Insulating Walls in Historic Buildings

It is crucial to identify opportunities and constraints within the context of the building’s significance, construction, condition and performance. For example, the location, exposure and vulnerability of a building or elevation to direct sunlight, wind-driven rain or climate change hazards, such as flooding, can influence what type of insulation (if any) may be suitable.

Suitable interventions will vary depending on whether a historic building is of modern or traditional construction, even though the same process and principles apply to decision making.

Different types of traditional construction include:

• thin solid masonry (for example, single-skin brickwork)
• composite masonry (rubble-filled walls usually greater than 400mm thick)
• earth walls
• timber-framed walls
• early cavity masonry walls or 'hollow walls'

A wall may contain several materials, which may have different performance characteristics. For example, soft, permeable chalk and hard impervious flint have contrasting properties but are commonly found within the same wall. The presence of voids, irregular bonding patterns and concealed timbers also needs to be considered.

Early cavity masonry walls or 'hollow walls' were used from the 19th century in exposed areas (particularly coastal locations) to provide as much protection as possible from the elements, especially wind-driven rain. They were built using two separate leaves of masonry tied together to provide enhanced stability and economy of materials. To mitigate risks associated with bridging the cavity with any type of cavity fill insulation material, early cavity walls should be treated like solid walls when considering insulation measures.

Learn more: Risks of Energy Efficiency Interventions.

If walls are persistently damp, potential problems include:

• reduced thermal performance
• poor indoor air quality and mould growth on interior surface
• movement of tars, soluble salts and other chemicals through the wall, causing staining or damage at the surface
• decay in timbers in contact with the masonry
• increased vulnerability of the external fabric of the wall to frost damage
• corrosion of metallic components in contact with, or buried within, the wall
• structural implications or failures

If a wall has been wet for a long time, it can take years for it to dry out. Before installing any insulation, the source of moisture ingress must be addressed, and the building must be allowed time to dry.

Learn more: Maintenance and Repair for Energy Efficiency.

There is a continual interchange of moisture between the building fabric and the environment. This is strongly influenced by air movement and heat, along with other physical forces that drive the movement of moisture within porous materials.

Although the vapour permeability of materials receives a lot of attention in building conservation and product literature, the movement of water as a liquid is of greater significance. The diffusion of water vapour through porous and permeable materials due to vapour pressure gradients is relatively slow. In comparison, liquid water moves quickly through the connected pores of a material towards an evaporative surface. Capillary forces also drive the movement of liquid water and can be strong enough to raise it against gravity. However, gravity influences the movement of water in larger pores and voids, causing it to move downwards, for example from overflowing rainwater goods.

Moisture in external walls comes from:

• external sources, such as rainwater, ground moisture, defective drainage and leaky water mains
• internal sources, such as moisture generated by occupants' activities or plumbing leaks

In buildings of traditional construction, the building envelope deals with sources of moisture in the following ways:

• Roof coverings, overhangs and wall claddings, alongside water deflecting details such as plinths, eaves, cills and hoods (with drip details), can be used to deflect rainwater into rainwater goods or disposal systems.
• Liquid water penetration can be minimised using material thickness, construction type and lime/earth-based renders and coatings.
• Moisture in both vapour and liquid forms can be absorbed, stored, transported and released:
  • Liquid water is transported within permeable materials by capillary action or gravity flow. This is the primary mechanism of moisture movement thrrough a permeable material.
  • Water vapour is absorbed and released by porous hygroscopic materials, contibuting to moisture buffering.
  • Water vapour is transported via vapour diffusion through the linked pore structure of a permeable material or via unobstructed air pathways (such as cracks or gaps) within the thickness of the building assembly

In historic buildings, earthen walls were constructed on masonry plinths, usually at a height of 650mm or more, to act as a barrier to the capillary rise of water from the ground and to reduce water splashback on the the earth walls. Wide overhanging eaves sheltered the walls from rain and shed water effectively; this is referred to as 'having a good hat and boots'. Earthen construction was rendered in lime/earth and lime washed.

Timber-framed buildings often have walls as thin as 100mm, with infill panels of permeable materials such as wattle and daub or brick. In many cases the walls were covered by full render coats of lime plaster, weather boarding or hung tiles in exposed locations or on exposed elevations.

Solid masonry walls generally rely on their thickness to prevent moisture penetrating to the interior face. Walls were often rendered to increase their resistance to moisture penetration. In exposed locations, roughcast (wet dash in the north, harling in Scotland) or the inclusion of aggregate in lime render coatings increased the surface area to aid evaporation.

In early cavity masonry walls, moisture is absorbed into the outer leaf and then freely evaporates out again in drier conditions. However, under severe and prolonged exposure to moisture, the outer leaf can become saturated to the extent that water runs down the inner face into the cavity. This cavity is intended to provide a break that prevents moisture reaching the inner leaf. Filling the cavity with insulation risks liquid moisture finding its way to the inside. Cavity fill insulation is, therefore, unsuitable and early cavity walls should be treated in the same way as solid walls when considering installing insulation.

Modern methods of protecting buildings against moisture, including using materials that are highly resistant to the passage of water vapour or liquid water, are not usually appropriate for a traditionally constructed building. These methods risk trapping moisture in the building fabric, increasing fabric decay and negatively impacting occupants' health.

Learn more: Damp in Historic Buildings.

Earth and mass masonry are inherently good insulators. They also have high thermal mass (ability to absorb, store and release heat) and high thermal inertia (ability to resist temperature changes), which can help to regulate internal temperatures. In warmer months, these characteristics can help reduce overheating (when paired with adequate nighttime purge ventilation) and minimise the need for mechanical cooling. In winter months, they may reduce the need for heating.

Learn more: Overheating in Historic Buildings.

Many soluble salts are hygroscopic, so they absorb moisture from the air and cause surfaces to feel 'clammy' to the touch. Salts may also recrystallise on a material's surface or within surface pores at drying faces, where the related expansion within the pores can turn sound masonry into powder.

The interface between existing walls and added insulation can be susceptible to cycles of evaporation, condensation and salt crystallisation. As these locations are often hidden from view, major deterioration may occur before anybody becomes aware there is a problem.

Unfortunately salts are notoriously difficult to remove effectively from porous builsing materials such as brickwork, masonry and plasters.

It is prudent to consider the whole life carbon cost of installing wall insulation, to ensure operational carbon savings are not outweighed by embodied carbon costs. Other noteworthy considerations include environmental impact, resource intensity and the release of VOCs or particulates from some materials.

More research is needed to help us fully understand how certain forms of construction and certain materials can mitigate or exacerbate the risks of energy efficiency interventions. In some cases, the technical risks of adding insulation to a building's walls will be too great. Alternative ways of providing a more cost-effective long-term solution to improving energy efficiency may be needed.