The Challenges of Insulating Old Buildings

The energy efficiency of old buildings is a hot topic nowadays. Unlike new buildings which are designed and built with adequate insulation in mind, the retrofit insulation of old and listed buildings – especially the ones with solid walls – poses significant challenges, as the materials used in this regard must satisfy multiple (often conflicting) requirements:

a1 wall page web
  • Resist moisture: due to their construction, old walls are often affected by multiple moisture sources. Wind driven rain can easily saturate external walls. Temperature differences can be an ongoing source of condensation on external walls. Moreover, the base of old walls is often subject to rising damp resulting in ongoing moisture and salt damage.
  • Breathable: in order to prevent the build-up of moisture behind the thermal insulation (which can seriously damage the building fabric long-term), the insulation MUST be breathable so the humidity can evaporate. 
  • Traditional look and feel: wherever possible, the use of traditional, building-friendly materials is recommended.

The vast majority of insulating solutions on the marketplace has been developed for newer buildings. They offer good thermal insulation, but modern materials – foam, plastic, metal foil, cement etc. – are non breathable and thus trap moisture.

However, the presence of moisture is one of the major problems. Many traditional breathable materials that can last for decades in a damp-free environment, deteriorate quickly in the presence of elevated humidity and salts. Solid brick or stone walls subject to moisture push traditional materials and lime plasters to their limit, resulting in a significantly shorter lifespan and decreased performance.

Due to increasing energy costs, government pressure as well as the need for greater comfort, breathability is often sacrificed, leading to severe dampness problems in old building years or decades later.

The Right Solution

Here is a traditional lime thermal insulation system that can be adapted to any building, old or new - ticking all relevant boxes without making any compromises or shortcuts.


Here are the main benefits of the MGN lime insulation system, which has:

  • Excellent thermal performance
  • Breathable throughout
  • Traditional look & feel
  • Suitable for listed buildings, heritage projects as well as green buildings
  • Healthy, made of natural materials, no chemicals
  • Stops liquid moisture and salts penetration
  • Very long lifetime (decades)
  • Internal or external application possible

Here is a sketch of both internal and external insulation.

internal insulation

Internal insulation

external insulation

External insulation

Here are at the various components of the insulation system in more detail.

1. Salt-Resistant Lime Base Coat

Most often, lime thermal insulation is applied directly onto brick or stone walls. Although this is a common practice, the presence of moisture and salts affects the thermal insulation long-term, reducing its longevity.

To mitigate the problem of dampness and salts, and thus to prolong the life expectancy of the thermal insulation, we always recommend the application of a special lime base coat under the insulation. This is a critical component of the insulation system, as it perform several important functions:

  • Acts as a selective moisture barrier that stops all forms of liquid water, whether sideways moisture penetration (underground), wind driven rain or rainwater (above ground). 
  • Acts a salt barrier, preventing the crystallization of salts into the thermal plaster. Salt crystallization is the main cause of crumbling and degradation of plasters and historic masonry - not moisture!
  • It's open pore and breathable, allowing the passage of vapours. 
  • Consolidates the frail, crumbly masonry, providing you with a dry, solid surface to build on. 
  • By stopping the liquid water helps reducing the internal humidity, contributing to a drier, more comfortable environment.

All this is achieved with a traditional lime plaster of which origins stretch back to ancient Rome. Being outstanding architects and builders, the Romans have discovered that by adding certain volcanic soils and other minerals to lime, they can significantly alter its properties, including its mechanical strength and water resistance, while retaining its breathability.

The materials most commonly added to lime were natural pozzolans (volcanic soils or rock fragments) and cocciopesto (milled bricks or terracotta fragments). These reacted chemically with the free lime, forming water resistant compounds. Such mortars were able to harden quicker not only in the presence of water but even underwater in the total absence of air, and they are known as hydraulic mortars (water limes).

These traditional pozzolanic Roman mortars differ significantly from today’s NHL (Natural hydraulic lime) mortars. NHL mortars are made of lime and clay. For the clay to react with the lime, the mix must be fired at higher temperatures (at about 1200 °C) than lime alone (which is fired at around 850-900 °C). The higher temperatures close off some of the pores, making the NHL plasters less breathable than normal limes. Thus, NHL plasters trade breathability for improved water resistance.

Natural pozzolanic lime mortars are made of lime and volcanic ashes / sands. Because the volcanic ingredients have already been pre-burned by the volcano, these are incredibly breathable and they readily react with the lime at low firing temperatures (850-900 °C), resulting in a very breathable final product.

natural pozzolan

Natural volcanic pozzolans have a highly porous, breathable pore structure.

Various volcanic lime mortars have been extensively used by the Romans in very demanding environments including sewers, ports, spas and aqueducts and they have survived for many centuries. They have also been widely used in Venice, well suited to the humid and aggressive environment of the Venetian lagoon.

2. Lime Thermal Insulation

The thermal insulation coat is the main coat responsible for the thermal performance of the building.

We have several thermal insulation materials, depending on the requirements of your project. Your choices on which thermal material(s) to use might depend on some of these factors:

  • Target U-value
  • Available internal space
  • Price / budget

Here are the most common thermal plaster choices:

  • Termointonaco 2020 MGN: an extremely lightweight (278 kg/m³) traditional lime thermal plaster with excellent insulating properties (λ = 0.04). It can be built up to a considerable thickness (up to 120 mm) in subsequent 20 mm coats.
  • Termorasante Aerogel 2020: this is a breathable superinsulator, a lime plaster mixed with Aerogel, one of the lightest solid materials known to man, with extremely low thermal conductivity and heat reflectance (λ = 0.01). Due to its high aerogel content, offers outstanding thermal insulation in a thin form factor ­– providing an elegant breathable solution against condensation problems in any building. The application of a thin (5­-10 mm) plaster coat can raise the surface temperature of walls by 3 to 5 °C. Thanks to its excellent heat reflectance, it reflects the heat back into the room, creating high thermal comfort indoors. 

These materials can be applied individually or in combination.

Here are some typical examples and achievable U-values. 


U-value example

3. Lime Finish

Because thermal plasters are light and somewhat soft materials, they need mechanical protection. So instead of a "general purpose" lime finish, we use a mechanically more resilient but equally breathable lime finish. Thus, the lime finish performs a dual decorative-protective function.

The finish can be made completely smooth or left slightly textured looking traditional lime finish.

Materials Used

Here is a list of materials used in the thermal insulation system.

  1. Rinzaffo MGN – Salt Resistant Protective Lime Base Coat
  2. Termointonaco 2020 MGN – High Performance Lime Thermal Insulation Plaster
  3. TermoRasante AeroGel MGN – Lime-Aerogel Superinsulating Lime Plaster
  4. Rasante B40 MGN – Protective-Decorative Lime Finish

Key Application Points

Here are some key points about the application of each plaster coat.

1. Rinzaffo MGN – Salt Resistant Protective Lime Base Coat.

  • For above ground level walls one single coat is recommended, in not less than 10 mm thickness. 
  • For underground walls or for walls subject to sideways water pressure two coats should be applied at not less than 20 mm combined.
  • Extra material should be allowed for uneven walls, which take up extra material.
  • Attention should be paid to compress the base coat and close off all pores.
  • The material dries quite quickly, typically in 1-2 days, depending on ambient conditions. 

2. Termointonaco 2020 MGN – High Performance Lime Thermal Insulation Plaster:

Thermal plasters behave differently from normal denser lime platers. While a regular lime plaster takes up about 20% water (e.g. 25 kg bag vs 5 litre of water), thermal insulation plasters take up significantly more, about 50% water (e.g. 10 kg bag vs 5 litres of water). All this water needs to evaporate, so drying / carbonation times of thermal plasters is longer.

  • Being a very light material, this thermal plaster is typically applied in 20 mm coats. 
  • Additional coats can be added up to about 120 mm combined thickness.
  • Hardening time between coats can be 3 days or more, depending on ambient conditions. 
  • Once the thermal coat has hardened, the next coat can be applied. The application of 40-60 mm of thermal insulation is quote common.
  • After the whole thermal layer is on the walls, the plaster needs to be left to harden and carbonate. This can take 2-3 weeks, depending on the total thickness of the thermal coat.

3. TermoRasante AeroGel MGN – Lime-Aerogel Superinsulating Lime Plaster

  • The aerogel-lime plaster is ideal for space-saving applications, such as window reveals and can significantly reduce the thermal bridging of hard-to-insulate areas.
  • It can be applied in 5-10 mm coats in one go.
  • The common thickness is 10 mm, but more can be applied if needed (within limit as it is fine granulation plaster). 
  • Hardening time between coats can be 3 days or more, depending on ambient conditions.

4. Rasante B40 MGN – Protective-Decorative Lime Finish

  • The decorative-protective finish should only applied after the thermal coat has hardened and carbonated, which can take 2-3 weeks, depending on the total thickness of the applied thermal coat.
  • For smoother finish the finishing coat is recommended to be applied in 2 x 2 mm coats, with a fine 4-5 mm fibreglass mesh to prevent surface shrinking and fine cracks. 
  • A lime finish should be painted with a breathable mineral paint. Plastic, film-forming emulsion paints should be avoided.