Cavity wall

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Cavity walls are masonry walls consisting of two masonry walls ('skins') separated by a small hollow space (cavity). The cavity may or may not be filled by other material.

1 Introduction
2 Insulation
3 Specification
4 History
5 References
6 External links

[edit] Introduction

The masonry skins of a cavity wall can be brickwork, blockwork or similar. Different masonry materials can be used on either side of the cavity. The cavity is initially empty but can be filled with insulation by various methods. Cavity walls are more time consuming –end therefore slightly more expensive– to build than a walls with the two skins bonded together, but they provided better sound and heat insulation and most importantly resistance to rain penetration.^[1] For instance, in South Africa the building code requires cavity walls for residential buildings in coastal regions that have higher rainfall.

[edit] Insulation

Even with the cavity left unfilled, the air trapped is a good insulator and dramatically reduces thermal energy lost through the walls. This is because, firstly, air is a poor conductor of heat and it must first be conducted through it onto the outer walls before being convected, as well as that the air pockets can hold heat for a longer time than the bricks, and therefore thermal energy does not move into the hotter environment (the cavity), but instead into colder regions. There is also a Damp proof Course which is laid in the cavity which prevents moisture from entering through the walls and causing damp in the house or rot.^[2]

With environmental conditions becoming more of an issue, people now take much more interest in reducing energy wastage and cavity wall insulation is a cost effective way to reduce the amount of heat (as much as 35%) lost from convection off walls. As well as being more environmentally friendly, it can reduce heating costs as more of the heat is used effectively and it is often used as a first step, due to its low payback time and smaller initial installation costs. As the demand for energy efficiency in buildings increases the issue of thermal bridging in cavity wall openings is becoming more prominent. Solutions to thermal bridging include cavity closers, an insulated frame which seals the cavity.

[edit] Specification

A cavity wall is often constructed with a "half brick" thick outer skin and a dense concrete blockwork inner skin (100 mm). The "half brick" actually in bricklaying parlance actually refers to wall made with bricks laid end to end which just as a normal single-skinned brick wall would be — the "half" in reference to normal thickness which usually equates to "half the length" of the brick. For example, a brick might be 203 mm (8.0 in) long and 92 mm (3.6 in) wide which results in a single-skinned wall as thick as the brick is wide (92 mm in this example) and a double-skinned (no cavity) wall as wide as the brick is long (203 mm). The single-skinned wall is therefore roughly half the thickness of the double-skinned wall, but only because most bricks are roughly twice as long as they are wide.

Whereas exposed brickwork consisting of visually appealing bricks are often used for the outer skin of a cavity wall, the inner skin is often constructed of cheaper "plaster bricks" or cement blocks, because the interior facing side it often plastered leaving no part of the inner skin visible.

The cavity may be partially or completely filled with thermal insulation from the damp-proof course upwards. The two leaves are connected by wall ties to spread lateral loads. Cavity sizes have to adhere to a certain minimum to prevent water penetration and typically are at least 50 mm to 100 mm. Sizes are increasing rapidly to accommodate super-insulating wall specifications, but the larger the cavity the more interior floor area is sacrificed.

[edit] History

The cavity wall method of construction was introduced into the United Kingdom during the 19th century and gained widespread use from the 1920s. In some early examples stones were used to tie the two leaves of the cavity wall together. Initially cavity widths were extremely narrow and were primarily implemented to prevent the passage of moisture into the interior of the building. The widespread introduction of insulation into the cavity began in the 1970s with it becoming compulsory in building regulations during the 1990s. ^[3].