Envelope and protection · Insulation
Hemp-Lime Cast Insulating Infill (Hempcrete)
Explains hemp-lime as a cast monolithic infill rather than a fitted product, and why the frame, the shuttering and the drying period are the subjects rather than fitting tolerance.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What hemp-lime infill is
Hemp-lime is made from shiv — the chopped woody core left after the fibre has been stripped from the hemp stalk — mixed with a lime binder and water. It arrives on site as separate constituents and is mixed to a damp, open consistency, then placed. It is not a fitted product at all: it is cast into shuttering, tamped in lifts, or sprayed onto a face and trimmed back, and when the shuttering comes off there is a continuous body of material with no joints in it.
It is frequently confused with hemp fibre insulation, which is a different material: that is a flexible quilt of bast fibre, blended with a binder fibre, supplied in batts and rolls and friction-fitted between studs. The two share a plant and nothing else. Hemp-lime uses the woody core rather than the fibre, arrives wet rather than dry, is placed rather than cut, has no fitting tolerance because it has no joints, and sets and dries over a period that has to be planned for rather than worked around.
It is not a structural material. The building is framed in something else — most commonly timber — and the frame passes through the infill, so the hemp-lime carries only itself while doing the thermal and hygric work of the wall body. The lime binder keeps the material open to water vapour and buffering of moisture is part of why it is chosen, which is why it is normally finished with a vapour-open lime render or plaster rather than lined with an impermeable board. The drying period after casting is the single largest programme consequence of using it, because the wall cannot be closed up or finished until the professional responsible is satisfied it has dried.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- Hempcrete
- Hemp lime
- Lime-hemp infill
- Hemp shiv and lime infill
- Cast hemp lime wall
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Cast infill to the external walls of a timber-framed new building, placed around the frame in shuttered lifts.
- Sprayed application onto a face or into a frame where shuttering the whole wall is impractical.
- Insulating infill to the panels of an existing timber-framed historic building where a vapour-open material has been specified.
- Internal insulating linings cast against solid masonry walls where a professional has designed a vapour-open build-up.
- Insulating floor and roof build-ups laid as a cast layer rather than as fitted boards.
- Non-load-bearing internal wall bodies where thermal mass and moisture buffering are wanted together.
Consideration
Where it is commonly considered
- Where the structure is framed separately and the infill is only ever asked to carry itself.
- Where a jointless wall body is wanted and the absence of fitting tolerances is the attraction.
- Where the programme can genuinely accommodate a drying period that cannot be shortened to suit a date.
- Where a vapour-open finish has been chosen with the material rather than decided later by someone else.
- Where the contractor and the designer have both worked with cast lime-based materials before.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- In any load-bearing role, since the material is placed around a structure rather than acting as one.
- Where the programme cannot absorb a drying period whose length depends on thickness, weather and ventilation.
- Where the wall is to be closed with an impermeable lining or finish, which is a different build-up from the one this material belongs to.
- Where the frame, its fixings and its protection within a wet lime-based material have not been designed.
- Where casting will happen in cold, frosty or persistently wet conditions without the work being protected.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The material is open to water vapour and takes up and releases moisture as conditions change, which is part of why it is chosen. It also arrives wet, so the building carries a large quantity of water until it has dried — two separate moisture subjects that are easily confused with each other.
- Thermal behaviour
- Thermal behaviour is discussed here only as a topic. It depends on the mix, the density placed, the thickness and the state of drying, and any figure belongs to the supplier's documentation and to a calculation for the actual wall.
- Installation dependency
- Mixing consistency, the density to which each lift is tamped, the lift height and the time shuttering stays on all shape the finished wall. There is no way to correct a badly placed section other than cutting it out and recasting it.
- Substrate dependency
- The frame, the shuttering and anything the material is cast against govern the result. How the frame is protected within a wet lime material, and how services and fixings pass through the infill, are designed before casting rather than resolved during it.
- Repairability
- There are no joints, so repairs are cut out and recast rather than replaced as units. A recast patch will differ in colour and density from the surrounding material until it is rendered over.
- Maintenance
- The finish, normally a vapour-open render or plaster, is what is maintained; the wall body behind it is only reached when the finish comes off. Keeping the finish sound is therefore how the wall body is looked after.
- Fire-related questions
- No fire characteristic is stated or implied here. Fire behaviour belongs to the complete wall build-up including the frame, the infill and the finish, and is a matter for the supplier's documentation, a qualified professional and the relevant authority.
Exposure
Exposure and environmental conditions
- What the weather is expected to do during casting and through the drying period, since both frost and persistent rain affect the work.
- How the building will be ventilated while the walls dry, and who is responsible for managing that.
- How exposed each elevation is to driving rain once the finish is on.
- Whether the base of the wall is protected from splash and ground water by the detailing around it.
- How freshly cast work is protected from sun, wind and rain before the finish goes on.
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The frame the material is cast around, its protection within a wet lime mix, and how it is fixed and braced.
- The shuttering arrangement, its stiffness and the sequence in which it is struck and moved up the wall.
- How services, fixings and openings pass through or are formed in a jointless wall body.
- The finish chosen with the material, which needs to be open to vapour for the build-up to work as intended.
- How the wall meets the ground, the roof and the openings, all of which are designed details rather than site decisions.
Appearance
Appearance and finish considerations
- The cast face is irregular and shows the shuttering, the lifts and the tamping, so it is normally rendered or plastered rather than left as found.
- Where a cast face is deliberately left visible, the shuttering becomes the formwork for a finish and needs to be treated as such.
- Wall thickness is set by the casting rather than by a board dimension, and it shows in reveal depth, cill projection and internal proportions.
- A vapour-open render or plaster over the infill governs the finished appearance, and its colour and texture are chosen at that stage rather than earlier.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- How the frame within the infill is expected to behave over time, and how that has been designed for.
- How the wall behaves at its base, where splash and ground water arrive.
- What happens at junctions between the cast body and other materials as the building moves.
- Whether the finish over the infill will remain sound and vapour-open, and what happens if it is later coated with something that is not.
- How fixings made into the wall later would be formed, given that the body is not a board and not a masonry unit.
Upkeep
Maintenance and repair considerations
- How a damaged area would be cut out and recast, and how the patch would be finished to match.
- What cleaning and repair methods are appropriate for the vapour-open finish that covers the wall.
- How a later decorative coating would be assessed before it is applied, given what the build-up depends on.
- What record exists of the mix, the lift sequence and the frame positions behind the finish.
- Who would be asked to assess a problem in a cast lime-based wall body.
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Consistent mixing, since the mix proportions and water content shape the density and the drying.
- Shuttering stiff enough for the tamping, and a striking sequence agreed before work starts.
- The lift height and tamping density agreed with the designer, because both are judged on site and both are permanent.
- Protection of fresh work from frost, sun, wind and rain.
- A drying period managed with ventilation, which is a programme item rather than a pause.
- Coordination with the frame, the services and the finish so that nothing has to be cut into the wall afterwards.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The supplier's description of the shiv and the binder supplied, and the mix they are intended to be used in.
- Guidance on mixing, placing, tamping density and lift heights for the intended application.
- Guidance on drying, ventilation and the conditions under which casting should not proceed.
- Documentation for the finish intended over the infill, and confirmation that it is compatible with the build-up.
- A record of the mix and the sequence actually used, since none of it is visible once the wall is finished.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What is carrying the building, and how does the frame pass through and out of the infill?
- How long is this wall expected to take to dry, what does that depend on, and what is the programme doing in the meantime?
- How is the building ventilated during drying, and who is responsible for managing it?
- What finish is going over the infill, and does the whole build-up stay open to vapour?
- How are services, sockets, fixings and openings formed in a wall body with no joints?
- How is the frame protected where it sits within a wet lime-based material?
- What weather conditions would stop casting, and how is fresh work protected?
- Which parts of this construction need confirming against local requirements, and who is confirming them?
Blind spots
Commonly overlooked points
- It is not a form of hemp fibre batt insulation; it uses a different part of the plant, arrives wet, and is cast rather than fitted.
- It carries nothing but itself, so a separate structure always exists and has to be designed with the infill around it.
- The drying period is a programme item, not a pause, and it governs when the building can be closed up and finished.
- Choosing an impermeable finish later undoes much of the reason for choosing the material in the first place.
- A jointless wall means every service, fixing and opening has to be planned before casting rather than cut in afterwards.
What this page does not do
- This entry states no thermal, structural, moisture or fire value and makes no performance claim.
- The material is placed around a structure rather than acting as one, and nothing here should be read as a structural conclusion.
- Whether a cast lime-based infill suits a particular building is a judgement for a qualified professional who has assessed the structure, the exposure and the programme.
- Build Design Hub does not test, approve, certify, specify or recommend any hemp-lime mix, binder, finish or wall build-up.
Related entries
Common comparisons
Entries met at the same decision but which behave differently, so the comparison is about the difference rather than a swap.
Related entries
Often used alongside
Entries commonly encountered together with this one. Co-occurrence only — never a claim that any combination is compatible or suitable.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
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