Last edited 30 Apr 2024

Insulating plaster

1 Introduction
2 Types of insulating plaster by binder and insulant
3 Related articles on Designing Buildings

[edit] Introduction

While all plasters, to some extent, improve the thermal performance of a wall because they increase its thickness, insulating plasters contain additives or aggregates to improve their resistance to the flow of heat. This improvement in thermal performance is usually not as significant as other layered board or insulation filled build-ups, and as such, it is usually used where other solutions are not possible due to construction, space, or planning restrictions.

Standard plaster usually has a k-value of around 0.50 W/mK, where as insulating plasters can have K values of between 0.074 to 0.127 W/mK meaning for every 1mm of insulating plaster that is applied the U-Value might be improved by 1%, depending on overall thickness and moisture content. Unlike the use of insulation board or block materials, which normally then receive a finish coat of plaster, insulating plaster contains additives within the plaster mix itself as part of the base or bonding coat that make up the wall.

Insulating plaster can consist of gypsum, lime, or cement based binders (or a mix of these) with additives such as cork, perlite, vermiculite, polystyrene, or hemp as fibres, balls, or powders, though these are generally not interchangeable as they have differing characteristics. Certain materials work better together; different binders and insulants have differing levels of breathability or hygroscopicity, i.e., they can to some extent absorb and release moisture and, as such, should be used together.

[edit] Types of insulating plaster by binder and insulant

There are a number of different types of plaster, depending on the binder that is used, certain insulants can be used with certain binders to create insulating plasters with differing characteristics.

[edit] Insulating gypsum plaster

Gypsum plaster, or ‘plaster of Paris’ (POP), is the most common form of plaster for interior walls. It is produced by heating gypsum to around 150°C (300 °F). When mixed with water, the dry plaster powder re-forms into gypsum. Unmodified plaster starts to set about 10 minutes after mixing, but it will not be fully set until 72 hours have elapsed. Gypsum plaster has good fire-resistant qualities. Although gypsum plaster does have some hygropscopic qualities it is not in general considered to be a breathing material when compared to lime.

[edit] Gypsum plaster with lightweight synthetic insulants

Gypsum plaster can be applied with a mix containing small polystyrene balls (around 3mm in size) to help improve the thermal performance by around 30-50%. In general gypsum is not considered to have high levels of hygroscopicity (although it does have some) and therefore the construction is not usually referred to as breathing, the same applies to polystyrene as a plastic polymer. Polystyrene has the advantage of being lightweight and highly insulating, thus decreases the overall weight of the plaster layer and improving thermal performance.

In general the K-value of these kinds of solution may vary between 0.191 and 0.116 W/mK for 10% and 30% addition of the smallest polystyrene balls, respectively. The lowest thermal conductivity value 0.047 W/mK might be achieved but for 80% EPS and 1.5% resin content, which in turn will reduce the density and thus hardness of the finish, but perform very well thermally.

[edit] Gypsum plaster with lightweight minerals

Gypsum plaster can be applied with a mix containing vermiculite or perlite minerals, these are lightweight and naturally aerated thus helping to improve thermal performance by, depending on sources, around the same or slightly less than polystyrene at about 0.10 W/mK.

[edit] Insulating cement plaster and render

Cement plaster and render is a mixture of sand, cement and water. It might be applied to masonry interiors for its structural capabilities and referred to as plaster or a parge coat (usually finished with gypsum plaster) but mostly it is applied externally forming the external render. While it is capable of achieving a smooth surface, external renders will often have a stipple effect, finished with masonry paint. Cement render offers greater moisture resistance than gypsum plaster, it is not considered to be a breathing construction.

[edit] Cement render with polystyrene

In the same way as with gypsum, insulating cement renders can be mixed with small polystyrene balls to improve thermal qualities. Cement is water resistant, heavy and hard, whilst the water resistant polystyrene is also lightweight with good thermal properties reducing the overall weight of the render, and maintaining water resistance and thermal performance.

The lowest thermal conductivity value is 0.05 W/mK with 80% of EPS, 20% of cement, and 1% of resin in the mixture granular aerogels. When better performance is required, other solutions concern the incorporation of granular aerogels in the plasters reaching values of thermal conductivity of 0.05 W/mK with 90% of aerogel in the mixture.

[edit] Cement render with lightweight minerals

Cement render can be applied with a mix containing vermiculite or perlite minerals, usually vermiculite. These are lightweight and naturally aerated thus helping improve the thermal performance whilst maintaining a weather seal.

[edit] Insulating lime plaster and render

Lime plaster is a composite of calcium hydroxide (lime) and sand (or other inert fillers). It may sometimes be strengthened with animal hair to prevent cracking and reduce shrinkage. Non-hydraulic lime (or quicklime) sets through contact with carbon dioxide in the atmosphere which transforms the calcium hydroxide into calcium carbonate (limestone), the product is softer and the setting process slower, but it improves with age. Hydraulic lime sets in part through a reaction with water. It is available with differing degrees of strength, but in general is faster and harder setting and more durable (at least initially) than non-hydraulic lime. In general hydraulic lime tends to be used for external render applications, whilst non-hydraulic lime is used internally. It is very common to used lime products on historic buildings.

Lime is often referred to as being a breathable construction, which means is has good levels of hygroscopicity, so can absorb and release moisture well, which can be important for historic buildings that tend not to have modern moisture protection details or damp membranes. This means the lower levels of walls are highly likely to absorb moisture. The breathability of the lime allows this moisture to enter the construction but also to evaporate and leave without causing damage, whereas the use of less breathable materials (especially in solid construction) might prevent to moisture from leaving.

[edit] Lime plaster with cork

Lime plaster is a composite of calcium hydroxide (lime) and sand (or other inert fillers). It may sometimes be strengthened with animal hair to prevent cracking and reduce shrinkage. It is typically more flexible and breathable than gypsum and cement plasters, and is most commonly used on older properties. The addition of fine cork particles can improved thermal performance.

Various sources and products will give varying figures for thermal performance in relation to moisture levels and the percent of cork aggregate in the mix, but in general K value figures range from 0.037 W/mK 0.045 W/mk.

[edit] Lime plaster with lightweight minerals

Lime plaster can be applied with a mix containing vermiculite or perlite minerals, these are lightweight and naturally aerated thus helping to improve thermal performance. As the aggregate is also porous it facilitates the continued breathability of the lime plaster.

Various sources and products will give varying figures for thermal performance in relation to moisture levels, whether it contains vermiculite or perlite aggregate and the percentage of aggregates in the mix, but in general K value figures range from 0.10 -0.19 W/mK though with increased levels of aggregate up to 40% this might be decreased to 0.059 W/mK.

[edit] Lime plaster with organic aggregate

Various sources and products will give varying figures for thermal performance in relation to moisture levels, the percent of hemp aggregates in the mix and how compact it is but in general K value figures for dry finishes range from 0.05 to 0.138 W/m K.

[edit] Lime render with cork

Lime render with lightweight minerals indicatively can have a thermal performance of 0.037 W/mK.

[edit] Lime render with hemp shiv

Lime render with hemp shiv generally has a thermal performance of 0.127 W/mk but it can be as low as 0.07–0.09 W/mK.

[edit] Clay and earth plasters

Many of the above products may also contain some clay. In general insulating clay and earth plaster products are not commonly available as stand alone products. Both types of plaster are considered more sustainable alternatives to modern plasters, with a lower embodied energy than gypsum, cement or lime based plasters.

Traditionally fibres have been added to both earth and clay plasters to increase tensile strength and reduce cracking, whilst there are a range of ‘natural’ colours that can also be added. Both clay and earth plaster are breathable and do not need to be painted, and in fact should not be painted. On its own the thermal conductivity of clay plaster is around 0.84 W/mK depending on moisture levels. Earth can vary but is similar.

Whilst standard products with insulating additives are not common, some natural builders recommend testing the addition of cork granules, perlite and fibres such as straw. Other suggestions to improve the thermal performance of clay or earth plaster might include adding sawdust, or paper pulp. In each case attention needs to be paid to the consistency of the over all mix and most likely the best results will be through a trial and error approach.