EXTERNAL WALLS AND FRAME STRUCTURE

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Walls and Building Structure

The external walls of a structure are vital to the integrity safety and aesthetics of the building. They are more often then not the load bearing walls and must also face the harsh conditions that the weather can throw at a building. External walls normally consist of two different designs in conventional style buildings cavity and solid.

Cavity walls

Cavity walls are normally made from brickwork and were introduced to the UK in the 19th century, becoming a commonly accepted building practice by the 1920s. Commonly a cavity wall is constructed with a 102mm half brick outer skin with a 100mm dense brick inner skin. The size of a cavity will vary. Traditionally sizes of between 50mm and 100mm were used but this is now being increased in modern builds to allow for extra insulation. The two walls are joined in places by wall ties to help spread lateral loads.

Brick

Brick can also be clad onto a modern eco timber structure. Some more attractive bricks such as older London Reds, used to match other street brickwork, are recycled material and so attractive to an eco builder; but the motive to use them is to create a uniform blandness of appearance. ('matching the existing street scene').

The air in a cavity wall acts as a poor conductor of heat. This has the effect of insulating a building, as the heat does not easily pass into the air of the cavity. The air in a cavity wall will also heat up during the day releasing heat at a slow rate helping to keep a building warm. A damp proof course laid through the cavity will prevent moisture from entering the building through the walls.

Whilst the air in a cavity provides greater insulation than a comparable solid wall would there are ways in which it can be further enhanced. The most common of these is to fill the cavity with insulating foam.

This traps in even more heat and makes it easier to regulate the temperature inside a building. Cavity wall insulation can be injected into any building with a cavity wall, the process usually takes three to four hours and can cost around 500GBP for an average house.

The savings that can be expected from insulating a cavity wall are around 90GBP a year allowing the insulation to pay for itself within 6 years. This translates to an annual carbon saving of 750kg.

Solid walls

Solid load bearing external walls can be made using a variety of different techniques.

The most common is to use masonry but other alternative construction methods are available such as rammed earth and cob building. Solid walls lose heat much more rapidly then cavity walls but they can be insulated from both the inside and outside.

Insulation

Outside insulation is called rendering and is often done decoratively. This type of insulation is not cheap but it does have the added bonus of helping to weatherproof a wall. Insulating a solid wall internally is a cheaper option. This often costs around £42/m2, and entails insulating boards being placed onto the wall. This type of insulation on a family home can save up to 300GBP a year and as much as 2.4 tonnes of carbon emissions.

Insulating Concrete Framework

Insulating Concrete Framework (ICF) walling is a construction method beginning to get a lot of popularity for its ease of construction and insulating properties. In an ICF system a framework made from insulating material (usually expanded polystyrene) designed to lock into each other similar to LEGO bricks allowing the build to not require binder materials such as mortar. Once the framework is in place concrete is poured in.

When this sets the structure is strong and has a high insulation value. The method of which ICF forms are created allow them to be moulded into various shapes and designs as well as requiring less labour and time to build. The use of thick concrete also provides exceptional sound insulation and prevents air leakage into a property.

Frame construction

Framed construction methods allow different materials to be used for external walls. In a framed construction the frame bears the load of the building instead of the walls. Frames can be made from various materials including wood and steel.

Timber framed

A wood framed building can be made from sustainably forested materials such as FSC sourced timber instead of energy intensive man-made materials such as concrete. Wood is not as good as an insulator as masonry and concrete. Wood's low density also stops it from acting as a heat store, requiring faster acting heating systems to maintain a constant temperature.

Timber framed buildings can be premade in sections in a factory. Design drawings are engineered to take account of load bearing sections etc, then the kit of parts is made and transported to site in a series of staged deliveries. The timber builders, not always from the manufacturing company, then assemble the building according to the instructions.

Another method is so-called stick building, where the timber builders make up the building from the engineered drawings, but the wood is measured and cut on site. This is a cheaper method.

Concrete frame construction

Above is a high rise under construction in London. This is using a 1960s method which is called RC. This is reinforced concrete framing. The concrete is set in situ, using ply boards with the metal reinforcing elements pre-positioned. Concrete is then poured into the mould. It sets to give the large panels and blocks. Anything else is made of reinforced concrete beams.

Steel framed construction

Steel-framed construction (shown opposite) is often coupled with a heavy use of glass. This is common in high rise buildings and in such a construction the frame bears all the weight. These structures can be coupled with Passive Solar Design (PSD) allowing a lot of heat to enter the building through solar radiation.

Cob

Cob building uses a mixture of clay and sand to create the walls of a building. This technique has been used extensively in the UK since before the 13th century. However modern materials and rapid industrialisation of the nation's cities during the late 19th and early 20th century led to many cob buildings being knocked down and replaced.

Cob is arguably the simplest and cheapest form of building construction with low cost raw materials, however it is very labour intensive requiring a lot of man hours compared to its rivals. Cob buildings are ideally suited to climates that do not have harsh winters and receive regular sun exposure. See the section on cob and traditional building techniques.

See the article on Cob building.

Rammed earth Rammed earth building is an evolution of cob building. Rammed earth relies on a mixture of damp earth, gravel, cement and sand which is formed into a mould of the desired shape. This is then compacted down eliminating all space in the mixture and creating a solid strong wall. Rammed earth offers huge environmental advantages over other building materials, the bulk of the material needed is earth which can be dug on site or locally. This reduces the ecological cost of transporting the material, compared to say concrete, which needs to be transported from external locations.

Rammed earth also has a good thermal mass which helps regulate the temperature within the building by heating up slowly during the day (and slowing the rate that a building heats up), reaching a peak temperature as the sun descends. This allows the rammed earth wall to slowly cool back down overnight, with the heat being released slowly, providing heat for the building.

In general natural walls such as cob and rammed earth will need to be built thicker to achieve the same heat insulation and strength of man made materials. This limits their use on sites with space limitations. Timber framed buildings are better suited to areas that don't get enough sun to warrant a brick or ICF design which can take full advantage of PSD (passive solar design) building.

See other articles such as Roof, Cob, Traditional building, Materials and Interior Walls for further information.