Cavity Wall Insulation

previous next

6 History of Cavity Walls

In the latter part of the 19th century cavity wall construction started to appear. In some towns and cities bye-laws required that 'hollow walls' be constructed with a half brick outer leaf, a cavity not less than two and a half inches wide (62mm), and a one brick thick inner leaf. The revised 1904 Model Bye-laws allowed two half-brick leaves (for modest houses which previously could have been built with a one brick wall). It was not, however, until the 1920s that this became the accepted form of construction; but with cavities slightly narrower than 62mm (most cavities from the 1930s to 1980s are 50 - 60mm). Cavity walls were cheaper than their solid wall counterparts; in addition they offer improved thermal insulation and better weather protection. The two halves of the wall were tied at regular intervals with steel or wrought iron wall ties. ). The external leaf of brickwork was usually laid in facing bricks, the internal leaf in commons.
A few very early walls were bonded with special bricks (left). 

Bricklaying mortars were usually based on lime although cement mortars (perhaps with the addition of some lime to improve its workability) steadily became more common during the 1920s and 1930s. 

The two images on the left are taken from Advanced Building Construction, anon, Longmans, Green & Company, 1899.

In prolonged wet conditions the outer leaf of a cavity wall can become quite wet. This is usually caused by water getting through the mortar joints. In sheltered conditions there may be slight dampness on the outer leaf's inner face (seepage mostly occurs through the vertical 'perpend' joints). In conditions of severe or very severe exposure (see map on next page) water can run down the outer leaf's inner face. It's vital, therefore, that the cavity is kept clear of any obstructions or debris which might allow water to cross to the inner leaf.     
Since the very early days of cavity construction, wall ties have included a 'drip' to shed any water trying to cross them. Ties should never slope inwards (ie towards the inner leaf) and should be free of mortar droppings. Where cavities were closed (usually around window and door openings) it was good, but not universal, practice to separate the inner and outer leaves with a vertical DPC.

The graphic on the left shows typical 1930s construction; the one below shows construction from the 1960s to 1980s. 

Blockwork has become very popular in the last 60 years or so because of its cost advantages over brickwork. There have been several aggregates used in their manufacture, such as crushed gravel, pulverised fuel ash, blast furnace clinker, gas coke breeze and pumice. Changes in industrial processes have meant that some of these aggregates are no longer available. Dense blocks, suitable for party walls, internal load bearing walls and foundation work, are made from cement, sand and crushed gravel. Until the 1980s they were also used for the internal leaf of cavity walls. Lightweight blocks are made from cement and various lightweight aggregates and are generally used for the internal skins of cavity walls. In modern construction aerated concrete blocks have become very popular. They are made from cement, lime, sand, pulverised fuel ash and aluminium powder. Once these materials are mixed with hot water the aluminium powder reacts with the lime to form millions of tiny pockets of hydrogen.
The construction at the head of a window can be quite complicated. A lintel or arch has to carry the loads safely across the opening; at the same time some form of cavity tray is required to prevent water in the cavity crossing to the inner leaf. The graphic on the left shows a number of solutions from the last 70 years or so.

In practice, patches of dampness which appear above windows can be quite confusing - there is often an automatic assumption that the problem is caused by penetrating dampness. In fact, the problem is often caused by condensation. This is explained in more detail on a later page. Similar problems can also occur the side of, and below, windows.        

In the 1970s the maximum acceptable U value fell to 1.0; it was further reduced to 0.60 in 1985 and 0.45 in the early 1990s. It has dropped much further since. Achieving 0.60 was relatively straightforward (see left hand graphic); it did not necessarily require cavity insulation. The reduction to 0.45, however, was normally achieved by adding some form of cavity insulation as the wall was being built - either partial or full fill.

Any wall constructed from the mid 1980s onwards may, therefore, have been built with some form of cavity insulation.       

A modern cavity wall has a ‘U’ value (a measure of a the wall’s ability to transmit heat) some 5 or 6 times better than its 1920s counterpart.

©2013 University of the West of England, Bristol
except where acknowledged
previous next