperature year-round, the Sprayed Hemp + Cork + Clay system is the pinnacle of sustainable building. It turns your walls from simple barriers into active climate-control devices.