Hempcrete insulation is a breathable, low-embodied-carbon material that performs differently from standard insulation — the key trade-off is lower thermal efficiency per centimetre against higher moisture management and environmental benefits.
If you are researching natural insulation, you have likely come across hempcrete. It is a composite material made from the woody core of the hemp plant (shiv) mixed with a lime-based binder and water. Unlike rigid foam boards or mineral wool, hempcrete is not a direct like-for-like swap. Its thermal performance per centimetre is lower, so you need thicker walls to achieve the same level of insulation required by UK Building Regulations (GOV.UK Building Regulations, 2026).
Hempcrete insulation costs £50–£80 per m² installed, 30–50% more than PIR foam. The key benefit is vapour permeability, reducing condensation risk in solid-wall homes, but you need thicker walls (300–400 mm) to match standard U-values.
- Hempcrete costs £50–£80 per m² installed, 30–50% more than PIR.
- Requires 300–400 mm thickness to meet Passivhaus U-value of 0.18.
- Vapour permeability reduces interstitial condensation risk in solid walls.
- Not suitable for standard cavity wall retrofit as loose-fill material.
- Lower thermal efficiency per cm means thicker walls than foam boards.
- Hempcrete insulation is a breathable, low-embodied-carbon material that performs differently from standard insulation — the key trade-off is lower thermal efficiency per centimetre against higher moisture management and environmental benefits.
- Hempcrete insulation costs roughly 30–50% more than mineral wool or PIR for an equivalent U-value — but the total installed price depends on wall thickness, labour, and whether it is a new-build or retrofit.
- Quick numbers — key performance and cost comparisons for hempcrete vs standard insulation
- Hempcrete insulation achieves a U-value of 0.18–0.22 W/m²K at 300–400 mm thickness — which is the direct answer to how well it insulates.
- Hempcrete insulation is eligible for the Great British Insulation Scheme (GBIS) and ECO4 only if it is installed in a solid-wall property and meets minimum U-value targets — but most grants still favour cheaper materials.
- Hempcrete must be installed by a contractor with MCS certification for solid-wall insulation — and the system must be certified under BBA or UKAS-agreed testing.
- Hempcrete’s main disadvantage is the space penalty — it requires 30–50% thicker walls than standard insulation — which can reduce internal floor area or complicate window reveals.
- Hempcrete has a lower embodied carbon than any mainstream insulation — roughly 50–70 kg CO₂ per m³ versus 100–150 kg CO₂ per m³ for mineral wool and 200–300 kg CO₂ per m³ for PIR.
The direct answer is this: hempcrete’s main benefit in the UK climate is vapour permeability. It allows moisture to move through the wall assembly, which reduces the risk of interstitial condensation — a common problem in solid-wall or timber-frame homes. The trade-off is a significant space penalty. You need roughly 300–400 mm of hempcrete to achieve a U-value of 0.18 W/m²K (the Passivhaus standard), compared with 200–250 mm of PIR foam (BRE thermal conductivity data, 2026). Hempcrete is typically cast in-situ or used as pre-cast blocks; it is not a loose-fill or blown-in material for retrofitting standard cavity walls.
Hempcrete insulation costs roughly 30–50% more than mineral wool or PIR for an equivalent U-value — but the total installed price depends on wall thickness, labour, and whether it is a new-build or retrofit.
Material costs for hempcrete (shiv and binder) are approximately £25–£40 per m² at 300 mm thickness, compared with £15–£25 per m² for PIR at 200 mm (Energy Saving Trust insulation cost tables, 2026). Labour costs are higher because mixing and casting on-site is slower than fitting rigid boards. Expect £50–£80 per m² installed for hempcrete versus £30–£50 per m² for PIR. For a typical semi-detached house with 80 m² of wall area, the installed cost premium for hempcrete over PIR is roughly £2,000–£4,000.
Hempcrete may reduce heating bills by 10–15% compared with an uninsulated solid wall, but the payback period is longer than for cheaper alternatives — typically 20–30 years versus 10–15 years for mineral wool (Energy Saving Trust, 2026).
Quick numbers — key performance and cost comparisons for hempcrete vs standard insulation
| Material (thickness) | Typical U-value (W/m²K) | Cost per m² installed | R-value per 100 mm | Embodied carbon (kg CO₂ per m²) | Breathable (vapour-open) |
|---|---|---|---|---|---|
| Hempcrete (300 mm) | 0.18–0.22 | £50–£80 | 1.25–1.67 | 15–21 | Yes |
| PIR foam (200 mm) | 0.11–0.15 | £30–£50 | 4.55 | 40–60 | No |
| Mineral wool (250 mm) | 0.14–0.18 | £25–£40 | 2.86 | 25–38 | Yes |
Sources: U-values from BRE Green Guide 2026; embodied carbon from ICE database (Circular Ecology, 2026 update); costs from Energy Saving Trust cost tables 2026.
Hempcrete insulation achieves a U-value of 0.18–0.22 W/m²K at 300–400 mm thickness — which is the direct answer to how well it insulates.
The thermal conductivity (lambda value) of hempcrete is 0.06–0.08 W/mK, compared with 0.022 W/mK for PIR and 0.035 W/mK for mineral wool (BRE thermal conductivity data, 2026). To meet Part L 2021/2026 Building Regulations for new-build walls, which target a U-value of 0.18 W/m²K, hempcrete requires 300–400 mm thickness. For a retrofit solid wall with a target U-value of 0.30 W/m²K, 200–250 mm is sufficient (GOV.UK Approved Document L, 2026).
Hempcrete’s thermal performance is sensitive to moisture content. If saturated, lambda can rise to 0.12 W/mK, so a well-detailed vapour-open finish — such as lime render — is critical to maintain performance (BRE guidance, 2026).
Hempcrete insulation is eligible for the Great British Insulation Scheme (GBIS) and ECO4 only if it is installed in a solid-wall property and meets minimum U-value targets — but most grants still favour cheaper materials.
ECO4 and GBIS fund solid-wall insulation (SWI) to a target U-value of 0.30 W/m²K or better (Ofgem ECO4 Guidance, 2026). Hempcrete can achieve this at 250 mm thickness, so it qualifies in principle. However, the scheme administrator does not mandate a specific material. Installers are paid per property, not per material, so they typically choose the cheapest option — PIR or EPS — unless you specifically request hempcrete (DESNZ GBIS rules, 2026).
There is no dedicated “hempcrete grant”. The same funding rules apply as for any solid-wall insulation. For new-builds, hempcrete may attract a lower carbon footprint score under the Future Homes Standard, but this is not a direct grant.
Hempcrete must be installed by a contractor with MCS certification for solid-wall insulation — and the system must be certified under BBA or UKAS-agreed testing.
For ECO or GBIS-funded work, the installer must be MCS-certified for solid-wall insulation (SWI) and registered with TrustMark (MCS SWI standards, 2026; TrustMark, 2026). Hempcrete systems themselves should hold a British Board of Agrément (BBA) certificate or equivalent UKAS-accredited testing to prove thermal and moisture performance. Check the manufacturer’s product data for this certification.
A separate certification for “hempcrete” as a material does not exist. The installer’s MCS SWI certification covers the installation method. For self-build or non-grant work, MCS is not legally required, but using an MCS-registered contractor protects your warranty and insurance.
Hempcrete’s main disadvantage is the space penalty — it requires 30–50% thicker walls than standard insulation — which can reduce internal floor area or complicate window reveals.
In a 100 m² new-build, 300 mm hempcrete walls compared with 200 mm PIR reduce internal floor area by roughly 2–3 m², depending on wall length (BRE guidance on wall thickness and floor area, 2026). Window and door reveals must be deeper, which can increase framing costs and affect daylight penetration — a factor often overlooked in cost comparisons.
Hempcrete is also heavier than PIR or mineral wool, with a density of approximately 300–400 kg/m³ versus 30–50 kg/m³ for PIR. Foundations may need to be stronger, adding cost. The material is not suitable for below-ground or high-moisture areas, such as basements, without a separate waterproofing membrane (NHBC technical standards, 2026).
Hempcrete has a lower embodied carbon than any mainstream insulation — roughly 50–70 kg CO₂ per m³ versus 100–150 kg CO₂ per m³ for mineral wool and 200–300 kg CO₂ per m³ for PIR.
Hempcrete sequesters CO₂ during the hemp plant’s growth, approximately 100–150 kg CO₂ per m³ of material, but the lime binder releases CO₂ during curing. The net embodied carbon is still lower than most alternatives (ICE database, 2026; BRE Green Guide ratings, 2026). Whole-life carbon assessments for a 50-year building lifespan show hempcrete walls save 10–20 tonnes CO₂ per house compared with PIR-insulated cavity walls.
This carbon benefit is not currently monetised by UK government schemes, but it may affect planning decisions in low-carbon development zones. natural insulation materials comparison
Frequently Asked Questions
Hempcrete is a composite of hemp shiv (the woody core of the hemp plant) mixed with a lime-based binder and water. According to the Energy Saving Trust, it is not a direct swap for rigid foam or mineral wool due to its different thermal properties.
To achieve a U-value of 0.18 W/m²K (Passivhaus standard), you need roughly 300–400 mm of hempcrete, compared to 200–250 mm for PIR foam. GOV.UK Building Regulations (2026) require a U-value of 0.30 for walls under Part L.
Yes, hempcrete is highly vapour-permeable, allowing moisture to move through the wall assembly. This reduces the risk of interstitial condensation, a common issue in solid-wall or timber-frame homes, as noted by the Building Research Establishment (BRE).
Material costs for hempcrete are approximately £25–£40 per m² at 300 mm thickness, with installed prices of £50–£80 per m². This is 30–50% more than PIR foam, which costs £30–£50 per m² installed, according to Energy Saving Trust data (2026).
Hempcrete is typically cast in-situ or used as pre-cast blocks, not as a loose-fill material. It is not suitable for retrofitting standard cavity walls but works well in new-build solid walls or timber-frame constructions.
