The Rise and fall of The Cavity Wall - A 20th Century Phenomenon

In spite of pioneering Modern Movement enthusiasm for pre-cast panels and lightweight frame construction, most housing in Britain is still built with bricks and blocks. The only significant change throughout the last hundred years, has been the widespread adoption of the cavity wall.

Prior to 1900, cavity walling was not common. Its ancestry may owe something to traditional stone masonry, some of which had since the Middle Ages been content with a rubble core sandwiched between two outer leaves. Much Nineteenth Century low-cost housing had one-brick external walls. In more exposed conditions – as in most of Scotland – these could be rendered to keep out the rain. Otherwise, where internal heating and ventilation were inadequate, damp plaster, falling wallpaper and mould were often an inevitable result.

The gradual introduction of cavity walling after 1900, achieved better weather resistance with little increase in cost. In two-storey building it was often restricted to the ground floor, resulting in the characteristic stringcourse at first floor level, which facilitated the reduction from 11 to 9 inches.

Using the same number of bricks, a properly designed and built brick cavity wall is better than a solid brick wall at keeping rain out and warmth in. On the down side, it is more difficult to build, is weaker and uses more land than the solid alternative. It necessitated the introduction of cavity ties and is now commonly associated with the use of steel wind posts. If not treated carefully, these structural adjuncts to slender, cavity construction compromise performance, buildability and durability.

To keep out rain, it was necessary to keep the cavity clean and drained. This meant preventing mortar from dropping into it, and providing weep-holes at the base and over lintols. There was debate about the desirability of filling the cavity below the damp proof course at the bottom, and of ventilating it at the top. Building textbooks spelt out in ever-greater detail how cavity walls should be constructed, and experience eventually led to the guidance given in British Standards and the Building Regulations.

In the housing boom after the Second World War, flats and maisonettes of three and four storeys were often still built in solid brick masonry, the external walls being brick-and-a-half thick. But with two-storey work, the cavity wall became the norm. Inner leaves of common brick were soon abandoned in favour of cheaper concrete blockwork. After the middle ’sixties, thermal insulation began to assume an importance which has now become dominant. This was another reason for the use of a blockwork inner skin. But improved thermal efficiency with blocks comes at the expense of strength, and can only go so far without increasing wall thickness. The solution was eventually sought in the use of non-load-bearing thermal insulants.

These insulants are susceptible to impact damage, and are therefore customarily now placed in the cavity, where the masonry leaves protect them. Completely filling the cavity increases the risk of water penetration (Building Research Establishment Digest 236/1980) and partial fill has become more common. This requires a wider cavity and longer cavity ties. Theoretically, a gap of around 10mm would stop water crossing a perfectly formed, sealed cavity. Practical experience has shown that designing for a 50mm clear cavity is required to stop rain penetrating to the inner leaf (Building Regulations C4.11). It is now common practice to fix partial fill rigid board insulation tight against the inner leaf.

In most walls in houses, much of the wall is window or door, and around each opening the cavity disappears, together with most of the insulation. Up to 50% or more of a typical house wall will therefore be relatively poorly insulated, and additional weepholes over lintol flashings will still further adversely affect thermal performance.

Before the introduction of cavity insulation, best practice required the use of a suspended batten below working level to catch any mortar droppings. The intention was to prevent bridging of the cavity. Mortar – especially where lodged on wall-ties - is a well-known cause of internal damp patches. With cavity insulation, this practice becomes doubly difficult. One consequence is a need to remove blocks above damp proof courses so that droppings can be cleared. There can be few, if any, partially filled cavities entirely free from unwanted debris.

It is, of course, possible for the insulating material to be placed internally. Damage to it can easily be prevented with a plaster lining. With a building occupied and heated only at intervals, this may be acceptable. But the loss of beneficial heat storage within the wall will otherwise detract from overall thermal performance and efficiency. It is difficult to maintain continuity in internal insulation at floors, ceilings, etc.
It is also possible to place insulation on the outside, where it could be protected with a suitable render. This is discussed in BS 5617, which insists that the render should be impervious and long-lasting. Suitable methods for this are covered by BS5262, which recommends the use of a reinforced render.
Adequate thermal resistance could readily be achieved with face insulation – inside or out – without the need for a cavity. If the insulation is placed inside, the problem of rain penetration remains and some cold bridging at floors, etc. is difficult to avoid. If it is placed under a water resisting coating on the outside of the wall, this is no longer so, but other issues arise. The most significant are susceptibility to impact damage and restraint.

Interstitial condensation may occur in both cavity and solid walls. The risk of condensation damage depends, amongst other matters, on the placement and thickness of thermal insulation and on the vapour resistance of the materials used in the walls. Generally, in the UK climate, the further out the insulation the lower the risk of condensate damaging building interiors. Unless continuous vapour control layers are formed to the inside of the walls, it is important to ensure that all parts of the walls, which are outside the insulation, are adequately vapour permeable. This limits the choice of water resisting coatings that can safely be used with external insulation systems.

To deal with interstitial moisture in externally insulated walls, self-draining, ventilated or pressure-equalised systems are offered. This may be in the form of bonded, mechanically fixed or framed construction. Available external finishes include a wide range of pre-formed panels as well as in situ finishes.

To comply with the regulations current at the end of the last millennium, cavity walls with partial fill insulation will be typically about 300mm thick of which 50mm is thermal insulation and 200mm is masonry. Equivalent thermal and better structural performance can be achieved with solid walls from thicknesses of about 250mm upwards using insulated render claddings.

Placing insulation on the outside has potential benefits in construction and repair. It allows specialist contractors to carry out the insulation work after the masonry construction is finished. Errors causing, for example, pattern staining due to cold bridging and condensation can, with some of the externally insulated systems, be corrected more readily and with less disturbance to occupants than with internal or cavity insulation.
Proprietary systems, now widely used throughout Europe and the USA, claim to have overcome all the difficulties that might prevent a reversion to solid external walling with external insulation. Such systems could greatly simplify and potentially cheapen masonry construction, and offers the prospect of the highest levels of thermal insulation. The English preference for fair-faced brickwork is by no means universal, and the difficulties associated with insulated cavity walling might now lead to its early demise.