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Hempcrete and lime plaster — finishing your walls

5 min read

How to finish hempcrete walls with lime plaster inside and render outside.

Finishing hempcrete walls well is not simply an aesthetic decision; it's a technical one. The finish you choose affects vapour permeability, moisture management, durability, and the long-term performance of the whole wall assembly.

The good news is that hempcrete is a highly accommodating base for wet-applied finishes. This article focuses on lime plaster and render, the most commonly used finish for hempcrete walls, covering why lime works so well, how to approach application, and what practical considerations to keep in mind before you start.

Why Lime is the Right Choice for Hempcrete

Hempcrete walls must breathe. Any finish applied to them, internally or externally, needs to be vapour permeable, allowing moisture to pass through the wall assembly rather than becoming trapped within it. This rules out impermeable coatings, vinyl-based paints, and conventional cement renders.

Lime finishes satisfy this requirement naturally. They're vapour permeable by nature, meaning the wall continues functioning as intended. Beyond permeability, lime offers additional benefits that make it particularly well-suited to hempcrete:

  • Moisture buffering. Lime finishes share, to some extent, the ability of clay plasters to absorb and release moisture in response to changes in indoor humidity. This contributes to a stable and comfortable internal environment.
  • Weather resistance externally. The re-carbonation process, whereby building lime reacts with CO₂ in the air and gradually hardens, gives external lime renders a durable, hard finish that withstands weathering while remaining vapour open.
  • Self-healing properties. Lime finishes based on air limes or feebly hydraulic limes have a unique ability to reseal small surface cracks if they appear. Free lime particles, lime within the render that hasn't yet fully carbonated, can move into fine cracks when wetted by rain, then carbonate on exposure to air and effectively seal the crack again. This is a significant practical advantage in an external render context.
  • Compatibility with the substrate. Cast hempcrete has an open-textured surface that provides an excellent mechanical key for lime plaster. Lime finishes require a strong key, and hempcrete delivers exactly that.

Lime finishes can be used both internally and externally. For internal walls, clay plasters are also a valid and often preferable choice, since they have superior moisture-buffering characteristics, but lime offers advantages in terms of cost and ease of application, and remains a sound option for most internal situations.

Hempcrete as a Plastering Base

Cast hempcrete formed with shuttering is widely regarded as an ideal background for plaster finishes. Several properties of properly cast hempcrete make the plasterer's job easier and keep material costs manageable:

  • Flatness. Shuttered hempcrete produces a very flat wall surface, which simplifies the application of finishes and reduces the thickness of material needed to achieve a level result. Since lime finishes can be expensive, particularly if applied by a specialist, minimising build-up thickness has a direct impact on cost.
  • Consistency. The surface quality is uniform across the whole wall, with no significant variation in suction or texture. This makes it straightforward to achieve an even finish without patchy results.
  • Open texture and key. The rough, open surface of hempcrete provides the mechanical grip that lime plasters need to bond properly. Unlike dense, smooth substrates that require keying agents or bonding treatments, hempcrete accepts lime plaster directly.

These qualities make cast hempcrete and lime plaster a natural pairing. The most commonly used finish for a hempcrete wall is a simple two-coat lime plaster system, and in most cases this is all that's required to produce a high-quality, durable result.

If you're plastering onto a carrier board such as wood-wool board rather than directly onto hempcrete, for example on internal stud partitions or where permanent shuttering has been used, the same general approach applies, but seek specific advice from your hempcrete binder supplier or natural building materials supplier, as the requirements may differ slightly between substrates. See The Basic Build Process for more on permanent shuttering and wood-wool board specifically.

One important exception: if you're plastering onto a cast hempcrete ceiling rather than a wall, a glass-fibre reinforcing mesh is required before applying the plaster coats. This is a non-negotiable step and shouldn't be skipped.

Applying the Finish

Watch it in practice: Lime rendering a hempcrete wall walks through the application process on a real build.

Matt, my co-founder at The Low Carbon Project, built this particular wall himself, on his own house, with me helping for most of the build. Matt's an experienced builder, but this was his first time working with lime. His take afterwards: lime "goes off a lot slower than normal plaster, which has pros and cons. A novice would find lime more forgiving, but to achieve a perfect finish is more difficult." That's a fair summary of the learning curve: the slower set gives you more working time and room to correct mistakes than gypsum plaster, but getting a genuinely polished final surface still takes practice.

Temperature and Working Conditions

Lime plaster is sensitive to temperature. The absolute minimum ambient temperature for working with lime is 5°C, but in practice the workable range is between 8°C and 22°C. Working outside this range, whether too cold or too hot, compromises the carbonation process and the quality of the finished result. Plan your plastering schedule accordingly, particularly on external elevations where you have less control over conditions.

Choosing the Right Lime Product

Seek advice on the most suitable finish from your hempcrete binder supplier and/or natural building materials supplier. There are meaningful differences between lime types (air lime, feebly hydraulic lime, moderately hydraulic lime), and the right choice depends on factors including the exposure of the wall, whether you're working internally or externally, and the specific hempcrete mix used. Don't assume any lime product will perform equally well in all situations.

Avoiding Incompatible Finishes

Switching to gypsum board and skim for internal walls in a hempcrete building is generally inadvisable. Beyond the impact on moisture buffering, it tends to create an awkward transition between material types that can look and feel disjointed. If you're committed to building with hempcrete, carrying natural finishes, lime or clay, through the whole building is both technically and aesthetically the more coherent choice.

Practical Takeaways

  • Use only vapour-permeable finishes. Lime and clay are your primary options. Cement renders and impermeable coatings aren't compatible with hempcrete walls.
  • A two-coat lime plaster system is the standard approach for cast hempcrete and is suitable for most internal and external applications.
  • Take advantage of hempcrete's natural key. No bonding agents are typically needed on a properly cast hempcrete surface.
  • Check temperatures before you start. Work only within the 8–22°C range for best results with lime.
  • Use a reinforcing mesh on ceilings. This is essential for plastered hempcrete ceilings and isn't required for walls.
  • Get product-specific advice. Consult your supplier about the most appropriate lime type for your application, internal or external, direct on hempcrete or on carrier board.
  • Be consistent throughout the building. Mixing natural and conventional finishes within a hempcrete building creates both technical and aesthetic problems.

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