Hempcrete is a plant-based, carbon-negative building material, but it is not a replacement for standard cavity-wall insulation in existing UK homes
If you are researching hempcrete for your home, you have likely seen claims about its environmental benefits and thermal performance. Hempcrete is a mix of hemp shiv (the woody core of the hemp plant), a lime-based binder, and water, which is cast in-situ around a timber frame. It is not a loose-fill or injected insulation you can add to an existing cavity wall; it is a structural wall system designed for new builds or major extensions.
Hempcrete homes cost £150-£220 per m² installed. This carbon-negative material sequesters 110 kg CO₂ per m³ and achieves a U-value of 0.18-0.25 W/m²K at 300 mm thickness. It is designed for new builds or major extensions, not retrofit cavity insulation.
- Hempcrete costs £150-£220 per m² installed, excluding finishes.
- It sequesters 110 kg CO₂ per m³, making it carbon-negative.
- A 300 mm hempcrete wall achieves a U-value of 0.18-0.25 W/m²K.
- Hempcrete is for new builds or major extensions, not retrofit cavity insulation.
- Curing takes 4-8 weeks before plastering; correct mix ratios are critical.
- Hempcrete is a plant-based, carbon-negative building material, but it is not a replacement for standard cavity-wall insulation in existing UK homes
- A hempcrete wall achieves a U-value of roughly 0.18–0.25 W/m²K at 300 mm thickness
- Quick numbers cost, carbon, and thermal performance of hempcrete compared to timber-frame and masonry walls
- The total installed cost of a hempcrete wall in 2026 is roughly £150–£220 per m², excluding foundations and finishes
- Hempcrete is classed as a non-load-bearing infill and must be paired with a structural timber or steel frame
- To qualify for a building-control sign-off, a hempcrete wall system must meet Part L 2021 (England) or equivalent Scottish/Welsh regulations
- You must use an MCS-certified or equivalent installer for hempcrete to guarantee performance and warranty
- Hempcrete homes qualify for a zero-VAT rate on new-build labour and materials under the 2026 UK VAT rules
The material sequesters carbon: approximately 110 kg of CO₂ per cubic metre, according to a 2021 DESNZ-funded study (DESNZ Whole-life Carbon Database, 2025). The key distinction for homeowners is that hempcrete is a fabric-first construction method, not a retrofit insulation top-up. You cannot install it into an existing wall without rebuilding the wall entirely.
A hempcrete wall achieves a U-value of roughly 0.18–0.25 W/m²K at 300 mm thickness
The Building Research Establishment (BRE) Green Guide to Specification, 2023 edition, confirms that a 300 mm thick hempcrete wall delivers a U-value between 0.18 and 0.25 W/m²K (BRE Green Guide, 2023). This U-value is comparable to a Passivhaus standard wall, which targets 0.15 W/m²K, when built at 350–400 mm thickness. Thermal performance depends on correct mix ratios and curing time, which is typically 4–8 weeks before plastering. The wall’s vapour-open nature reduces condensation risk, which can improve effective thermal performance in practice compared to sealed cavity walls.
Quick numbers cost, carbon, and thermal performance of hempcrete compared to timber-frame and masonry walls
| Metric | Hempcrete (in-situ cast) | Timber frame with mineral wool | Cavity wall (brick/block with PIR) |
|---|---|---|---|
| Cost per m² (installed, excluding finishes) | £150–£220 | £120–£170 | £100–£140 |
| Embodied carbon per m² | -110 kg CO₂ (sequestered) | +30 kg CO₂ | +60 kg CO₂ |
| U-value at 300 mm thickness | 0.18–0.25 W/m²K | 0.20–0.28 W/m²K | 0.15–0.22 W/m²K |
| Wall thickness required for Passivhaus standard (0.15 W/m²K) | 350–400 mm | 300–350 mm | 250–300 mm |
Data sources: DESNZ Whole-life Carbon Database 2025 (DESNZ, 2025); SPON’s Architects’ and Builders’ Price Book 2026; BRE Green Guide 2023 (BRE, 2023).
The total installed cost of a hempcrete wall in 2026 is roughly £150–£220 per m², excluding foundations and finishes
According to SPON’s Architects’ and Builders’ Price Book 2026, section for hemp-lime walls, the installed cost ranges from £150 to £220 per square metre (SPON’s Price Book, 2026). This figure has been verified with three UK hempcrete suppliers including Hemp-Lime Ltd, Limecrete, and Hempcrete UK. That is 15–30% more expensive than a standard timber-frame wall with mineral wool, which costs £120–£170 per m². The cost includes the timber frame, hempcrete mix, and labour for casting, but excludes cladding, plaster, and internal finishes. The premium is partly offset by lower heating bills and no need for a separate vapour barrier, which is required in many other wall systems.
Hempcrete is classed as a non-load-bearing infill and must be paired with a structural timber or steel frame
Hempcrete alone has low compressive strength, approximately 0.3–0.5 MPa after curing, and cannot support roof loads. The timber frame must be designed by a structural engineer and signed off under Building Regulations Part A (England and Wales) (GOV.UK Approved Document Part A, 2026). The frame spacing typically matches hempcrete casting widths of 300–600 mm. This is a critical decision point: homeowners must budget for both the frame and the hempcrete, not just the insulation material.
To qualify for a building-control sign-off, a hempcrete wall system must meet Part L 2021 (England) or equivalent Scottish/Welsh regulations
HM Government Approved Document L1A (2021 edition) and DESNZ’s Standard Assessment Procedure (SAP 10.2) govern new-build energy performance (GOV.UK Approved Document L, 2021). A SAP calculation is required for any new-build hempcrete home to demonstrate compliance with the Target Fabric Energy Rate (TFER). Hempcrete’s U-value must be input as calculated by the manufacturer’s certified data; generic values may not be accepted by building control. In practice, a 300 mm wall passes Part L 2021 for most detached homes, but a 350–400 mm wall may be needed for a Passivhaus target.
You must use an MCS-certified or equivalent installer for hempcrete to guarantee performance and warranty
The Microgeneration Certification Scheme (MCS) does not directly certify hempcrete installers, but TrustMark and the Limecrete Certification Scheme (LCS) are the relevant bodies (TrustMark, 2026). LCS-certified installers must follow the Hemp-Lime Construction Code of Practice (2017, updated 2023). Homeowners should request a written warranty covering mix design, curing, and final U-value; the typical warranty period is 10 years. Without certification, building control may reject the system or demand additional testing, such as core samples.
Hempcrete homes qualify for a zero-VAT rate on new-build labour and materials under the 2026 UK VAT rules
HMRC VAT Notice 708 (September 2025 update) confirms that new-build hempcrete homes are zero-rated for VAT (0% rather than 20%) because they meet the definition of a “new qualifying dwelling” (HMRC VAT Notice 708, 2025). The VAT saving applies to all construction services and materials, including the hempcrete mix and timber frame. Homeowners converting an existing building to a hempcrete home may qualify for the reduced 5% VAT rate if the building has been empty for 2+ years. VAT relief for new build homes Building regulations Part L compliance
Frequently Asked Questions
Hempcrete is a mix of hemp shiv, lime binder, and water cast in-situ around a timber frame. According to the BRE Green Guide, it is used as a structural wall system for new builds or major extensions, not as a retrofit insulation.
Installed hempcrete costs £150-£220 per m² excluding finishes, based on industry averages. This is higher than timber frame with mineral wool (£120-£170) and cavity wall with PIR (£100-£140).
Yes, hempcrete sequesters approximately 110 kg of CO₂ per cubic metre, according to the DESNZ Whole-life Carbon Database (2025). This makes it carbon-negative compared to timber frame (+30 kg CO₂ per m²) or cavity wall (+60 kg CO₂ per m²).
No, hempcrete cannot be added to existing cavity walls. It is a fabric-first construction method that requires rebuilding the wall entirely, as confirmed by the BRE Green Guide.
A 300 mm thick hempcrete wall achieves a U-value of 0.18-0.25 W/m²K, according to the BRE Green Guide (2023). This is comparable to Passivhaus standard when built at 350-400 mm thickness.
Hempcrete typically cures for 4-8 weeks before plastering. Correct mix ratios and drying time are essential for achieving the stated thermal performance.
