External loadbearing wall design and Part L

When considering the design of external walls to non-framed buildings there are generally four types to consider. These are solid, cavity, layered and clad. There are hybrids of course but the basics design constraints remain the same. In all forms of construction the wall needs to satisfy some basic functional requirements. These include the ability to resist moisture penetration, conserve energy, resist condensation, remain structurally stable and durable, non toxic, fire resistant, sound resistant, sustainable and secure. There are a string of other matters that need to be considered but the changing requirements of the building regulations over the last few years is bringing about a change in the generally accepted solutions.

The use of solid walls has always been a design solution. The design satisfies many of the basic functional requirements including resistance to the penetration of moisture. However, solid walls often rely on robust materials for durability and as a consequence are poor insulators. The problem of cold bridging and damp proofing at openings has always been a problem.

The use of a clad walling system by the use of an applied finish such as rendering or a fixed finish such as tiling or rain screen can overcome the problems associated with damp penetration as the external finish effectively provides a barrier to water penetration. This enables greater choice in the design of the wall in terms or structural stability and thermal insulation. It also enables the external finish to be maintained and/or replaced. It often requires good detailing especially where the main structural support is timber frame. This form of construction is one solution that provides various solutions to Part L. The use of materials susceptible to decay such as timber needs to be carefully designed to avoid problems with interstitial condensation. Externally insulated systems tend to remove many on the problems especially where it is used in conjunction with such materials as aerated concrete blockwork. Externally insulated structures overcome many of the problems associated with cold bridging and are a preferred choice in the refurbishment of some existing structures suffering from decay.

Cavity walls have been the favoured solution to the design of external walls for the greater part of the 20th century. Cavity walls were introduced to overcome any risk associated with the penetration of moisture particularly at window and roof junctions. They rely on good quality design, workmanship, and quality control on site. Like clad walls they permit the use of a robust outer leaf and an inner leaf that can be structurally as well as thermally efficient. This has enabled the use of aesthetically agreeable outer leafs that match the vernacular Architecture of the area, an attribute that is sometimes difficult to achieve with clad walls. The functional demand on cavity walls has increased with the need to comply with ever stringent thermal insulation requirements. This has resulted in the use of timber frame solutions or masonry solution with partially fill cavity construction. The timber frame solution can provide very good thermal performance and this in combination with prefabrication techniques has brought additional economies in construction. It should be born in mind that there has been criticism associated with the erection of timber frame dwellings and quite recently there has been some notable outbreaks of fire at the construction stage. Modern timber frame construction incorporating a masonry outer leaf requires good detailing to overcome problems associated with the bridging of the cavity, interstitial condensation and spread of fire within the cavity. To provide a thermally efficient masonry cavity wall inevitably increases the width of the structural cavity and as consequence it has implications for structural stability, moisture penetration, quality control and costs. There are particularly demands to avoid cold bridging, air tightness and unplanned bridging of the cavity.

Layered walls as the name suggests are contiguous layers of material that satisfy the functional requirements of the wall. The use of cavity walls where the cavity is completely filled with insulating material is probably the best example. With this type of construction there is a risk that moisture will penetrate the outer layer(s) and migrate vertically to accumulate and penetrate the inner layers to the inside of the building. For this reason many layered walls incorporate the same detailing as cavity walls and incorporate damp proof courses and cavity trays etc. The insulation layer can be formed as the wall is erected or injected after the wall has been formed. Unless the outer leaf(s) can be shown to be fully water resistant the material ‘filling the cavity’ must resist the penetration of moisture. This is usually demonstrated through testing and certification. The advantage of this form of construction is that the overall thickness of the wall is reduced whilst facilitating good thermal performance. It also means that the width of the structural cavity can be reduced and thereby increasing stability. The main disadvantage is that the use of the insulation material within the cavity effectively bridges the cavity and there is a risk of moisture penetration.

The increased demand for lower U values will inevitably bring about further changes to external wall construction and the use of ‘traditional’ cavity wall construction may pass.

The new requirements of part L will have a profound effect on external wall construction. If you have any comments to make on future developments then please let us know.