Hempcrete behaves differently from most modern construction materials, and nowhere is that difference more consequential than in its relationship with moisture. Understanding how hempcrete manages water vapour matters for anyone specifying, building with, or living in a hempcrete structure, since it underpins the healthy indoor environment the material is known for.
What breathability actually means
"Breathability" is a term that gets used loosely in construction, but in the context of hempcrete it describes something precise: the ability of the wall to allow water vapour to pass through it, absorb it temporarily, and release it again, without being damaged by that process.
This behaviour stems from two related properties: vapour permeability and hygroscopicity.
- Vapour permeability refers to the open matrix structure of hempcrete, which allows water vapour molecules to move through the wall rather than being blocked or trapped.
- Hygroscopicity refers to the ability of the hemp shiv to attract and hold water molecules from humid air, and to release them again when the surrounding air dries out.
Both properties are characteristic of organic materials, materials derived from living things, which have an inherent affinity with moisture. Hemp shiv is no exception: its porous surface allows water vapour to condense on the interior surface of its pores when humidity is high, and reverses that process as humidity falls. That's the material working as intended, not a defect to design around.
How hempcrete manages vapour within a wall
When relative humidity in the air adjacent to a hempcrete wall rises, the wall absorbs water vapour. Some of this vapour condenses to form liquid water on the interior surfaces of the pores within the hemp shiv. This continues until those pores are saturated. When relative humidity subsequently drops, the water evaporates back out of the wall and into the air.
This cycle of absorption and release has a critical practical consequence: it inhibits the formation of condensation on the wall surface, and therefore suppresses the conditions in which mould and fungi can establish themselves. In conventional buildings constructed from non-porous synthetic materials, water vapour that cannot be absorbed tends to find cold surfaces and condense there, creating the familiar problems of surface mould, damp patches, and degraded air quality. Hempcrete's hygroscopic capacity disrupts that process.
A hempcrete wall will never be completely dry in the way a synthetic material might be described as dry. A natural equilibrium, sometimes called the resting moisture content, is always present. After drying following construction, a hempcrete wall typically settles at around 14–16% moisture content. This figure fluctuates slightly in response to changing humidity levels both inside and outside the building, driven by weather externally and human activity internally. That variation is the material doing exactly what it's supposed to do, not a sign of a damp problem. See Hempcrete drying and curing for how this equilibrium is reached in the first place.
The link between moisture and thermal performance
Hempcrete's moisture behaviour doesn't operate in isolation from its thermal performance; the two are closely intertwined, a phenomenon known as hygrothermal behaviour. Because water conducts heat roughly 20 times better than still air, moisture condensing within the wall's pores changes its thermal conductivity in real time, which is part of why a hempcrete wall's real-world thermal performance doesn't match a single static U-value calculation. Hempcrete thermal performance and U-values covers this interaction, and the research behind it, in full.
Why this matters for indoor air quality and occupant health
The moisture-buffering capacity of hempcrete has direct implications for occupant health. Indoor relative humidity isn't merely a comfort issue; it has measurable effects on respiratory health and susceptibility to infection.
The Hempcrete Book cites an optimal indoor relative humidity range of 40–60%. Outside this range:
- Below 40% RH: increased risk of allergies, asthma, and respiratory irritation
- Above 60% RH: increased risk of mould, fungi, and dust mites
- At either extreme: elevated risk of bacterial or viral respiratory infections
By absorbing excess moisture when humidity rises and releasing it when humidity falls, hempcrete walls act as a passive buffer, keeping indoor humidity within the healthy range without mechanical intervention, something non-hygroscopic synthetic insulation simply can't replicate.
Hempcrete also contains no toxic materials and doesn't off-gas chemicals into the indoor atmosphere, unlike some synthetic insulation products. Combined with its humidity-regulating properties, that's a real benefit for indoor air quality.
Working with the material, not against it
Two things follow directly from how hempcrete handles moisture. First, never seal it: vapour-impermeable membranes, renders, or paints will block the hygrothermal cycle and can trap moisture, causing damage, so every finish, internal and external, needs to be breathable and compatible with the wall's vapour-open nature. Second, give it time: the wall needs to dry sufficiently before it's enclosed or finished, after which the resting moisture content will continue to fluctuate gently as a normal, permanent feature of the material rather than something to chase down and eliminate.
Sources
- Stanwix, W. & Sparrow, A., The Hempcrete Book