Hempcrete carries no weight. That one fact decides everything else about it — the frame goes up first and the hempcrete is cast around it, so what you end up with is an extremely good coat rather than a structure.
Almost every costly hempcrete mistake starts by reading it as a kind of concrete. It shares a syllable and nothing else.
Hempcrete has a fraction of the compressive strength of even a weak concrete block, and no useful strength in tension at all. It is never the structure.
Cast around a timber frame it becomes insulation, airtightness layer, thermal mass and plaster backing at once — a monolithic wall with no gaps to miss.
By weight, not by volume — hemp shiv is so light that a bucket of it tells you almost nothing. Different jobs want genuinely different mixes.
| Use | Binder : shiv | Water | Density | Feels like |
|---|---|---|---|---|
| Walls | 2 : 1 | ~2.5× shiv | 275–330 kg/m³ | Damp, crumbly, holds a squeezed shape |
| Floors | 3 : 1 | ~3× shiv | 400–500 kg/m³ | Wetter, denser, more binder |
| Roofs and ceilings | 1.5 : 1 | ~2× shiv | 220–275 kg/m³ | Lightest, most insulating, least strong |
Six steps, and the third is the one people get wrong.
The timber frame carries the building and is up, braced and square before any hempcrete is mixed. Studs sit centrally in the wall thickness so the material wraps them on both faces — that wrap is what removes the cold bridge.
Temporary boards either side, usually 600–900 mm tall. You are casting in horizontal lifts and moving the shuttering up, so it comes off the same day.
Fill in loose handfuls and firm it just enough to hold its shape and touch the frame everywhere. Do not compact it. The air is the insulation — tamping hempcrete like concrete is the single most common way to build a wall that performs like a much worse wall.
It comes off within hours, not days. It has to: the face needs air to start drying, and a wall left in its shutter overnight dries from one side only.
Weeks, not days — six to ten for a 300 mm wall in decent weather, and considerably longer in a wet autumn. Keep the rain off and the air moving. It gains strength slowly and goes on doing so for months.
Lime render straight onto the surface, once the wall is genuinely dry through. Rendering a damp wall seals the water in and both layers suffer.
This is the honest disadvantage and it is worth knowing before you start rather than in October. A hempcrete wall is not finished when it is cast; it is finished when it is dry, and drying is weather. A build that would take a fortnight in block takes a season in hempcrete, and the season has to be the right one.
Cast in spring or early summer. Casting into autumn means a wall that goes into winter wet, and a wall that goes into winter wet does not dry until spring.
Compacting the mix drives the air out, and the air was the insulation. A hard-tamped wall can lose a third of its performance while looking neater than the one that works. Firm, not compressed.
A wet mix slumps, takes far longer to dry and cures weaker. The squeeze test takes two seconds and is the only quality control most sites will ever apply. Do it every batch.
Lime does not set below about 5 °C, and frost on a green wall destroys the surface. Cast in spring or early summer, not in the last dry week of October.
Cement is far less permeable than what is behind it, so it traps the moisture the wall is designed to move. It also cracks against a softer background and lets water in behind itself. Lime, always.
No. Something has to carry the roof and the floors, and hempcrete will not. The frame is not optional or an extra — it is the building, and the hempcrete is what you wrap it in.
300 mm is the usual starting point for a heated room and 350–400 mm is common where the standards are tighter. Below about 250 mm you are giving up the thing you came for.
Not on the hempcrete itself. Anything with weight goes into the frame, so noggins go in before casting, at the heights you will want them. That list is much harder to make after the wall exists.
It performs very well in fire — the lime coating and the low density mean it chars and resists rather than burns. It is still a building element that must be assessed as part of a whole wall, not a product with a rating of its own.
Many people do, and it is one of the more forgiving materials to learn on because mistakes are visible before they are permanent. Hire a forced-action mixer; a drum mixer will not mix it properly and you will spend the day fighting it.