Thermal-Comfort Plasters
1 December 2025
So-called thermal-comfort plasters are made from a binder (lime, clay or gypsum) combined with either a plant fibre (straw, hemp shiv or flax) or a lightweight aggregate (expanded cork or pozzolan), which provides thermal improvement thanks to the air trapped within the material. They can be applied by hand in thicknesses of up to 6 cm, installed by shuttering between a timber frame, or sprayed using specialist equipment (fibre blower and spray machine) in thicknesses ranging from 10 to 20 cm. They are most commonly applied indoors, although they can also be used externally. The best-known examples include hemp-lime, hemp-clay, lime-cork and straw-clay plasters. All of these mixtures require a significant drying period to avoid defects and moisture-related issues. As a rule of thumb, hemp-lime plasters are often expected to dry at a rate of approximately 1 cm per week, although the actual drying time may extend to a year and a full summer season depending on thickness and the chosen finish.
The thermal performance of these plasters depends on the combination of binder and aggregate used, as well as the density of the aggregate itself. As a general rule, the higher the binder content, the lower the thermal performance of the mix. A material is considered insulating when its thermal conductivity (λ) is below 0.06 W/mK. Since these mixes generally exceed this value, they are not classified as insulation materials, despite their contribution to thermal comfort. As a reminder: R (m²K/W) = thickness (m) ÷ λ (W/mK) For example, Saint-Astier reports an R-value of 4.80 m²K/W for a thickness of 35 cm using its Batichanvre and Isocanna hemp-lime system, corresponding to a thermal conductivity of λ = 0.073 W/mK. Chanvriers en Circuits Courts reports λ-values ranging from 0.061 W/mK to 0.090 W/mK for hemp-clay mixes, depending on density. The thermal conductivity coefficient (λ) is determined under laboratory conditions, with controlled temperature and humidity, in accordance with French standards. As a result, these values are generally available only for industrial products or for materials that have been formally tested by organisations within the sector.
Thermal-comfort plasters cannot replace conventional insulation materials, whether bio-based or not. However, they offer a number of other benefits: reducing the cold-wall effect, regulating indoor moisture levels through their ability to sorb and release water vapour, capillary action (allowing a certain amount of liquid water and water vapour to move through the material), and acoustic improvement thanks to their mass and porous, textured structure. For these reasons, they are particularly well suited to the renovation of traditional buildings.
