Insulating External Walls

Wall Insulation

Thermal transfer through external walls is a significant concern to designers who are looking to achieve good ratings on their SAP Calculations, which is why optimal insulation is so important. Heat loss can be substantial; energy wastage due to inadequate insulation can account for thirty percent of all thermal inefficiency. It follows that selecting the correct materials used in the construction of external walls is critical if you intend to lower energy bills.


The insulation of walls is dealt with in Part L un the UK Building Regulations, which requires walls to have a U-Value of 0.3W/m2K. Three specific methods dominate contemporary thinking on wall insulation:

• Insulation of internal walls
• Insulation of external walls
• Cavity filling

Any of these approaches will result in an adequately insulated, comfortable feeling, draught-free living space. Two of the methods—internal insulation and cavity insulation—have the advantage of not altering a building’s external appearance, but this does not imply that exterior insulation is without merit, far from it.

Four reasons to choose external wall insulation

The elimination of condensation

In external wall insulation, the dew point is reached outside the building’s facade. However, with internal wall and cavity approaches, there is a danger that condensation may occur within walls without great care being taken—a highly undesirable state of affairs.


The whole structure is weather-proof, ensuring that load-bearing elements are protected from large temperature variations and damage caused by freeze-thawing. There is also less risk of thermal bridging.

The preservation of precious internal space

External wall insulation does not affect a building’s interior. As a bonus, retrofitting work may be executed without disruption inside the space. No need, therefore, for occupants to move out during refurbishment.

Aesthetic advantages

External wall insulation offers the perfect opportunity to enhance the outward appearance of structures. The materials come in a broad palette of colours and finishes. This especially applies when renovation work is carried out, yet even new-builds can benefit from seamless facades on rendered finishes.

Components for external wall insulation

Substrate EWI systems are versatile enough to facilitate installation on a range of substrates: brick and block, or sheathing on either steel or timber.


Normally, expanded polystyrene, and mineral fibre board are the materials of choice here, with polystyrene having the edge on cost-effectiveness while, at the same time, facilitating design flexibility. For example, the creation of complicated forms or curves.

Polystyrene’s environmental credentials are excellent. Polystyrene boards contain no chlorofluorocarbons or hydrochlorofluorocarbons and come with an A+ BREEAM rating. They also come in a variety of thicknesses.

Mineral fibre board, manufactured from rock, does not combust and provides high-quality resistance to fire. Also, its vapour-permeability offers some noise-reduction advantages.


Both types of insulation may be repaired, either mechanically or through the use of an adhesive—sometimes by combining both, depending on the precise circumstances (substrate type, wind, and budget, for example.) Typical adhesives include polyurethane foam and cement-based powder, although polyurethane foam has certain advantages.


Easily pistol-applied, efficiently handled, and transported.

Quick setting

More storable than non-compressed foam products. Less packaging is needed too.

Thermally equal to EPS

Having only one ingredient, no mixing will be required. Adhesives in powder form more often than not supply adhesive and reinforcement functions. On the downside, they do require mixing in water and will have a longer cure time. Occasionally, for example, with a substrate, mechanical fixing proves necessary in addition to adhesive measures.

The Reinforcement Layer

This layer comprises a wet product and a strengthening mesh. The combination of these materials spreads stress uniformly across the insulation surface. Reinforcement layers come in mineral and synthetic configurations (cement-free).


This type of reinforcement features high impact resistance and is chiefly used with fibre glass. And a thin coat application.

Mineral reinforcement

This type is suitable for any thickness of application and permits small irregularities in the insulation surface. It can be used with glass fibre or metallic mesh.
Amongst the available materials, you’ll find renders and decorative elements. Cost-effective mineral cement or renders based on lime have the advantage of offering effective vapour permeability. However, they lack strength, come in a small range of colours, and can exhibit efflorescence.

Silicone renders offer excellent resistance to soiling and water. They are water-repellent and have above average vapour permeability. Synthetic material, which is acrylic-bound, has a high capacity to be bent or curved. They are available in many attractive and vibrant colours.
Hard cladding

Resin brick finishes mimic a natural brick finish, though they have the advantage of being light and are therefore easy to install. They also have excellent impact resistance.

Design issues with insulating external walls

Wind load

Often, the most significant force exerted on a building’s exterior. Several factors are involved in determining the actual force: Location, shape, size, and nearness to other structures or features, all playing a part. Wind load may result in unwanted building movement, destruction of all or part of the external cladding, and particularly vulnerable windows and roofs.

Optimal external wall insulation design is achievable only when informed by precise wind load data. Only a structural engineer is qualified to derive the calculations needed to accurately plot wind loads from information such as:

• Accurate building plans
• Building elevation data
• Geographical placement
• Setting (urban or suburban)
• The height of the structure compared to sea level.
• Surface finish
• Data on all significant building apertures
• Data on nearby structures

A bespoke fixing method is determined for each project following wind-load calculations.
Fire Although recently, interest in fire management has intensified, it has always been a critical design issue, most notably in the construction of high rise structures. Top of the range fire fighting defences can prove costly, which demands the earliest possible intervention in the design phase. Building Regulations (B) sets out the fire safety issues for construction projects.

External wall insulation systems have to be constructed to halt the fire’s spread up the exterior of tall buildings. The essential requirement is for horizontal fire-breaks from the 2nd storey up. Vertical fire-breaks may also be mandated to recent the room-to-room spread. Furthermore, cavity fire breaks must seal the cavity at each opening (windows, for example.) Precise details may vary but must, in every instance, meet both fire-officer and building regulation requirements.

Detailing Facade insulation requires detailing of sills and windows. Measures must be robust here, and to ensure weather-tightness, a sealant is necessary around window/door openings.
Under the ground level, external wall insulation must prevent water infiltrating insulation. Drainage underneath the system plays a part in minimising rainwater splashing, striking the lower section of the facade. Insulation beneath the damp-proof course commonly uses extruded or expanded polystyrene.