Cavity wall

Cavity walls consist of two 'skins' separated by a hollow space (cavity). The outer skin is commonly masonry. Masonry is an absorbent material, and therefor will slowly draw rainwater or even humidity into the wall. The cavity serves as a way to drain this water back out through weep holes in the base of the wall system. A cavity wall with masonry as both inner and outer skins more commonly referred to as a double wythe masonry wall.

= 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. [cite web
year=
month=
url=http://www.maconline.org/tech/design/cavitywalls/cavitywalls.html
title=Design Guide for Taller Cavity Walls
publisher=
accessdate=2007-09-14 ] For instance, in South Africa the building code requires cavity walls for residential buildings in coastal regions that have higher rainfall.

Insulation

An added benefit of cavity wall construction is that it provides the ability to more adequately insulate the building. A continuous layer of rigid insulation is easily fitted between the cavity and the inner skin of the wall. The insulation does not fill the cavity but rather slip in behind it. The cavity itself also helps in insulating the building by acting as a thermal break between the two skins.

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 at apertures for doors and windows. [cite web
year=
month=
url=http://www.cavalok.com/why_cavity_closers.html
title=Explanation of cavity closers
publisher=
accessdate=2008-09-29]

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 convert|203|mm|in|abbr=on long and convert|92|mm|in|abbr=on 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.

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. [ [http://www.aecb.net/forum/index.php?topic=561.0 AECB The sustainable building association - AECB Forum ] ] .

References

External links

* [http://www.cavitywall.net/ Cavity wall building system]