Lime Plaster: Types, Standards, Vapour Permeability and Carbon Data

Lime plaster is a wall finish whose binder is lime: either air lime, usually supplied as putty, or natural hydraulic lime (NHL). It is specified for interior walls, for lime renders and for repairs to older buildings.^[1][2] The useful questions are which lime is in the bag, which EN 998-1 categories the product declares, and how it behaves while it cures.

This guide sets out what the documents we read say. For the earth-based alternative see our profile of clay plaster rather than a comparison here. For a lime-based paint read from company documents, see our article on Graphenstone paint.

What is lime plaster, and how do air lime, NHL and hydrated lime differ?

According to the ASBP Lime Group's July 2026 briefing paper, limestone is heated to make quicklime, which is slaked with water to make the lime used in mortars, plasters, renders and limewash. The two most common families are non-hydraulic lime, often supplied as putty, which sets mainly by carbonation, and NHL, which sets by hydraulic reactions, can set in damp conditions, and contains free lime that continues to carbonate.^[1]

Air lime and putty

Historic Environment Scotland (HES) writes that most lime used for internal plastering in traditional buildings was non-hydraulic. Its guide has slaked lime sieved to putty and matured for a minimum of three months, then mixed with sharp sand for backing coats ("coarse stuff", typically one part putty to two parts coarse sand by volume, with hair) or with fine sand for finishing coats ("setting stuff", equal parts putty and fine sand).^[2] The Tŷ-Mawr guide describes its fat-lime plasters as made from high-calcium, non-hydraulic lime, aggregates and, in the base coats, fibres.^[13]

Natural hydraulic lime

The ASBP paper says NHL is made from limestone that naturally contains clay or silicate impurities, and that the classes NHL 2, NHL 3.5 and NHL 5 are based on the compressive strength of standard mortars at 28 days. It describes NHL 2 as generally used for sheltered or internal work and NHL 3.5 as a widely used general-purpose option for many external renders, and adds that the strength labels are not a substitute for design judgement.^[1] Saint-Astier's NHL 3.5 data sheet lists a 28-day compressive strength of 3.5 to 15 MPa as the EN 459 requirement and 7 MPa as its typical value, and states free lime of at least 25 % (typical value above 28 %).^[8]

Putty versus hydrated powder

The Scottish Lime Centre Trust defines hydrated lime as slaked lime, in the form of a dry powder or of putty, and warns that the hydrated lime sold by builders' merchants as a plasticiser in cement:lime:sand mortars is not suitable for making lime mortars. It also warns against confusing "hydraulic" with "hydrated".^[3] HES says builder's lime supplied as a powder can be soaked to produce a putty but is generally considered inferior and not suitable for the repair of traditional plasterwork.^[2]

What do EN 459-1 and EN 998-1 say about lime plaster?

EN 459-1: the lime itself

EN 459-1 covers building lime used to prepare binders for mortar, to make other construction products and for civil engineering; it defines the types of lime and sets chemical and physical requirements and conformity criteria. The current edition is 2015-07, which replaced the 2010-12 edition.^[5] We read the DIN Media catalogue page, not the standard.

EN 998-1: the factory-made plaster

BS EN 998-1:2016 applies to factory-made rendering and plastering mortars based on inorganic binders, for external and internal use on walls, ceilings, columns and partitions. Air lime as principal binder falls inside it; mortars whose principal binder is calcium sulphate fall under EN 13279, and site-made mortars are excluded.^[6] Per the Mineral Products Association's guide (Issue 5, October 2023), the standard classes mortars by use, with general purpose (GP), lightweight (LW), coloured (CR), one-coat external (OC), renovation (R) and thermal insulating (T) types.^[7]

The same guide's Table 2 gives the hardened-mortar categories: compressive strength at 28 days CS I 0.4–2.5, CS II 1.5–5.0, CS III 3.5–7.5 and CS IV above 6 N/mm²; capillary water absorption W0 (not specified), W1 (c ≤ 0.40 kg/m² min^0.5) and W2 (c ≤ 0.20 kg/m² min^0.5); and thermal conductivity T1 (≤ 0.1 W/m·K) and T2 (≤ 0.2 W/m·K).^[7] We have not read the standard's own table.

What do manufacturers declare for lime plaster?

Declared values are product-specific and the products below are not equivalent: a damp-wall renovation plaster, a bonding coat, a filling plaster and a finishing coat. We show each in its own document and do not decide between them.

• According to Volteco's declaration of performance for Calibro NHL (No. 0014-CPR-2017/03/30, Rev. 1.1, 19 May 2026), a rendering and plastering mortar to UNI EN 998-1:2004, Type R, the declared values are reaction to fire class A1, CS II, adhesive bond ≥ 0.1 N/mm², capillary absorption ≥ 0.3 kg/m² after 24 h, vapour permeability coefficient µ ≤ 15, tabulated thermal conductivity 0.40 W/mK and apparent bulk density 1.1 to 1.2 kg/l.^[10]
• According to Womersleys' UKCA declaration for Bonding Coat NHL3.5 (28 October 2025), a general-purpose mortar to BS EN 998-1:2016 for internal and external use, the declared values are Euroclass A1, adhesion 0.1 N/mm² (fracture pattern C), water absorption W0, µ 15/35 (tabulated) and thermal conductivity 0.83 W/mK (tabulated).^[9]
• According to Secil's Reabilita Cal RB data sheet (undated copy), a filling plaster made exclusively with natural hydraulic lime, the hardened product is CS II (EN 1015-11) with adherence ≥ 0.2 MPa, capillarity ≥ 0.3 kg/m² (24 h), µ ≤ 15 (EN 1015-19), class A1, thermal conductivity 0.61 W/(m·K) (P = 50 %) and density 1450 ± 150 kg/m³.^[11]
• According to Saint-Astier's EcoMortar Finish data sheet (edition June 2024, type CR), dry density is 1,400 kg/m³, compressive strength 1.2 MPa at 28 days and vapour permeability 0.85 g/(m²·hour·mmHg), a unit that differs from the µ values above and that we have not converted.^[12]

The values for the first two products differ (µ ≤ 15 and 0.40 W/mK against µ 15/35 and 0.83 W/mK), and both thermal conductivities are declared as tabulated values. Kreidezeit's finishing coat is a different case: its product page states an sd value below 0.03 m at 2 mm layer thickness (DIN EN ISO 12572), a pH above 12.5, and a composition of marble sand, quartz sand, marble lime hydrate, cellulose, methyl cellulose and plant starch.^[15]

How long does lime plaster take to cure?

The documents we read give different answers, and each is for a different product or era.

• Tŷ-Mawr's application guide for its fat-lime plasters says lime plasters need carbon dioxide in the presence of moisture to harden and can carbonate only while moist. It gives one to two weeks for the first coat, one week for the second and a few days for the finish coat before the next coat goes on, varying with weather and substrate, and says not to use dehumidifiers or heaters to speed the set.^[13]
• Saint-Astier's NHL 3.5 data sheet gives a working temperature of 8 °C to 30 °C and asks for wet mortar to be protected from frost for at least 10 days.^[8] For EcoMortar Finish it gives 2 to 6 hours before scratching a 12 to 15 mm layer and 2 to 4 hours for a 10 mm floated finish, depending on conditions.^[12]
• U.S. Heritage Group's NHL3.5 plaster sheet says the plaster cures by hydration and by carbonation, should be kept damp for the first three days and protected from freezing for 28 days.^[14]
• Kreidezeit's finishing coat is "dry and safe to coat" after at least 24 hours per millimetre of layer thickness at 20 °C and 65 % relative humidity; the page does not state a carbonation time.^[15]
• The US National Park Service says historic lime plaster could take more than a year to dry and was traditionally left for at least a year before painting or papering.^[4]

For the first coat, HES adds that the rate of drying is controlled by minimising draughts and suction and by fine misting, and it says that cracking, crazing and friability can also result from poor background preparation and aftercare.^[2]

How much carbon is in lime plaster?

The lime cycle, as sources state it

The ASBP paper describes the cycle: calcination releases CO₂ as limestone becomes quicklime, and carbonation later turns slaked lime back into calcium carbonate, "effectively becoming stone again".^[1] The National Park Service brief makes the same chemical statement.^[4] In the author manuscript of Laveglia and co-authors, calcination contributes the most to CO₂ emissions, at 0.79 t CO₂ per t CaO, which is inevitable without carbon capture, and the paper puts limestone decomposition at around 65 % of total emissions.^[18] For a lime-and-cement dry render they modelled, they write that even full carbonation over the life cycle could never compensate all cradle-to-gate emissions.^[18] We do not net the two.

Declared EPD figures

Two product EPDs on the International EPD System are available. Volteco's Calibro NHL (inorganic binders 30–35 %, fillers 60–65 %) declares GWP-total for A1–A3 of 0.592 kg CO₂e per kg, with packaging not included in the declared unit.^[16] Adesital's Adesan NHL Intonaco (binders 0.15 kg and fillers 0.84 kg per kg) declares GWP-total A1–A3 of 0.0864 kg CO₂e per kg, made up of GWP-fossil 0.120 and GWP-biogenic −0.0341, with packaging included in the declared unit.^[17] The compositions and declared units differ, both declarations state that EPDs may not be comparable, and neither declares carbonation uptake.^[16][17] We did not find an EPD for an air-lime putty plaster.

What do sources say about lime plaster and humidity?

The ASBP paper, a trade-body document, describes lime plasters as vapour permeable and as taking up and releasing humidity.^[1] HES says lime plaster is a vapour-permeable finish that contributes to the breathability of solid masonry walls.^[2] Measured moisture buffering is less simple: a peer-reviewed study tested clay, gypsum and lime plasters with the NORDTEST protocol and compared the results with a single-room hygrothermal simulation. The two methods gave different estimates of buffering potential, which the authors attribute to factors the laboratory test does not capture, such as ventilation and moisture transport through the whole wall assembly.^[19] We read an abstract-level summary only and have no moisture buffer value for a lime plaster.

We did not open an emissions test report or an emissions label for any lime plaster, so we make no statement about indoor-air emissions. Kreidezeit's finishing coat declares organic additives (cellulose, methyl cellulose, plant starch).^[15]

Where is lime plaster used?

Suppliers whose documents we read

Frequently asked questions about lime plaster

The bottom line on lime plaster

Lime plaster is a family of products, not one: the binder type, the EN 998-1 categories, the declared µ and the curing regime all differ by product. The product's declaration of performance and datasheet are the documents that hold its performance values, an EPD holds its carbon figures, and heritage guidance such as HES's covers repairs to historic plasterwork.

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