Hempcrete has been gaining serious traction among architects, builders, and self-builders looking for alternatives to conventional construction materials. But what exactly is it, how does it work, and is it right for your project? This guide covers the fundamentals, grounded in what the material actually is, not the hype surrounding it.
What hempcrete is, and what it isn't
Hempcrete is a hemp-lime composite construction material. At its most basic, it consists of two ingredients: the chopped woody stalk of the industrial hemp plant (known as hemp shiv or hemp hurd) mixed with a lime-based binder. The result is a natural, low-embodied-energy building material used primarily as insulation and wall infill in timber-framed buildings.
It's not a structural material in the conventional sense: it doesn't replace concrete in load-bearing applications. The lime binder's job isn't to create a dense, strong matrix the way it would in ordinary concrete or mortar. Instead, its purpose is simply to coat the hemp particles and bind them to one another. This produces a finished mix that retains a significant amount of void space, and that void space is precisely what gives hempcrete its most useful properties.
Those voids are responsible for hempcrete's performance as an insulation material, contributing to both its thermal resistance and its ability to manage moisture within a wall assembly. Because of this, hempcrete is used for roof, wall, and floor insulation, and can be formed into a variety of building components depending on the application.
Hempcrete sits within a broader resurgence of interest in natural and traditional building methods, but unlike some historic techniques with limited applicability today, it's been deliberately refined for contemporary use, rooted in older traditions while built to meet modern construction needs.
How hempcrete is made
The mix itself is straightforward: hemp shiv combined with a lime-based binder and water. Choosing the right binder, though, is more nuanced than simply picking a lime product off the shelf.
The role of the binder
The binder is the active ingredient that drives the set. It needs to do several things simultaneously, and most standard limes can't fulfil all of these requirements on their own. According to The Hempcrete Book, a hempcrete binder must:
- Provide an initial hydraulic set, strong enough to support the weight of the hempcrete wall while it's still drying, allowing shuttering to be removed after a defined period
- Remain vapour permeable, so that water introduced during mixing can continue to evaporate out of the hemp shiv after the binder has initially set
- Achieve a full long-term set through carbonation, as the binder reacts with CO₂ in the air over time
That long-term carbonation process has one practical consequence worth knowing about: because moisture introduced during mixing needs to gradually evaporate out of the wall, hempcrete made with certain binders will shrink slightly as it dries, often by around 2–3mm at the edges. This isn't a problem for the material itself, but it does have implications for airtightness detailing in some situations.
Binders available in the UK
At the time The Hempcrete Book was written, three tested and reliable binders were available for hempcrete construction in the UK:
- Batichanvre®, manufactured by CESA/St Astier in France
- Prompt Natural Cement, manufactured by Vicat in France
- Tradical® HB, developed in France, produced under licence in the UK by Lhoist UK
These three products differ in their practical characteristics. Tradical® HB has received Local Authority Building Control (LABC) pre-approval, with a warranty available as part of the Tradical® Hemcrete® walling system, making it the most straightforward option for projects requiring formal building control sign-off in the UK. The other two binders have been used in UK buildings that received building control approval, but don't carry LABC pre-approval.
The binders also differ in working temperature guidance from manufacturers, initial set times, drying times before finishes can be applied, and suitability for spray application, factors that will influence your choice depending on the specifics of your build programme and method. See Where to buy hemp hurd and lime binder for a fuller breakdown of current suppliers.
Why hempcrete matters
Hempcrete's growing profile isn't simply a result of fashion. It addresses a set of genuine challenges that conventional construction materials struggle with.
The Hempcrete Book describes hempcrete as a natural, healthy, sustainable, low-embodied-energy building material that can often be sourced locally. Each of those qualities carries weight:
- Natural: the primary component is an agricultural crop, and the binder is mineral-based
- Low embodied energy: compared to energy-intensive materials like fired brick, steel, or Portland cement concrete, hempcrete's production requires significantly less energy
- Moisture management: the void structure of the material gives it hygroscopic properties, allowing it to absorb and release moisture and contribute to healthy internal environments
- Thermal performance: those same voids provide insulation value, reducing heat loss through walls
It also represents a material that can be locally sourced in many regions, reducing transport-related emissions compared to globally traded industrial materials, though how far your specific materials travel matters more than people often assume. See Is hempcrete carbon negative? for more on why local sourcing can make or break the material's carbon case.
That said, hempcrete isn't a universal solution. It requires a structural frame to carry loads, it needs careful detailing to manage airtightness, and its drying times mean build programmes need to account for adequate curing before finishing work begins. Understanding these constraints matters as much as understanding the material's benefits.
Practical takeaways for beginners
If you're new to hempcrete and considering it for a project, here are the key points to carry forward:
- Hempcrete is an insulation and wall-fill material, not a structural one; it always works alongside a load-bearing frame.
- The binder choice matters. Different binders have different set times, working temperature requirements, and approval statuses; get advice specific to your project and location.
- Drying time is part of the process. Factor in adequate time for the hempcrete to cure before applying renders or other finishes.
- Lime safety is non-negotiable. The lime content of the binder is caustic and can cause burns; proper protective equipment and access to washing facilities on site are essential, not optional.
- Building control approval is achievable. UK projects have successfully obtained approval using all three of the main binders, and pre-approved routes exist for some systems.
Hempcrete is a well-documented, tested material with a growing track record in the UK and beyond. The learning curve is real, but the fundamentals are straightforward, and the resources to build with it confidently continue to grow.
Sources
- Stanwix, W. & Sparrow, A., The Hempcrete Book