The Basic Build Process

The Basic Build Process

Hempcrete construction follows a broadly similar sequence whether you're doing a new build or a retrofit. Here's an overview of each stage.

Stage 1: Frame Construction

Because hempcrete is not load-bearing, the structural frame comes first. Common frame types include:

• Timber stud frame: the most popular choice; hempcrete fills between the studs
• Post-and-beam timber frame: hempcrete wraps around the posts inside formwork
• Steel frame: less common but used for larger commercial builds

The frame needs to carry hempcrete at its heaviest, which is at the point of casting, before the wall has had any chance to dry. A freshly cast 300mm wall typically weighs 200–400 kg/m² wet, depending on mix and moisture content. Once dried and cured, that same wall settles at closer to 60–120 kg/m², reflecting hempcrete's typical dry density of 200–400 kg/m³. Design the frame and fixings around the wet figure; the dry weight is what the building actually lives with long-term.

Watch it in practice: Building the timber frame, from our own 2022 build.

Foundation and Damp Protection

Hempcrete must be kept off the ground. The base of any hempcrete wall should sit at least 200–300mm above external ground level on a moisture-resistant plinth: stone, brick, or concrete. A physical damp-proof membrane between the plinth and the first lift of hempcrete is strongly recommended.

Stage 2: Mixing

Equipment

Two types of mixer are used in practice, and the right choice depends on scale. Forced-action (paddle) pan mixers are the professional standard for larger projects, since they give the most consistent results batch after batch. But a standard drum mixer (typically 100–130 litres) is widely and successfully used on smaller self-build projects, and it's what many self-builders reach for first simply because it's what's already on site. The key with a drum mixer is care over batch size and mixing time rather than avoiding it altogether. See How to mix and apply hempcrete for the full step-by-step comparison of both mixer types and both mixing sequences.

Process

1. Add the lime binder to the dry drum
2. Add a measured amount of water (approximately 0.5–0.6 litres per kg of binder)
3. Mix for 30–60 seconds until the binder is hydrated
4. Add the hemp shiv, typically 2 parts shiv to 1 part binder by volume
5. Mix for 2–3 minutes until every shiv particle is coated and glistening
6. Check: the mix should hold together when squeezed but not release lime water

This binder-slurry method (binder and water first, then shiv) is one of two valid mixing sequences. The alternative, dry-blending the hemp shiv and binder together before adding water, is covered in detail in the mixing guide. Either approach can produce a good mix; what matters is getting an even coating with no dry pockets.

Do not over-mix. Excessive mixing degrades the hemp fibres.

Watch it in practice: How to hempcrete shows the mix ratio and a drum mixer in use on a real build.

From our build: We used a timber frame with permanent shuttering on the interior, on the thinking that it would let us start internal plastering sooner. In practice it turned out not to be necessary: by the time the roof was on and the outside was rendered and painted, the hempcrete in the walls was already dry. Worth knowing before you commit to permanent shuttering purely for programme reasons: it can roughly double drying time in some cases, so weigh that against how much time your other trades will realistically take anyway.

Mix Ratios by Application

This table is the reference ratio set used across HempcreteHub; How Hempcrete Works refers back to it rather than quoting a separate figure.

Stage 3: Casting

Formwork

Hempcrete is typically cast in lifts of 200–300mm between temporary formwork panels. The formwork is:

• Clamped or strapped to the frame
• Removed after 24–48 hours (or kept in place longer in cold weather)
• Moved up to cast the next lift

Plywood, OSB, or proprietary hempcrete formwork systems can all be used. Ensure the formwork is lightly oiled or lined with polythene to prevent sticking.

Temporary vs permanent shuttering. The sequence above assumes temporary formwork, removed and reused as each lift cures. Some builders instead leave shuttering permanently in place, most commonly on an internal face using a wood-wool board, on the logic that it lets internal plastering start sooner without waiting on the hempcrete to dry through. That logic doesn't always hold up in practice: permanent shuttering can roughly double drying time in some cases, and depending on your programme, the rest of the build (roofing, external render, decoration) may already take long enough that the wall dries out regardless. Weigh this against your actual programme rather than assuming permanent shuttering saves time by default. Wood-wool board also then serves as the plaster carrier board on that face; see Hempcrete and lime plaster for how plastering onto a carrier board differs from plastering directly onto cast hempcrete.

Watch it in practice: Permanent shuttering with wood-wool board, from our own 2022 build.

Placement and Compaction

Hempcrete is placed by hand or with a shovel and tamped lightly, not rammed. The goal is to eliminate voids without compressing the material, which would destroy its insulating properties. A typical tamping board or gloved fist works well.

Spraying

For larger projects, hempcrete can be spray-applied using modified plastering equipment. This is faster for large wall areas but requires more skill and specialist kit.

Watch it in practice: Spray-applying hempcrete, a collaboration with Adyta Builds.

Stage 4: Curing and Drying

After casting:

1. Remove formwork after 2–7 days
2. Protect from rain for the first few weeks; a breathable membrane or temporary cover is sufficient
3. Maximise ventilation, opening windows and doors to allow moisture to escape
4. Avoid heating artificially for the first 4–6 weeks; too much heat causes surface cracking

A standard 300mm wall should be sufficiently cured for rendering after 6–12 weeks in good drying conditions. In cold or damp weather, allow longer. See Hempcrete drying and curing for a fuller picture of what's happening inside the wall during this stage.

Stage 5: Finishing

Hempcrete is typically finished with a breathable lime render, never cement render, which would trap moisture inside the wall.

External Render

• 2–3 coat lime render system, 15–25mm total
• Scratched between coats for mechanical key
• Coloured lime wash or silicate paint as a finish

Internal Finish

• 1–2 coat lime plaster, 8–15mm
• Lime wash, distemper, or breathable paint

Window and Door Reveals

Pay careful attention to the junction between hempcrete and the frame at reveals. A metal or timber bead provides a clean edge, and the render should lap onto the frame with a flexible joint.

Common Mistakes to Avoid

• Too much water in the mix: causes the shiv to float and creates weak spots
• Over-compacting: destroys the insulating air pockets
• Rushing the cure: applying render too early traps moisture and causes cracking
• No damp protection at the base: leads to rising damp and deterioration
• Cement render or cladding: prevents the wall from breathing

Summary

Hempcrete construction isn't complicated, but it doesn't reward rushing. Protect the base from moisture, get the mix right before you cast, give the wall time to cure, and finish with breathable materials.

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Sources

• Stanwix, W. & Sparrow, A., The Hempcrete Book