Understanding how hempcrete dries and cures is one of the most important aspects of planning and executing a successful build. Get the timeline wrong, apply finishes too early, or add too much water during mixing, and you can extend your programme significantly or compromise the wall altogether. This article walks through what actually happens after hempcrete is placed, what "drying" really means for a lime-based material, and how to plan your schedule around it.
Setting, drying, and carbonation: three distinct stages
These terms are often used loosely, but they describe different things, and confusing them leads to problems on site.
Setting
Setting refers to the initial hardening of the binder after the hempcrete has been placed. Most hempcrete binders have a relatively short set time, typically overnight. After this period, the cast material has gained enough strength to support its own weight, meaning the shuttering can be removed. This isn't the same as the wall being dry or structurally complete; it simply means it can stand without formwork.
Removing the shuttering promptly is important, since it allows the drying process to begin. Leaving shuttering in place unnecessarily traps moisture against the wall face and delays progress. See The Basic Build Process for the trade-off around permanent shuttering specifically, where the formwork is deliberately left in place.
Drying
Once the shuttering is off, the wall begins to dry: the evaporation of excess water introduced during mixing. It typically takes several weeks before the wall is dry enough to accept finishes, and this timeline is highly sensitive to several variables:
- Time of year and ambient temperature: warmer conditions accelerate drying; cold and damp conditions slow it significantly
- Wall thickness: thicker walls contain more moisture and take longer to dry through
- Binder type: different binders have different setting and drying characteristics; a slow-setting binder extends the timeline
- Weather conditions during and after casting: an exceptionally wet summer, for example, can push finishing works back by months
Critically, the relationship between wall thickness and drying time isn't linear. You can't simply double the expected drying time for a wall twice as thick; the actual time required may be considerably more. This makes early planning essential.
A hempcrete wall also never fully dries out, and that's entirely normal: it settles at a resting moisture content that fluctuates gently with the surrounding environment, a feature of hempcrete's hygroscopic properties rather than a defect. See Moisture, breathability and vapour management for the full picture of what that equilibrium looks like and why it matters.
From our build: We used permanent shuttering (wood-wool board) on the internal face, thinking it would let us start plastering sooner. It turned out not to matter either way: by the time the roof was on and the outside was rendered and painted, the hempcrete was already dry. If your programme has other trades taking a similar amount of time, permanent shuttering may not save you anything, and it can roughly double drying time in some cases. Worth factoring in before you commit to it.
Carbonation
Carbonation is distinct from drying, though the two overlap in the early life of the wall. It's the chemical process by which the lime binder gradually reabsorbs carbon dioxide from the atmosphere, converting back to calcium carbonate and hardening over time. This long-term curing process continues for months and years after the wall is built. A new-build community centre referenced in The Hempcrete Book, cast during an exceptionally wet summer with a slow-setting binder, wasn't rendered until a full year after the hempcrete was cast.
Carbonation is also part of what makes hempcrete carbon-sequestering: the lime binder re-absorbs CO₂ as it cures, offsetting a portion of the emissions produced during its manufacture.
Why drying times are so critical to your project schedule
Slow or problematic drying is the most commonly reported issue with hempcrete builds, and the evidence points clearly to one primary cause: insufficient planning. Most problems arise not from any inherent weakness in the material, but from builders who are unfamiliar with lime and natural plant-based materials, and who haven't given adequate thought to timing, weather, and drying management before work begins.
Well-documented examples, including cases that have appeared on television, involve novice builders who failed to account for drying time in their programme, leading to delays and, in some cases, applying finishes before the wall was ready.
The practical implication is straightforward: drying parameters need to be established at the very start of project planning, not halfway through the build. This means making decisions about:
- Material choice: binder type and its expected set and dry times under your local conditions
- Wall specification: thickness has a direct impact on drying duration
- Programme sequencing: knowing when plastering, cladding, or other finishing works can realistically begin
- Contingency: building time buffers into the schedule, particularly for builds in cooler or wetter climates
One useful note: if you're using a vented cladding or rain screen finish rather than a direct render, drying time becomes less critical. The vented cavity continues to draw air across the face of the hempcrete wall, assisting drying, and surface staining, a common concern with direct renders applied too early, doesn't affect a cladding finish.
Measuring moisture and knowing when to apply finishes
Judging readiness for finishing works isn't a matter of guesswork or elapsed time alone. Moisture readings should be taken to confirm the wall has reached a suitable moisture content before any finish is applied. Applying finishes prematurely is one of the more consequential mistakes on a hempcrete build: lime render in particular needs a stable substrate, and trapping moisture behind a render coat can cause staining, adhesion failure, or prolonged drying problems that become expensive to rectify.
Hempcrete is a material that rewards careful planning and patience. Built well, with realistic timelines and appropriate management of the drying environment, it performs exceptionally. The problems that make headlines almost always trace back to insufficient preparation, not to the material itself.
Sources
- Stanwix, W. & Sparrow, A., The Hempcrete Book