Clay plaster, also called earth or loam plaster, is an interior wall finish made from building clay, sand and, in some grades, straw or cellulose fibre. The useful questions are what the standards guarantee, how much the humidity claim delivers, and where the material stops working.
This guide sets out the sourced numbers, the standards behind them and the caveats. Rammed earth appears only as context.
What is clay plaster, and what does DIN 18947 say?
Clay minerals bind the sand when the mixing water evaporates. Nothing sets chemically, so cured plaster softens again when wetted: easy to repair, but it needs protection from water.[1]
What the standard covers
DIN 18947, "Earth plasters – Requirements, test and labelling", covers factory-made earth plasters for walls and ceilings indoors and in weather-protected outdoor areas. The current edition is 2024-03.[11] Trade press summarising the 2018 edition says it applies to layers of at least 3 mm, and thinner clay coatings fall under a separate Dachverband Lehm bulletin (TM 06).[12][13] The standard has two strength classes, S I and S II, and Dachverband Lehm guidance expects plasters for normally used rooms to meet S II.[14] Claytec's declaration cites quality assurance under DIN 18942-100,[1] so ask for the test report. A companion standard, DIN 18948:2024-03, covers earthen boards.[15]
Vapour sorption classes
DIN 18947 also grades vapour adsorption. In the 2018 edition WS I, WS II and WS III require at least 35, 47.5 and 60 g/m² taken up in 12 hours when humidity rises from 50 % to 80 %, and a declared plaster must reach at least WS I.[9] We read this table in a research paper, not in the standard, and have not checked the 2024 edition; DIN Media's list of changes for 2024 names starting materials and formats, a life-cycle annex and radionuclide values, and does not mention the sorption table.[11]
Strength in practice
Claytec's S II products declare compressive strength of 2.0–3.0 N/mm² and adhesion of 0.10–0.20 N/mm².[2][3] A peer-reviewed comparison found earth plasters weaker than gypsum and cement mortars but the most hygroscopic of the five plasters tested.[16]
Does clay plaster regulate indoor humidity?
Yes, measurably, but "regulates humidity" describes a material test, not a promise for a room.
What was measured
In a Nordtest moisture buffer test, two clay plasters at 25 mm gave 1.56–1.89 g/(m²·%RH), "good" on a scale where above 2.0 is "excellent".[10] A study of 114 unfired clay samples, blocks and plasters, concluded that the soil, meaning its mineralogy and particle size, matters more than density, preparation or stabilisation.[17] Clay mineralogy also drives vapour adsorption.[18] We found no product-specific MBV in supplier documents.
What reduces it
Finishes and additives change the result. Beeswax and linseed-oil treatments cut the moisture buffer value of an earth plaster by 29–50 %.[19] In an FH Potsdam master's project (not peer-reviewed), cement (22 %), lime (about 18 %), gypsum (15 %), starch and methyl cellulose (about 25 %) cut vapour sorption, and an epoxy resin cut it by about 40 %,[13] and stabilised rammed earth lost buffering in peer-reviewed lab work.[20] Wallpaper is less clear-cut: at MFPA Weimar, common woodchip and renovation wallpapers had little or no effect on 12-hour sorption, while experimental coated textiles reduced it, though the plaster still met WS II.[9] Claytec's clay paint is vapour-open (sd 0.04 m) but contains up to 3.5 % dispersion binder.[6]
In a real room
A Claytec-led, DBU-funded project with TU Dresden simulated a museum room with ten visitors. With all walls in an experimental high-sorption clay plaster, the humidity swing was 47 % for hour-long visits against 68 % with a non-storing plaster, and the benefit shrank as visits lengthened.[14] That plaster is a prototype with lower adhesion, and the report is by the manufacturer. Read it as damping swings in proportion to exposed area.
Is clay plaster healthy for indoor air?
"Healthy" and "VOC-free" need product evidence.
Emissions
Claytec says its plasters were tested under EN 16516 and ISO 16000; the report is held by the programme operator.[1] An eco-INSTITUT certificate and SHI "Indoor Air Quality Certified" status cover them; the SHI passport rates LEED v4.1 low-emitting materials compliant and marks BREEAM, DGNB, QNG and BNB not relevant for these products.[8] Ecoclay's EPD lists VOC as 0 without units or a report, so treat it as a declaration.[4] Claytec's declaration says natureplus criteria are met,[1] but we did not verify a natureplus licence. Plant-fibre grades can grow mould if they dry slowly.[7][14]
Pollutant removal
A peer-reviewed field and lab study found two clay wall coatings may remove ozone passively, with by-product emissions that decayed within two months.[21] That is ozone only. Broader claims, such as a supplier's statement that its plasters "absorb toxins", are company claims we could not verify.[22]
How low-carbon is clay plaster?
Per kilogram the footprint is low but varies with production route and thickness.
Declared figures
Claytec's verified declaration gives GWP-total (A1–A3, per kg) of 0.0127 kg CO₂e for earth-moist mineral plaster and 0.047–0.063 for dry-dosed grades, which use dried raw materials. Earth-moist grades with plant fibre come out slightly negative because straw stores biogenic carbon.[1] Ecoclay's EPD reports 0.0547 as an average of three mortars and warns that EPDs from different programmes may not be comparable.[4]
Per square metre
Thickness dominates. Topcoat fine 06 uses about 1.47 kg per m² per mm, mineral plaster 20 about 2.0 (earth-moist).[2][3] By our arithmetic, 3 mm of topcoat 06 at the declared 0.0632 kg CO₂e/kg is about 0.28 kg CO₂e per m². A lapsed Clayworks EPD declared 0.136–0.185 kg CO₂e/m² for a 1.5 mm coat; those figures do not follow from its own per-kg values (0.048–0.068) at 2.3 kg/m², so treat them with caution.[22]
"Zero-carbon" and "100 % recyclable"
The negative figures depend on a straw recycling scenario. Reuse is real but conditional: recovered plaster must be free of old paint and of gypsum, cement or lime repairs.[1] Ecoclay's EPD models end of life as landfill.[4]
What are the limits of clay plaster?
Water
Claytec's declaration says to protect it from standing or running water and permanent dampness.[1] Its clay board is rated for water exposure class W0-I, which covers bathrooms except showers,[5] and tiles in showers need a different plaster.[7]
Cracking and drying
Declared linear drying shrinkage is 2–4 % for Claytec grades[1] and 1.8–6.2 % in Ecoclay's EPD.[4] Too much water or too fast drying can cause cracks; slow drying invites mould. Claytec's worksheet gives 24–48 hours for 2–3 mm coats and about a week for 1 cm, in both its 2018 English and 9-2026 German editions.[7]
Adhesion and loads
Clay plaster adheres only mechanically, so the substrate must be stable, dry, rough and absorbent.[2] Fixing heavy wall-hung loads is not covered by the sources we read; Claytec's 9-2026 worksheet gives only a figure for tiles, saying clay-plastered drywall build-ups can generally carry a tile covering of 25 kg/m².[7] For fire, check the test report for your build-up: Claytec declares its fibre grades B2 (DIN 4102-1) and Ecoclay's EPD lists its fibre base coat as A2-s1,d0.[1][4]
What is clay plaster used for?
Interior wall and ceiling plasters
Best for: architects · interior designers
Mineral plaster 20 goes on at 5–15 mm (base) or 6–10 mm (top coat),[3] fine top coats at 2–3 mm,[2] designer plasters at about 2 mm (Claytec claims 146 shades in its current worksheet and brochure, 140 in the 2018 English worksheet).[7][23] They can finish hemp walls too: Claytec publishes an application sheet with IsoHemp for plastering hemp blocks (see hemp).[23]
Clay boards and clay paints
Best for: interior designers · architects
Claytec's clay dry plasterboard is 16 mm thick, about 700 kg/m³ with μ 18, and needs a reinforcing layer before plaster or paint; humidity must stay at or below 70 % after installation.[5] Its clay paint is not for splash-water areas and also contains titanium dioxide and potassium silicate.[6] The same range includes cork boards.[23]
Rammed earth, for context
Best for: architects (structure)
Rammed earth is compacted soil used for walls, not a plaster; Claytec lists a rammed-earth mix.[23]
Clay plaster in use: projects
Walled Garden
Nimtim Architects · London, UK · completion year not stated
Clayworks Rustic Arakabe finish covers the walls and ceilings of the main kitchen and living space, and Clayworks Demi-Rustic is used through the rest of the house. This is the manufacturer's case study.[24]
100 Acre Wood
Denizen Works · Scotland, UK · completion year not stated; RIBA National Award 2023 (as stated by Clayworks)
Inside, the space is finished with Clayworks Demi-Rustic clay plaster in a warm earth tone with mica specks. The manufacturer's page lists the project as a 2023 RIBA National Award winner.[25]
House Giebel
Peter Grundmann · Brandenburg, Germany · 2024
In the architect's description, the old brick walls on the ground floor were plastered inside with clay and partly outside with lime. Divisare is an aggregator, so treat this source as medium confidence.[26]
Where to buy clay plaster in Spain and Europe
Manufacturers selected by our editors and checked against company documents and certifier or registry records.
| Supplier | Products | Country | Certifications |
|---|---|---|---|
| Claytec (Claytec GmbH & Co. KG) | Base and top-coat plasters · clay boards · clay paints · designer plasters | Germany | eco-INSTITUT 0117-11340-002 (to 01/2027) and SHI Indoor Air Quality Certified (to 31 Jan 2027).[8] The eco-INSTITUT-Label database (checked 8 Oct 2026) lists that certificate for the plaster and clay mortars, and a second one, 0413-11340-001 (to 04/2027), for YOSIMA designer plasters, clay filler and CLAYFIX paint.[27] Dachverband Lehm UPD_LPM_CLAY2023006 (to 5 Dec 2028).[1] Some datasheets in Spanish.[23] |
| Ecoclay (Minercer S.L.U.) | Clay plasters (Base+Fibre, Base, Finishing) · clay paints | Spain (Vila-real; clay quarried in Crivillén, mixed in Alcañiz, Teruel) | EPD-IES-0013373, International EPD System, third-party verified, valid to 8 Dec 2029; the registry entry (checked 7 and 8 Oct 2026) shows status Valid, version :002 of 15 Jan 2025, with the owner named as Minera Sabater S.L., whereas the 9 Dec 2024 PDF we read names Minercer S.L.U. Test reports behind its listed values not seen.[4] |
| Clayworks (Clayworks Ltd) | Clay plasters (Rustic, Demi Rustic, Smooth, Tonal) | United Kingdom (Helston, Cornwall) | EPD S-P-02278, valid to 8 Nov 2025; the registry entry (checked 7 and 8 Oct 2026) shows status Expired, and a library search for Clayworks (8 Oct 2026) lists no newer clay-plaster EPD.[22] |
Certificates are summarised from manufacturer documents, certifier passports and EPD entries; confirm on the certifier's registry before specifying. Listing policy: editorial selection, not paid.
Frequently asked questions about clay plaster
What does DIN 18947 cover?
Requirements, tests and labelling for factory-made earth plasters indoors and in weather-protected outdoor areas. The current edition is 2024-03. Manufacturer datasheets and the 2018 edition use strength classes S I and S II and sorption classes WS I to WS III; we have not read the 2024 text.
Does clay plaster regulate humidity?
Measurably: two clay plasters at 25 mm gave 1.56–1.89 g/(m²·%RH), rated good. In a room the effect depends on thickness, exposed area and finish, and it does not replace ventilation.
Is clay plaster VOC-free?
Not automatically. Emissions depend on the finished product, its fibres and its paint. Ask for a product-specific certificate and test report.
What is the thermal conductivity of clay plaster?
Claytec declares 0.91 to 1.1 W/(m·K) for standard grades and 0.19 for its lightweight insulating plaster.
Can clay plaster be used in bathrooms?
Only where it stays free of standing or running water. Claytec's clay board is rated for water exposure class W0-I, which excludes showers.
Is clay plaster carbon negative?
Some straw grades are slightly negative in A1–A3 in Claytec's declaration, but the credit depends on a recycling scenario. Mineral grades are small positive values.
Does paint or wallpaper ruin the humidity buffering?
It depends. Beeswax and linseed oil cut the moisture buffer value by 29–50 % in one study; common wallpapers had little effect on sorption in another.
The bottom line on clay plaster
Clay plaster is a credible interior finish when specified product by product: the datasheet for strength, λ and fire class, the EPD for carbon, an emissions certificate for indoor air, and test data for the humidity claim. Its buffering is real but finish-dependent, and it needs dry conditions.
Related materials
Sources
- Dachverband Lehm e.V. / Claytec GmbH & Co. KG, "Umweltproduktdeklaration Lehmputzmörtel nach DIN 18947", UPD_LPM_CLAY2023006_PKRÜ5-DE (in German), issued 6 Dec 2023, valid to 5 Dec 2028; EN 15804, external verification (Dr.-Ing. Horst Schroeder), also listed in the Dachverband Lehm UPD register (checked 8 Oct 2026); declared unit 1 kg; 2022 plant data (Viersen, Ransbach-Baumbach). Source of the property table, fire classes, GWP A1–A3 values, shrinkage, end-of-life and natureplus statement. The GWP text and charts in the declaration carry some labelling inconsistencies, so only the charted totals are used. Manufacturer-commissioned. PDF
- Claytec, "Clay topcoat fine 06" product sheet, item 05.113 / 10.113, as of 2026/9 (DIN 18947, LPM 0/1 f, S II, 1.8): λ 0.91, μ 5/10, WS III, strength, adhesion, consumption, substrate. Manufacturer data. PDF
- Claytec, "Mineral clay plaster 20" product sheet, item 05.030 / 10.130, as of 2026/9 (DIN 18947, LPM 0/4 f, S II, 2.0): λ 1.1, μ 5/10, WS III, layer thicknesses, consumption. Manufacturer data. PDF
- MINERCER S.L.U. (Ecoclay), Environmental Product Declaration "Clay plasters", EPD-IES-0013373, International EPD System, EN 15804+A2, published 9 Dec 2024, valid to 8 Dec 2029; individual verifier Eva Martínez Herrero (CTME); declared unit 1 kg; average of ecoclay Base+Fibre, Base and Finishing; quarry in Crivillén and plant in Alcañiz (Teruel); owner's address in Vila-real (Castellón). Property table values are manufacturer-declared. EPD (PDF)
- Claytec, "Clay dry plasterboard" product sheet, item 09.010, as of 2026/9: 16 mm, about 700 kg/m³, μ 18, water exposure class W0-I (DIN 18534-1) except showers, humidity limit 70 %. Manufacturer data. PDF
- Claytec, "ClayTec clay paint / roll-on plaster ready-to-use" product sheet, items 13.005 / 13.105, as of 2026/9: sd 0.04 m, wet abrasion class 3, dispersion ≤ 3.5 %, composition, not for splash-water areas. Manufacturer data. PDF
- Claytec, "Clay plasters" worksheet: English edition issued 07-2018 (stated validity 12 months; still the English file on Claytec's download page on 8 Oct 2026) and German edition "Lehmputze", Ausgabe 9-2026 (the current one): drying times (24–48 hours for 2–3 mm, about a week for 1 cm, same in both), shrinkage cracks, mould, tiling in showers (needs another plaster; tiles only where occasionally splashed, W0-I), tile load of 25 kg/m² on clay-plastered drywall (9-2026 only), designer plaster (YOSIMA, 2 mm; 140 shades in 2018, 146 in 9-2026, company claims). Manufacturer guidance. EN 2018 (PDF) · DE 9-2026 (PDF)
- Sentinel Holding Institut (SHI), Product Passport 12018-10-1016, "ClayTec Lehmputz (Unterputz mit Stroh, Mineral 20, SanReMo, Oberputz fein 06, Oberputz mit Stroh)", dated 28 Sep 2026, verification by EVIDANA GmbH; includes eco-INSTITUT certificate 0117-11340-002 (Köln, 4 Sep 2025, label programme, criteria eIL 05.02 (03/2025) mineral building products, valid/monitored to 01/2027) and the SHI Indoor Air Quality Certified assessment (valid to 31 Jan 2027). Certifier document hosted by SHI. PDF
- Liebrich, C., Grützner, T. & Wagner, R. (MFPA Weimar) (2020). "Lehmwände tapezieren – (K)eine gute Idee?!", in LEHM 2020, Dachverband Lehm e.V. (in German). Reproduces DIN 18947:2018-12 Table A.1 (WS I ≥ 35.0, WS II ≥ 47.5, WS III ≥ 60.0 g/m² after 12 h; RH 50→80 %) and tests wallpapers on a WS III plaster (mean 61.56 g/m²; scatter about one class). Conference paper. PDF
- Nutt, N., Kubjas, A. & Nei, L. (2020). "Adding waste paper to clay plaster to raise its ability to buffer moisture." Proceedings of the Estonian Academy of Sciences 69(3):179–185. Nordtest MBV of two clay plasters of Saviukumaja OÜ at about 25 mm: 1.56–1.89 g/(m²·%RH)@8/16 h; classification after Rode et al. doi:10.3176/proc.2020.3.01
- DIN Media / DIN, DIN 18947:2024-03 "Lehmputzmörtel – Anforderungen, Prüfung und Kennzeichnung" (Earth plasters – Requirements, test and labelling): current edition, replaces DIN 18947:2018-12. Catalogue pages only, including DIN Media's list of changes against the 2018 edition (starting materials and formats, life-cycle annex, radionuclide values; checked 8 Oct 2026); the standard itself is paid and was not read. din.de · dinmedia.de
- Baunetzwissen, "Normen für den Lehmbau" (trade-press summary of DIN 18942–18948 as of the 2018-12 editions; page read 8 Oct 2026: DIN 18947 applies to layers of at least 3 mm and not to stabilised earth plasters). baunetzwissen.de
- FH Potsdam, "Reduction of Water Vapour Sorption of Clay Plasters through Stabilising Agents", master's thesis project of P. Schlacht, 2020–2021, supervisors K. Pistol and C. Ziegert. University project page, not peer-reviewed; reductions of about 15–25 % for cement, lime, gypsum, starch and methyl cellulose and about 40 % for epoxy resin; mentions TM 06 for clay thin-layer coatings. fh-potsdam.de
- Breidenbach, P. & Röhlen, U. (Claytec), "Entwicklung und Erprobung eines optimierten Lehmputzmörtels zur Reduktion der aktiven Klimatisierung im Museumsbau", final report to Deutsche Bundesstiftung Umwelt, Az. 39380/01, 28 Mar 2025 (in German; simulation by TU Dresden, Institut für Bauklimatik). Source of the two strength classes, TM 01 statement and the museum-room simulation. Company-authored report of an experimental plaster. PDF
- DIN Media, DIN 18948:2024-03 "Earthen boards – Requirements, test and labelling": current edition, replaces DIN 18948:2018-12. Catalogue page only. dinmedia.de
- Santos, T., Gomes, M.I., Silva, A.S., Ferraz, E. & Faria, P. (2020). "Comparison of mineralogical, mechanical and hygroscopic characteristic of earthen, gypsum and cement-based plasters." Construction and Building Materials 254:119222. Abstract read; full text not accessed. doi:10.1016/j.conbuildmat.2020.119222
- McGregor, F., Heath, A., Shea, A. & Lawrence, M. (2014). "The moisture buffering capacity of unfired clay masonry." Building and Environment 82:599–607. 114 samples (compressed earth blocks or plasters), Nordtest; abstract read, full text not accessed. doi:10.1016/j.buildenv.2014.09.027
- Lima, J., Faria, P. & Santos Silva, A. (2020). "Earth Plasters: The Influence of Clay Mineralogy in the Plasters' Properties." International Journal of Architectural Heritage 14(7):948–963. Abstract read. doi:10.1080/15583058.2020.1727064
- Santos, T., Faria, P., Sotomayor, J., Silvestre, J. & Santos Silva, A. (2024). "Effect of Different Surface Treatments on the Performance of Earth Plasters." Coatings 14(12):1537. Abstract read (via Crossref); beeswax and linseed oil cut MBV by 29–50 %. doi:10.3390/coatings14121537
- Arrigoni, A., Grillet, A.-C., Pelosato, R., Dotelli, G., Beckett, C.T.S., Woloszyn, M. & Ciancio, D. (2017). "Reduction of rammed earth's hygroscopic performance under stabilisation: an experimental investigation." Building and Environment 115:358–367. Bibliographic data checked on Crossref; the finding that stabilisation reduced moisture buffering comes from repository-record summaries, full text not accessed. doi:10.1016/j.buildenv.2017.01.034
- Darling, E.K. & Corsi, R.L. (online 2016; vol. 27, 2017). "Field-to-laboratory analysis of clay wall coatings as passive removal materials for ozone in buildings." Indoor Air 27(3):658–669. Abstract read. doi:10.1111/ina.12345
- Clayworks Ltd, Environmental Product Declaration "Clay Plasters (Rustic, Demi Rustic, Smooth, Tonal)", S-P-02278, International EPD System, EN 15804+A1, published 9 Nov 2020, valid to 8 Nov 2025 (the EPD library lists it as expired); GWP-total A1–A3 0.048 / 0.068 kg CO₂e per kg, 0.185 / 0.136 kg CO₂e per m² for a 1.5 mm coat; BRE fire tests (BS 476); marketing claims such as "absorb toxins" are the company's. EPD (PDF) · EPD library
- Claytec download page (product range and datasheets, including Spanish-language sheets, cork lining and insulation boards, rammed-earth mix) and the Claytec / IsoHemp application sheet for plastering hemp blocks. Manufacturer listing. claytec.de · IsoHemp sheet (PDF)
- Clayworks, "Walled Garden" residential project page: architect, products and use. Manufacturer case study; no completion year given. clay-works.com
- Clayworks, "100 Acre Wood" residential project page: studio, product, location (Scotland) and award year. Manufacturer case study; RIBAJ and Dezeen could not be read. clay-works.com
- Divisare, "House Giebel" by Peter Grundmann (2024): project description. Aggregator; the architect's own page was not reached. divisare.com
- eco-INSTITUT-Label, database "Certified products" (search for 11340, checked 8 Oct 2026): Claytec entry lists certificate 0117 — 11340 — 002, valid until 01/2027, for ClayTec clay plaster and masonry mortars for interiors, clay adhesive and reinforcing mortar, clay filler and joint filler, and certificate 0413 — 11340 — 001, valid until 04/2027, for YOSIMA designer plasters, colour filler and CLAYFIX paint (certificate PDF read: criteria eIL 08.02 (03/2025), dated 2 Oct 2025). Certifier registry. eco-institut-label.de