Insulating Solid-Walled Homes Safely: Myths, Facts, and Expert Advice

Recent headlines have drawn attention to the widespread failures associated with insulation installed under government-backed energy efficiency schemes. According to a National Audit Office (NAO) investigation, almost all external wall insulation fitted through the Energy Company Obligation (ECO) and Great British Insulation Scheme (GBIS) requires repair or replacement. Of the approximately 23,000 homes that received external wall insulation, 98% were found to have defects requiring remedial work.

The report also identified significant issues with internal wall insulation, with between 9,000 and 13,000 homes—representing around 29% of installations—experiencing major problems. These findings highlight systemic shortcomings in design, material selection, and installation quality across both external and internal insulation programs.

The problems largely stem from underskilled specifiers, poor workmanship, and a fundamental misunderstanding of how buildings naturally manage moisture. Many of these issues could have been avoided with proper adherence to PAS2030 and PAS2035 standards. Having attended some of the government-funded training ourselves, however, it’s easy to see how gaps in knowledge can arise despite these frameworks.

However, it isn’t all doom and gloom. Insulation can perform very effectively when installed correctly with appropriate materials. The key is ensuring that both installers and specifiers possess a thorough understanding of how buildings naturally manage moisture.

 

Read the report here: Energy efficiency installations under the Energy Company Obligation - NAO report

How a solid walls works

Understanding a property's moisture management system is essential for the successful installation of insulation. Moisture will inevitably enter a wall, but by addressing it properly with the right materials, it can be managed effectively and will not cause problems

When there is a temperature difference between the warm interior and the cold exterior of a wall, moisture vapour can condense within the wall structure — a process known as interstitial condensation. The use of impermeable materials, such as plastic vapour barriers or certain modern insulation products, can exacerbate this issue by trapping moisture either inside the wall or within the living space.

A more effective approach is to use vapour-open, breathable materials that allow moisture to move through and evaporate naturally. This helps the wall regulate humidity levels and remain dry, reducing the need for mechanical ventilation while maintaining a healthy indoor environment.

In fact, allowing moisture absorption within the wall assembly is beneficial. This process contributes to hygroscopic buffering — the ability of natural materials to moderate indoor humidity levels by temporarily absorbing and releasing moisture. This stabilising effect helps maintain a comfortable and healthy indoor environment, reducing the likelihood of surface condensation, mould growth, and associated health risks.

The moisture absorbed by the wall — typically in vapour form — will naturally dry out over time. Depending on environmental conditions, this moisture either returns to the interior as indoor humidity drops or migrates outward toward the external surface, where it safely evaporates.

Is insulation bad?

Insulation and Moisture Balance in Traditional Buildings

With rising energy costs, it is understandable that homeowners are seeking ways to improve thermal efficiency and reduce heat loss through insulation. However, recent reports have suggested that certain insulation methods can disrupt the natural moisture balance within traditional buildings. When this balance is disturbed, the building’s ability to manage moisture is reduced, increasing the risk of dampness and subsequent fabric deterioration.

In our view, much of the press surrounding these concerns can be misleading. It is essential to distinguish between different insulation types and to understand which are appropriate for use with solid walls.

A solid wall has a natural capacity to store and release a significant amount of moisture before its thermal performance is affected. When hygroscopic, vapour-permeable materials—such as lime-based insulating plasters—are used, the wall’s natural moisture movement and diffusion continue largely unchanged. Problems typically arise only when impermeable insulation materials are applied, or when insulation is installed in excessive thicknesses in an effort to meet modern building regulations or achieve U-values comparable to new constructions. In such cases, moisture can build up within the wall, resulting in reduced thermal efficiency and long-term deterioration of the building fabric.

What we propose and why

Do not be scared by insulation. The correct materials installed well can have a lot of benefits.

External insulation offers several key advantages in the thermal and moisture management of solid wall construction. Unlike internal insulation, it does not increase the moisture content of the wall. By placing the insulating layer on the exterior, the wall structure remains warm and dry, allowing any internal moisture to diffuse outward and evaporate naturally.

In addition to maintaining the wall’s moisture balance, external insulation significantly reduces heat losses by minimizing thermal bridging and stabilizing internal surface temperatures. The performance of external insulation generally improves with increased thickness; in most cases, the thicker the insulation layer, the greater the thermal resistance and energy savings achieved.

When designed and detailed correctly— with vapor-permeable, breathable materials—external insulation systems can enhance both the thermal efficiency and the durability of the building fabric without compromising its ability to manage moisture.

Internal insulation presents a higher risk when applied in thick layers, as required to meet certain building regulations. However, thinner applications can be highly effective and offer additional benefits by maintaining warmer internal surface temperatures. This has a secondary advantage: up to 80% of heat loss occurs through radiant exchange with interior surfaces rather than through convective air movement. Consequently, even a modest layer of internal insulation can significantly improve comfort levels and reduce heating demand. Traditional lime plasters already provide some of this effect due to their composition of fine, porous aggregates, unlike modern plasters made with dense, washed sands. Further improvements can be achieved by incorporating insulating, porous aggregates into the plaster mix.

Material Choice

Hemp Boards/Batts and Wood Fibre Insulation

Hemp boards, hemp batts, and wood fibre insulation are highly effective natural insulators. Thin layers — typically up to 40–60 mm — perform well without the need for a WUFI test (a computer model used to assess specific wall build-ups and ensure moisture safety). Even at just 20 mm, these systems can significantly improve thermal performance.

These materials are installed directly onto the wall with no cavity, allowing unhindered vapour transmission while minimising space loss. They can also be used externally, offering versatile application options.

Insulating Lime and Clay Plasters

Although insulating lime and clay plasters are generally less effective in terms of pure thermal resistance, they carry far lower moisture risks due to their high capillarity and vapour permeability. Even thin internal layers of around 5 mm can provide noticeable improvements to thermal comfort and surface temperature, making them a valuable component in breathable wall systems.

PIR Insulation and Moisture Risks

Polyisocyanurate (PIR) insulation products, such as Celotex, have no capacity to absorb or move moisture. As a result, they trap any moisture present within the wall and condensation is created on the cold side of the insulation. To mitigate this, PIR systems are typically specified with an internal air gap between the insulation and the wall, allowing trapped moisture to either vent out or drain to the floor.

However, this design introduces several problems. Moisture levels within the cavity can become very high, creating ideal conditions for mould growth. These mould spores can become airborne, posing potential health risks to occupants. Additionally, the inclusion of an air gap increases the overall wall thickness, leading to further space loss.

Another major issue with air gaps is thermal bypass, where heat moves around rather than through the insulation layer as well as convection currents. This significantly reduces the overall thermal performance of the wall.

Through experience of resolving damp problems in solid wall buildings that have used PIR insulation, it is evident that damp and moisture-related issues are a common outcome with this type of system.

Always Seek Expert Advice

When carried out correctly, insulation can offer substantial benefits — improving comfort, reducing heat loss, and lowering energy costs. However, as recent reports have highlighted, specialist advice and experienced installers are essential to ensure that insulation is compatible with your property’s construction and moisture dynamics.

We strongly recommend that homeowners take the time to research different insulation materials and methods, and seek guidance from professionals familiar with traditional and breathable building systems.

It’s not all doom and gloom — with the right approach, insulation can be both safe and highly effective, enhancing energy performance while preserving the health and longevity of the building.