Technical

The detail
behind the wall.

U-values, build-ups, finishes and Building Regulations — the technical detail our PassivHaus-certified team works to on every project.

The material

What is
hempcrete?

Hempcrete is a mixture of hemp shiv (the pithy woody centre of the hemp plant stalk), water, and a lime binder. The mixture is created on site and either installed within formwork to form a new wall, or sprayed directly against an existing wall for internal or external insulation.

Hempcrete has excellent moisture buffering properties because it is vapour-open and hygroscopic — it actively manages moisture rather than resisting it. It also has a high thermal mass relative to its physical mass, which means it provides insulation while also reducing the risk of overheating.

Thermal performance

What is a
U-Value?

A U-Value is the rate of heat flow in Watts through 1m² of structure when there is a temperature difference of 1 degree (K or °C) from inside to outside. The lower the U-Value, the more insulated the wall and the less heat is lost.

What U-Values does hempcrete achieve for Building Regulations?

For retrofit, existing walls being insulated need a U-Value of between 0.3W/m²K and 0.7W/m²K. For new build, walls need to achieve 0.18W/m²K. Hempcrete at 100mm typically meets retrofit requirements; new build walls need approximately 400mm around a timber frame.

Application Hempcrete thickness Approximate U-Value
Retrofit IWI (existing solid wall) 80mm ~0.45 W/m²K
Retrofit IWI (existing solid wall) 100mm ~0.35 W/m²K
New build (timber frame) 300mm ~0.25 W/m²K
New build (timber frame) 400mm ~0.18 W/m²K

Adyta uses a thermal conductivity of λ = 0.07 W/m.K — conservative compared with the BBA-certified λ = 0.06 W/m.K from Lime Technology Tradical® Hemcrete®.

The 0.07 figure accounts for binder variation, installation variation, and spray vs hand-cast application.

Actual U-Value depends on the existing wall construction, substrate, and external conditions.

Building regulations: Part L governs U-value requirements. Retrofit: 0.3–0.7 W/m²K. New build: 0.18 W/m²K.

Approved Document L — GOV.UK ↗

Finishing hempcrete

Does it need
plastering?

Yes — we recommend lime or clay plaster. Unplastered hempcrete is friable and not airtight; air movement through the material reduces its insulation performance. Make hempcrete airtight including around service penetrations and window junctions. We also recommend considering mechanical ventilation — it improves both thermal comfort and energy performance.

Allow 4–6 weeks drying time after installation before applying plaster. Hempcrete continues to dry over a long period, so all finishing plasters and paints must be vapour-open to allow this to continue.

Internal finishes

Avoid NHL (Natural Hydraulic Lime) internally — it is harder than the lime binder in the hempcrete. Recommended products:

Breathable paint brands: KEIM, Earthborn, Graphenstone, AURO. Look for an SD value below 0.1. Send us any paint options you're considering and we'll check.

External finishes

In exposed or very wet locations, conceal hempcrete under cladding to protect against wind-driven rain. If rendering externally, use NHL 2 maximum (NHL2 has minimum hardness after 28 days) — preferably air lime or non-hydraulic lime.

Suggested external spec: 12mm Warmcote + 10mm Limecote + KEIM paint or limewash, with mesh at midpoint and stress points as per standard plastering detail. Talk to lime plasterers and your architect before finalising external finishes.

Got a technical question?

Talk to
our team.

Build-up queries, moisture risk, U-value calculations — Annie and the team are happy to talk through the detail before you commit to a specification.

Get in touch

info@adytabuilds.com · @adyta.builds on Instagram · East Anglia & UK-wide