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How Hempcrete Works

6 min read

The science behind hempcrete — how hemp and lime combine, how it cures, and what makes it an effective insulating material.

How Hempcrete Works

Understanding hempcrete requires a little chemistry, a little physics, and an appreciation for slow processes. It behaves differently from almost every other building material you may have encountered.

The Chemistry: Lime Carbonation

The core process that makes hempcrete work is lime carbonation. When you mix lime binder with water, the calcium oxide (CaO) in the lime reacts with water to form calcium hydroxide (Ca(OH)₂). This initial slaking reaction generates heat.

Over time, the calcium hydroxide reacts with CO₂ in the air:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

This produces calcium carbonate, essentially limestone. As the lime carbonates, it:

  1. Binds the hemp shiv particles together
  2. Hardens the matrix progressively over weeks and months
  3. Absorbs CO₂ from the atmosphere in the process

This is why hempcrete is described as carbon negative: the hemp stored carbon as it grew, and the lime absorbs more as it cures. See Is hempcrete carbon negative? for the fuller picture, including the caveat that not every mix qualifies.

The Mix

The lime-to-shiv ratio varies by application. As a starting point, roughly 1 part lime binder to 1.5–2 parts hemp shiv by volume is typical for external walls, with proportionally more shiv relative to binder for internal walls and roof insulation, since a lower density suits those applications better. The Basic Build Process has the full ratio table broken down by application; this article uses that same reference range rather than a separate figure.

Water is added just enough to coat the shiv without pooling. The exact ratio ultimately comes down to binder type, shiv grade, and your manufacturer's guidance. Getting it right matters: too much lime binder makes the mix heavy and unnecessarily expensive, too little and it won't bind properly.

Curing and Drying

Hempcrete does not "dry" the way concrete or plaster does. It undergoes a two-stage process:

Stage 1: Initial Set (Days 1–28)

The free water evaporates and the lime begins to harden around the shiv. The wall gains enough strength to be de-shuttered after 2–7 days, but remains fragile.

Stage 2: Carbonation (Months to Years)

Lime carbonation continues for months to years, progressively strengthening the wall. A hempcrete wall is effectively still curing for its first year, which is why good ventilation during and after construction is essential.

Thermal Performance

Hempcrete is primarily valued as an insulating material. Its thermal performance comes from:

  1. Low density: the air trapped within the hemp fibres and lime matrix reduces heat transfer
  2. High thermal mass: the calcium carbonate matrix absorbs and releases heat slowly, damping temperature swings
  3. Phase-change buffering: moisture absorption and release in the wall moderates both temperature and humidity

Typical lambda values (thermal conductivity) range from 0.06 to 0.12 W/m·K depending on density and moisture content. That's noticeably higher, meaning worse, than dedicated insulation boards like wood fibre, which typically sit around 0.038 to 0.045 W/m·K. Thickness for thickness, purpose-built insulation outperforms hempcrete on this figure alone. What closes the gap is that a hempcrete wall provides insulation across its full thickness, with no separate structural layer eating into that depth, so the whole-wall U-value ends up competitive even though the raw lambda value doesn't win on paper. See Hempcrete thermal performance and U-values for the fuller picture, including why real buildings tend to outperform these steady-state figures.

Breathability and Moisture Management

One of hempcrete's most important properties is its vapour permeability. Unlike synthetic insulation with vapour barriers, hempcrete walls breathe:

  • Water vapour can pass through the wall
  • The hemp shiv absorbs excess moisture and releases it when conditions change
  • This reduces condensation risk and improves indoor air quality

This hygroscopic behaviour is sometimes called moisture buffering, and it's why hempcrete buildings typically have stable indoor humidity levels without mechanical ventilation.

What Hempcrete Cannot Do

Hempcrete is not suitable for:

  • Direct load-bearing: it must always be combined with a structural frame
  • Below-ground use: it must be kept off wet soil and away from standing water
  • Thin sections: it requires a minimum of roughly 100mm to perform; thinner applications lack structural coherence

Summary

Hempcrete works through a combination of carbonation chemistry, hygroscopic physics, and low-density thermal buffering. It's a slow material by design, one that gets stronger and more stable over time, provided it's protected from prolonged moisture and given time to breathe.

Next: The Basic Build Process →

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