CAVITY WALL 

CAVITY WALL 

Generally speaking, cavity wall insulation (CWI) acts like a blanket that stops heat escaping. It is made up of two walls with a gap in between, known as the cavity. The gap between them can be filled with insulating material. The outer leaf is usually made of brick and the inner layer of brick or concrete block. If your house was built after the 1920s, it is likely to have cavity walls. If so, the bricks will usually have an even pattern with all the bricks laid lengthways. A way to tell if you have a cavity wall or not is to check the brickwork on the outside of your home. You can also tell a cavity wall by measuring its width. If the wall is more than 260mm thick, it is most likely a cavity wall. A much narrower wall suggests that it's a solid wall. If solid walls, the bricks will have an alternating pattern, with some bricks laid across the wall so you can see the smaller ends from the outside. Cavity wall insulation can be in the form of slabs or sheets fitted as the walls go up (the most common option on a self-build), or retrofitted material that’s blown in once the main envelope is constructed.

CAVITY WALL INSULATION

Cavity wall insulation is an insulating material which is placed in the gap between the inner and outer wall of your property, making it more energy-efficient. Most common types of cavity wall insulation used are:

• Polystyrene Beads,
• Blown Mineral Fibre,
• Mineral Wool Slabs,
• Rigid Foam.

In the case of cavity wall insulation, you must first of all consider water retention and fire resistance as well as the quality of air indoors for your home. This is the reason why these four materials are used more often in concurrence with cavity walls.

POLYSTYRENE BEADS

These are pushed into the cavity of the walls, which is mixed with an adhesive which sticks the beads together to avoid being spilled outside of the wall. This type of insulation is usually used in narrower cavity walls rather than using mineral wool insulation and is also preferred in some stone built houses or properties. Polystyrene beads may be supplied loose or in a light sticky resin to hold them together. Polystyrene granules will stick together of their own accord due to their rough shape. Polystyrene has low thermal conductivity, which makes the material have a long term R-value, meaning it is resistant to heat flow and an ideal product for thermal insulation. EPS bead does not absorb moisture and its thermal properties are not affected by damp, humidity or moisture. Its closed-cell means it cannot absorb water and the bead will maintain its shape, size, structure and appearance. Polystyrene beads are more popular than alternative materials – mineral fibre and expanding foam – because they are more effective at keeping the heat in, and don't absorb moisture. Beads are also quicker to install as they can be blown easily and far into the cavity, reaching awkward areas. 

BLOWN MINERAL FIBRE

Blown mineral fibre consists of strands of fibreglass or mineral wool that are forced into the cavity using compressed air. This material can be used in any part of the country and is covered by British Board of Agrement (BBA) certificates. 

MINERAL WOOL SLABS

Mineral wool insulation is one of the most commonly used materials when it comes to cavity wall insulation. It’s easy to slide between the joists, can be packed tightly and also pre-cut to suit joist widths. The flexibility of glass mineral wool batts affords the capability to accommodate small variations in cavity width, such as those caused by mortar snots. Glass mineral wool products benefit from the batts being effectively knitted together, offering a continuous thermal envelope. Glass mineral wool products such as those in the ISOVER range can achieve Euroclass A1 Reaction to Fire classification and have excellent acoustic insulation properties. The thermal performance of glass mineral wool is typically around 0.034W/m.K, depending on the product and when installed across the whole width of the cavity wall, achieving a thermal performance roughly equal to partial fill PIR cavity wall insulation.

RIGID FOAM BOARDS

If you were building a new property or extending you would normally install it in the cavities or in the stud work during construction. They come in three main types:

• PIR Boards,
• Phenolic Boards
• EPS Boards.

PIR BOARDS

PIR boards are made by mixing chemicals with a blowing agent. This forms large rigid blocks which are low density, closed cell insulation sheets. The gas which is trapped in the closed cells of the insulation has a very low thermal conductivity. This low thermal conductivity makes the insulation perform better than if it just had air trapped in the cells. They come in different types but the most common type has an aluminium foil backing to each side which makes them easily recognisable. The addition of an aluminium foil facer helps improve the thermal performance of any air cavity the PIR board faces into. Moreover, they are very nearly as thermally efficient as phenolic boards and cheaper then phenolic boards. PIR boards tend to offer the optimum performance vs price ratio as they’re pretty close to phenolic boards when it comes to r-value and yet they’re cheaper. Although you will need a slightly thicker board to achieve the same r-value.

PHENOLIC BOARDS

Phenolic board are probably the most thermally efficient insulation products used at present. It has a slightly higher r-value and a slightly thinner profile than PIR boards due to the manufacturing process. It offers the thinnest insulation boards for a given specific u-value. As they’re closed cells they shouldn’t absorb moisture or water vapour, unlike open cell products. However, phenolic boards will likely be the most expensive of all the rigid foam insulation options. If you wanted an R-value of 4.5 for each type of rigid foam insulation boards, you’d need the following approximate thicknesses: PIR – 100mm ǀ Phenolic – 90mm ǀ EPS – 170mm

EPS EXPANDED POLYSTYRENE

EPS is the most basic cavity insulation board and the one with the lowest R-value. It is also the cheapest of the three rigid insulation boards. Board doesn’t have a closed cell structure as it is basically a mass of polystyrene balls moulded together into a block. However, it can allow water to penetrate over time and to achieve similar r-values or u-values to PIR or Phenolic boards you’ll need a much thicker block of insulation.

TYPES OF CAVITY WALL

Choosing between full and partial fill installation and subsequently selecting the correct insulation type is essential in terms of compliance with Building Regulations and is crucial to the energy performance and the carbon footprint of a building. There are two types of Cavity Wall Insulation : Full Fill Cavity or Partial Fill Cavity.

FULL FILL CAVITY

Full fill cavity insulation fits closely against the brick and block of the wall construction with no sagging or slumping. As a rigid full-fill insulation solution, the product ensures a resistance to fire spread between and within cavities. It acts as a barrier when tightly fitted between masonry leaves where an insulated wall connects with an uninsulated wall cavity. Full fill cavity insulation must be weatherproof to provide protection against bad weather, and it must be water repellant to resist the passage of moisture. Full fill cavity wall insulation differs from a partial wall in that the entire space of the cavity is filled with insulation without leaving an air gap. Full fill cavity wall insulation allows more of the cavity between the inner and outer leaf to be filled with insulation, providing a much better insulated structure without increasing the wall width. Full-fill built-in insulation solutions are installed as the walls are built, with slabs being friction-fitted between the inner and outer leaves of the wall and in between wall ties. There is a variety of products available for full fill cavity insulation: PIR, Glass Mineral Wool and Blown Insulation. All of the solutions are designed to prevent moisture movement through the cavity.

PARTIAL FILL CAVITY

A partial fill cavity features thermal insulation secured back to the inner SFS frame, leaving a cavity of air between the insulation and the masonry outer leaf and can be used on buildings of any height. It simply involves leaving a gap between the outer face of the insulation and the external leaf. Using insulation below the slab helps maintain steady internal temperatures in the building by bringing the ground bearing concrete slab into the insulated envelope of the building making use of the concrete thermal mass. Partial fill wall insulation typically requires a 50mm cavity to be maintained. Here are some examples of the available insulation for full and partial fill cavity wall installation:

• DriTherm Cavity Insulation Slab- Knauf
• CW4000 Partial Fill Cavity Wall Insulation Board-Celotex
• Full Fill Cavity Batt-Rockwool
• High Performance Partial Fill Cavity Slab-Rockwool
• Xtratherm XT/CW Thin-R 
• Xtratherm CT/PIR CavityTherm Thin-R 

COST OF CAVITY WALL INSULATION

Insulating your cavity walls is a simple and effective way to reduce your heating bills and have a warmer home in winter. You should be able to make back the installation cost in five years or less due to the yearly energy bill savings you will make. With fuel costs on the rise, now is a good time to invest in cavity wall insulation. You might not see the same pound-for-pound reduction in energy bills as you once would have, but you will be using less energy – so you’ll be protecting yourself against upcoming energy price hikes. Cavity wall insulation can cut your heating bills so much that it will pay for itself quickly.

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Video credit: https://www.youtube.com/watch?v=wdsZVhieZrc