Hemp yields long fibres and the woody core of the stalk, called shiv. The shiv, mixed with a lime binder, makes hempcrete. The fibres, thermobonded into mats, make insulation batts.[1][4] The useful question is not whether hemp is "green" but which hemp product, with which data, fits a given job. This guide says which family each number refers to and tests the common hempcrete claims against verifiable sources.
What are hemp and hempcrete?
According to BRE, industrial hemp is planted in April and harvested at the end of August in the northern hemisphere, reaches 3–4 m, and needs no pesticides or herbicides after planting. The fibres go to textiles and composites; the shiv is shredded and graded for construction.[1]
Hempcrete (hemp-lime) is most commonly hemp shiv, a formulated lime binder and water, cast in formwork, sprayed or pressed into blocks. It is a non-structural infill used with a frame.[1] Hemp-fibre insulation is batts and mats of fibre, held together by polymer or bio-polymer binder fibres and treated for fire protection, and goes between structural members.[4][5]
Is hempcrete carbon negative?
Sometimes. It depends on which life-cycle stages are counted.
What a verified EPD reports
IsoHemp's third-party-verified B-EPD for its 36 cm hempcrete block declares a GWP-total of −17.38 kg CO₂e per m² of wall in A1–A3. In the EPD's tables, about −62 kg CO₂e of biogenic carbon in the hemp is offset by about +62 kg CO₂e of fossil emissions in raw-material supply (A1). The A1–A3 total also takes credit for lime carbonation during curing (13.91 kg CO₂ per m²) and for the biogenic carbon in the wooden pallets (11.54 kg CO₂ per m²). Counting installation and early carbonation (A1–B1) gives −11.45 kg CO₂e/m². The full cradle-to-grave result (A1–C4) is +64.42 kg CO₂e/m², because the standard treats biogenic carbon as re-emitted at end of life and the EPD assumes landfill, a scenario it describes as an assumption.[3]
What peer-reviewed studies say
Ip and Miller modelled a 1 m², 300 mm hemp–lime wall with timber frame over 100 years and reported 82.7 kg CO₂ sequestered and a net life-cycle reduction of 36.08 kg CO₂e.[8] Arrigoni and colleagues, assessing a block wall without end of life, found raw-material production the main source of impact, with transport distance and binder amount and composition changing results considerably. Their X-ray tests showed slower carbonation than earlier studies assumed: after 240 days only the outer layers were significantly carbonated. They still found a negative footprint.[7]
Why the binder matters
Quicklime is made by heating limestone in kilns, which releases CO₂, and formulated hempcrete binders often add cement in quantities producers do not always disclose.[1] The IsoHemp EPD assumes 85% of the lime's theoretical carbonation, the cap in EN 16757 Annex G (IsoHemp itself estimates 90%), and relies on the manufacturer's estimate that 61% of the lime is carbonated after the two-month cure.[3] We read the carbonation credit as the least certain term. Ask for the binder composition and the EPD, not the slogan.
How well does hempcrete insulate?
Thermal conductivity depends on density
BRE reports that commercial hemp-lime walls tend to achieve dry densities of 270–330 kg/m³ and λ of 0.07–0.09 W/(m·K), a U-value of about 0.21 W/(m²·K) at 300 mm.[1] IsoHemp declares 0.071 W/(m·K) at 50% RH, and 0.067 dry, for its 36 cm block at 320 kg/m³.[3] Collet and Prétot measured 0.09–0.16 W/(m·K) at 23 °C and 50% RH across sprayed, moulded and precast mixes, and found λ rises about 54% when density rises by two-thirds.[6] Density, not the word "hempcrete", sets performance. Against hemp-fibre batts at about 0.04 W/(m·K),[4][5] hempcrete needs roughly twice the thickness for the same resistance (our arithmetic), but it also forms the wall.
Is hempcrete "breathable"?
It is vapour-open: the IsoHemp block has a vapour resistance factor μ of 2.8.[3] That is not the same as waterproof. BRE says finishes must stay vapour-permeable, because trapped moisture can degrade the shiv and the timber frame, and a soaked wall will eventually pass water inside.[1] IsoHemp's EPD requires protection from rising damp and a suitable finish or cladding, with outdoor masonry starting at least 200 mm above ground.[3] Moisture also costs performance: λ rises by less than 15–20% from dry to 90% RH.[6] Clay plaster is another mineral finish we profile; confirm its vapour permeability and compatibility with the specific hemp build-up.
Indoor air
IsoHemp's EPD states that no air-quality measurement was made and argues that similar products show no VOC emissions.[3] That is a statement, not a test; ask for an emissions report on the specific product.
Is hempcrete structural?
No. BRE's paper covers non-loadbearing hemp-lime walls with a typical compressive strength of 0.1–0.2 N/mm², most often with a timber frame.[1] IsoHemp's datasheet gives 0.2 MPa and says its blocks never contribute to the load-bearing capacity of a floor or roof.[2] The 2024 US model code likewise limits hemp-lime to nonstructural infill between or around framing.[9]
Is hempcrete fireproof?
Not as a blanket claim. Reaction to fire and fire resistance are different tests.
- Reaction to fire: IsoHemp's EPD lists B-s1,d0 for the uncoated block and A2 with a non-flammable render.[3]
- Fire resistance: IsoHemp's datasheet claims 45 to 240 minutes by block thickness, plastered on the fire side. We have not seen the test report; treat it as a manufacturer claim. The Belgian approval ATG 3169 confirms B-s1,d0 but lists no fire-resistance figure, and the German general approval Z-17.25-1331 classifies the 36 cm infill masonry as F30-B (fire-retardant, 30 minutes) up to 4 m high under German rules. Ask which rating applies to your build-up.[2][17][20]
- US model code: hemp-lime walls have no fire-resistance rating unless tested to ASTM E119 or UL 263.[9] A US producer reported a one-hour result in July 2024 for a 12-inch spray-applied wall with an internal frame; that belongs to that system, not to hempcrete in general.[10]
- Fibre insulation: Euroclass E for both examples below.[4][5]
How long does hempcrete take to dry?
In BRE's cast-in-place sequence the shuttering is struck after about 24 hours, then the wall dries for 4–8 weeks depending on weather before windows and finishes go on. It must be sheltered from rain while drying.[1] IsoHemp cures its blocks for two months before delivery, and its EPD estimates that lime carbonation continues for three to five years in place.[3]
What about hemp-fibre insulation?
Two example products
IndiNature's IndiTherm declares λ 0.039 W/(m·K) at 45 kg/m³, μ 1.3 and Euroclass E.[4] The BBA certificate confirms λD 0.039 and 45 kg/m³ and Euroclass E, adds a water vapour resistivity of 6.52 MN·s/(g·m), and limits the wall product by building height (storey of 18 m or more in England, Wales and Northern Ireland; 11 m or more in Scotland and for residential buildings in England).[19] HempFlax's Thermo Hanf Premium declares λD(23,50) 0.041 W/(m·K) (0.038–0.043 across its three variants in the 27 March 2026 version) at 35–48 kg/m³, μ 1–2 and Euroclass E.[5] IndiTherm's EPD reports −3.70 kg CO₂e per m² at 100 mm in A1–A3 (−7.86 biogenic, +4.15 fossil), with end-of-life incineration adding +6.08; the document is dated valid to 13 April 2026. We searched the EPD International library for its registration number on 8 October 2026 and found no entry, so treat the declaration as expired unless IndiNature supplies a renewed one; it told us on 14 October 2026 that its updated EPD is in verification.[4] Other bio-based boards, such as cork, have their own profiles; compare datasheet against datasheet.
Is hemp insulation "100% natural"?
No. IndiTherm contains 4.05 kg hemp, 0.405 kg polymer binder and 0.045 kg fire retardant per 4.5 kg unit, so about 90% hemp by mass (our arithmetic).[4] The Thermo Hanf assessment describes polymer or bio-polymer binder fibres and a fire-protection treatment.[5]
What is hemp used for in building and design?
Cast or sprayed hempcrete walls
Best for: architects · building envelope
BRE finds it suited to projects with a rendered or rain-screened finish, good insulation and minimal thermal bridges. It should not be used below ground level.[1]
Blocks and panels
Best for: architects · renovation
IsoHemp's blocks serve as infill masonry, envelopes, partitions or floor insulation.[2] BRE noted in 2011 that blocks need more binder than cast walls, lowering thermal performance and raising cost.[1]
Is hempcrete allowed by building codes?
United States
The 2024 International Residential Code added Appendix BL, Hemp-Lime (Hempcrete) Construction, which applies only where a jurisdiction adopts it. It covers nonstructural infill at 200–400 kg/m³ and sends certain buildings in higher seismic categories to engineered design.[9]
France, the UK and the EU
Hempcrete in France has been covered by professional rules ("Construire en chanvre") since 2012. The AQC's C2P commission now lists the June 2024 revision, limited to vertical walls, partitions and linings, as accepted from 1 July 2024; the 2012 text, which also covered floor and roof insulation and plasters, is listed as no longer accepted.[18] A trade-association article of June 2024 adds that several hempcrete processes hold an Avis Technique.[11] BRE noted in 2011 that some hemp-lime products hold BBA certification; check current status.[1] IndiTherm holds BBA Agrément Certificate 23/7060 (walls, pitched roofs and suspended timber floors), first certified on 21 December 2023 and supplied to us as Issue 2 of 8 May 2024,[19][13] and Thermo Hanf has European Technical Assessment ETA-05/0037, current version 27 March 2026.[5] IsoHemp's block holds the Belgian technical approval ATG 3169 and, for 36 cm infill masonry, the German general approval Z-17.25-1331.[17][20] We found no harmonised European product standard for hempcrete itself; confirm with the authority having jurisdiction.
Hemp in use: projects
Hemp House, Upper Norwood
Nimtim Architects · Croydon, London · Dezeen, 8 February 2025 (completion year not stated)
A timber frame infilled with hempcrete forms the single-storey rear extension. The hempcrete is left exposed in places inside, with a natural render outside. Working with the specialist firm Hempcrete UK, the team used a mix of pre-dried hemp blocks and hemp hand-pressed on site in layers.[14]
Ein Hod house
Tav Group · Ein Hod, Haifa District, Israel · Dezeen, 6 July 2017 (completion year not stated)
The main-floor walls are hempcrete, a mix of hemp hurds and hydraulic lime cast in timber frames, covered with an earth-based plaster. Dezeen reports that Tav Group says this is the first use of hempcrete in Israel, which is the architects' claim.[15]
Mat chair collection
Foersom & Hiort-Lorenzen for Normann Copenhagen · Dezeen, 6 February 2024
The hemp version's shell is made from milled hemp stems mixed with a binder, formed into sheet material and compression-shaped. It was developed with the Danish Technological Institute. This is hemp fibre, not hempcrete.[16]
Where to buy hemp building materials
Manufacturers selected by our editors, one for hempcrete and two for fibre insulation, checked against their own product data.
| Supplier | Products | Country | Certifications |
|---|---|---|---|
| IsoHemp | Hempcrete blocks (7.5–36 cm), interlocking blocks, thin-joint mortar[2] | Belgium | B-EPD 025-0078-002 (PAL36EX), third-party verified, valid 6 Oct 2025 to 6 Oct 2030.[3] Belgian technical approval ATG 3169, valid 6 Feb 2026 to 5 Feb 2031; German approval Z-17.25-1331 (36 cm infill masonry), valid 20 Jan 2026 to 20 Jan 2031.[17][20] |
| HempFlax Building Solutions (brand Thermo Hanf) | Hemp and hemp-jute insulation mats, 30–220 mm[5] | Germany | ETA-05/0037, current version of 27 March 2026, issued by DIBt.[5] |
| IndiNature (Industrial Nature UK Ltd) | IndiTherm hemp batts, 20–100 mm (BBA); the EPD states 20–140 mm[19][4] | United Kingdom | BBA Agrément 23/7060, Product Sheets 1–3, Issue 2 of 8 May 2024 (first certified 21 Dec 2023); ISO 9001 as listed in the EPD. EPD-IES-0020226 (EPD International) was valid to 13 Apr 2026 and has no current library entry.[4][19] |
Certificates are summarised from manufacturer datasheets, EPDs and certifier case studies; confirm on the certifier's registry (B-EPD, UBAtc, EPD International, DIBt, BBA) before specifying. The approvals for IsoHemp were confirmed on the UBAtc and DIBt registers. Listing policy: editorial selection, not paid.
Frequently asked questions about hemp and hempcrete
Is hempcrete carbon negative?
At the factory gate, sometimes. IsoHemp's verified EPD reports −17.38 kg CO₂e per m² of wall in A1–A3 but +64.42 kg CO₂e per m² cradle to grave, because biogenic carbon is counted as re-emitted at end of life. Ask for the EPD, binder composition and transport distance.
Is hempcrete structural?
No. It is a non-structural infill, about 0.1–0.2 N/mm² in compression, and the frame carries the loads.
What is the thermal conductivity of hempcrete?
BRE reports 0.07–0.09 W/(m·K) at 270–330 kg/m³ for commercial walls; IsoHemp declares 0.071 at 320 kg/m³. Conductivity rises with density, so use the datasheet of the mix you specify.
Is hempcrete fireproof?
Not as a blanket claim. IsoHemp's block is B-s1,d0 uncoated and A2 with a non-flammable render, but in the US model code hemp-lime walls have no fire-resistance rating unless the assembly is tested. Hemp-fibre batts are Euroclass E.
How long does hempcrete take to dry?
In BRE's cast-in-place sequence, shuttering is struck after about 24 hours and the wall dries for 4–8 weeks, depending on weather, before finishes.
Is hempcrete allowed by building codes?
It depends on the country. The 2024 US model code has Appendix BL, which applies only where a jurisdiction adopts it; France has had professional rules since 2012, revised in 2024. We found no harmonised European standard for hempcrete, so confirm locally.
Is hemp insulation 100% natural?
No. IndiTherm is about 90% hemp by mass, with a polymer binder and a fire retardant, and Thermo Hanf uses polymer or bio-polymer binder fibres.
The bottom line on hemp
Hemp is credible when specified product by product. Take λ and density from the datasheet, carbon from a verified EPD read stage by stage, and fire behaviour from classified test reports, then design a frame, a vapour-open finish and a drying schedule around hempcrete. Those documents tell you more than the phrase "carbon negative" does.
Related materials
Sources
- Sutton, A., Black, D. & Walker, P. (2011). "Hemp lime: an introduction to low-impact building materials." BRE Information Paper IP 14/11. Crop calendar; non-loadbearing walls; typical dry density 270–330 kg/m³, λ 0.07–0.09 W/(m·K), compressive strength 0.1–0.2 N/mm²; 4–8 weeks drying; lime kiln CO₂ and cement in formulated binders; BBA and building-control notes (2011, may be out of date). BRE (PDF)
- IsoHemp S.A./NV, "Technical data sheet – Hemp block", updated October 2024. Dimensions, bulk density 320 ± 10% kg/m³, λ values, compressive strength 0.2 MPa, fire resistance claim by thickness. Manufacturer data, copy hosted by a distributor. The manufacturer's own sheet "Product sheet. Hemp blocks", updated 4 September 2026, gives the same values for the 36 cm block (λUi 0.071, μ 2.8, 0.2 MPa, B-s1,d0, fire resistance 45/60/120/240 min). datasheet 2024 (PDF) · datasheet 2026 (PDF, isohemp.com)
- B-EPD 025-0078-002, IsoHemp PAL36EX hempcrete block (Belgium). EN 15804+A2 and B-EPD PCR, external third-party verification by Naeem Adibi (WeLOOP), 19 Aug 2025, published 6 Oct 2025, valid to 6 Oct 2030. Declared unit 1 m² installed wall (115.2 kg of blocks, 320 kg/m³, 36 cm). GWP-total A1–A3 −17.38 kg CO₂e/m²; A1–B1 −11.45; A1–C4 +64.42. Landfill end of life assumed. No indoor-air measurement (section 11.1). Copy hosted by ULiège (read in full; document in French): EPD (PDF). The federal B-EPD database requires a login, so registration on b-epd.be could not be confirmed.
- EPD International, EPD-IES-0020226, "IndiTherm" (Industrial Nature UK Ltd), version 2025-04-14, "valid until 2026-04-13", EN 15804+A2, individually verified (Metsims). Declared unit 1 m² at 100 mm, R 2.56 m²K/W, 4.5 kg. λ 0.039, 45 kg/m³, μ 1.3, Euroclass E; GWP-total A1–A3 −3.70, GWP-fossil +4.15, C4 +6.08 kg CO₂e; composition; BBA certificate number 23/7060; ISO 9001 certificate listed. The document warns that the product was recently on the market. On 8 October 2026 the EPD International library had no entry for this registration number, and the validity date had passed; no renewed version found. EPD (PDF, ASBP copy)
- Deutsches Institut für Bautechnik, European Technical Assessment ETA-05/0037 of 27 March 2026 (replaces the version of 6 May 2025; the latest on the DIBt listing), "THERMO HANF PREMIUM", "PREMIUM PLUS" and "COMBI JUTE", manufacturer C-HempFlax Building Solutions GmbH, Nördlingen, under EAD 040005-00-1201. Euroclass E; λD(23,50) 0.041 / 0.043 / 0.038 W/(m·K); 35–48 kg/m³ (Combi Jute 34–46); μ 1–2; mould resistance level 0; thickness 30–220 mm. Earlier versions gave different Combi Jute values (0.037 in July 2023, 0.043 in May 2025), so cite the current one. English translation by DIBt. ETA (PDF) · DIBt listing
- Collet, F. & Prétot, S. (2014). "Thermal conductivity of hemp concretes: Variation with formulation, density and water content." Construction and Building Materials 65:612–619. 90–160 mW/(m·K) at 23 °C and 50% RH; +54% for +2/3 density; +15–20% from dry to 90% RH. Figures from the abstract and the open preprint. doi:10.1016/j.conbuildmat.2014.05.039 · arXiv:1406.3310
- Arrigoni, A., Pelosato, R., Melià, P., Ruggieri, G., Sabbadini, S. & Dotelli, G. (2017). "Life cycle assessment of natural building materials: the role of carbonation, mixture components and transport in the environmental impacts of hempcrete blocks." Journal of Cleaner Production 149:1051–1061. Findings taken from the abstract (full text paywalled). doi:10.1016/j.jclepro.2017.02.161
- Ip, K. & Miller, A. (2012). "Life cycle greenhouse gas emissions of hemp–lime wall constructions in the UK." Resources, Conservation and Recycling 69:1–9. 1 m², 300 mm wall with timber frame, 100 years: 82.7 kg CO₂ sequestered, net reduction 36.08 kg CO₂e. Figures from the abstract (via RePEc). doi:10.1016/j.resconrec.2012.09.001
- International Code Council, 2024 International Residential Code, Appendix BL "Hemp-Lime (Hempcrete) Construction", read on the ICC digital codes site (scope BL101.1, density 200–400 kg/m³ in BL103.1, fire-resistance rating BL105.1; the appendix states its provisions are not mandatory unless referenced in the adopting ordinance). The text matches the Knoxville Residential Code 2024 copy on UpCodes. Adoption status per HempBuild Magazine. Read the appendix itself, including its building-height (25 ft) and weight limits, before design.
- HempBuild Magazine, "Hempcrete Passes 1-Hour ASTM Fire Test" (Americhanvre Ereasy spray-applied hempcrete, Intertek, York, Pennsylvania, July 2024). Trade press; the test report was not available to us. hempbuildmag.com
- Fédération Française du Bâtiment, "Bétons biosourcés : en marche vers la certification", 13 June 2024. Hempcrete framed by professional rules ("Construire en chanvre") since 2012 (the 2012 version, since replaced; see [18]); several processes hold an Avis Technique and are on the C2P green list. Trade-association article; the Avis Technique statement was not checked against the CSTB or green-list registers. ffbatiment.fr
- Shelly, D., Lee, S.-Y. & Park, S.-J. (2025). "Hemp fiber and its bio-composites: a comprehensive review part I—characteristics and processing." Advanced Composites and Hybrid Materials 8(3):252. Statements taken from the abstract. doi:10.1007/s42114-025-01314-0
- British Board of Agrément, "IndiNature: IndiTherm" case study, April 2024. States that IndiTherm can be specified from January 2024 following a full BBA Agrément Certificate, and that it is made in the Scottish Borders from Yorkshire-grown hemp (company claim, reported by the BBA). case study (PDF)
- Ella Jessel, Dezeen, 8 February 2025, "Nimtim Architects creates \"textured envelope\" for Hemp House extension": architect, location, hempcrete use, specialist contractor. Independent trade press. www.dezeen.com
- Eleanor Gibson, Dezeen, 6 July 2017, "Tav Group builds rural artist's house in Israel with \"cannabis walls\"": architect, location, wall build-up and the first-use claim. Independent trade press. www.dezeen.com
- Amy Frearson, Dezeen, 6 February 2024, "Mat eelgrass and hemp chairs launched by Normann Copenhagen": designers, material and process of the Mat chairs. Independent trade press. www.dezeen.com
- UBAtc (Belgian Union for Technical Approval in Construction), ATG 3169 "Isohemp", hemp concrete masonry unit, holder IsoHemp S.A., valid 6 Feb 2026 to 5 Feb 2031 (approval operators SECO and Buildwise, certification operator BCCA). Used as non-load-bearing infill or facing masonry. Blocks B09–B36: reaction to fire B-s1,d0, mean compressive strength 0.20 N/mm², λUi 0.071 W/(m·K), gross dry density 270 kg/m³ (7.5 cm block: 310 kg/m³, λUi 0.080), μ below 3.0; no fire-resistance figure. Registration confirmed on the UBAtc listing; text read from the copy on isohemp.com (PDF).
- Agence Qualité Construction (AQC), C2P list of professional rules accepted, updated January 2026. Lists "Exécution de parois verticales (murs, cloisons et doublages) en bétons de chanvre" (June 2024, Association Construire en Chanvre), accepted with feedback from 1 July 2024, and shows the 2012 rules ("Exécution d'ouvrages en bétons de chanvre": walls, floor insulation, roof insulation, plasters) as accepted 1 July 2012 to 30 June 2024. Same revision announced in the C2P semi-annual publication of July 2024. qualiteconstruction.com
- British Board of Agrément, Agrément Certificate 23/7060, Product Sheets 1 (timber frame walls), 2 (pitched roofs and lofts) and 3 (suspended timber floors), all Issue 2 of 8 May 2024, originally certified 21 December 2023, Industrial Nature UK Ltd t/a IndiNature. Nominal characteristics: 1,200 mm long, 370, 440 or 570 mm wide, 20, 30, 50, 70, 80 or 100 mm thick, 45 kg/m³. Declared λD 0.039 W/(m·K); reaction to fire Euroclass E (EN 13501-1, System Laboratories UK report 466 issue C, 6 November 2023); water vapour resistivity 6.52 MN·s/(g·m) (EN 12086); mould growth test passed; dimensional stability passed (70 °C, 90% RH, 48 hours, change 3% or less); service life equivalent to the structure under normal conditions. Wall sheet: not for buildings with a storey 18 m or more in height in England, Wales and Northern Ireland, nor residential buildings with a storey 11 m or more in England; in Scotland not for buildings with a storey 11 m or more, nor within 1 m of a relevant boundary unless the cladding is A1 or A2 and fire-resistance and unprotected-area requirements are met. The wall and floor sheets require the product to be contained by a fire-resistant lining board (BS EN 520). Copies of the three PDFs were supplied by IndiNature's marketing manager by email on 14 October 2026. The certificate states that readers must check for the latest issue; we have not checked it on the BBA website. The BBA's own homepage carried a notice on 8 October 2026 that its UKAS accreditations were temporarily suspended. bbacerts.co.uk
- Deutsches Institut für Bautechnik, general building authority approval and general construction technique permit (abZ + aBG) Z-17.25-1331, "Ausfachungsmauerwerk aus IsoHemp Hanfstein" (infill masonry from IsoHemp hemp stone), applicant IsoHemp SA-NV, valid 20 Jan 2026 to 20 Jan 2031 per the DIBt register. 36 cm blocks with IsoHemp adhesive mortar, infill masonry for buildings up to 18 m; reaction to fire B-s1,d0; section 3.6 classifies the non-load-bearing infill masonry up to 4,000 mm high as fire-retardant (F30-B). German original read; an English translation is on isohemp.com. approval (PDF, DIBt) · DIBt listing