Cavity Wall Insulation - more information

What is a cavity wall?

House-builders started building houses with cavity walls from around 1920-1930, but there are many examples of this construction method being used earlier than that. Most houses built after 1930 will be a cavity wall construction. Prior to this houses were made with solid walls – two layers of brick forming a 9 inch solid wall.

A cavity wall is made of two distinct layers with a gap in-between. The gap or cavity is usually between 60-70mm wide (just less than 3”) but can be as little as 45mm-50mm, or up to 100mm or even greater.

How does Cavity wall insulation work?

Cavity wall insulation is a tried and tested technology. Material, shown in yellow here, is blown or injected into the cavity between the two layers that make the external walls of your property. A trained technician / installer will do this from the outside of your property.

There are a variety of different materials that can be used, but they all work in the same way. The principle is the same for loft insulation - air is trapped and the insulation acts as a barrier to prevent heat loss.

For insulation to be really effective a cavity should be over 50 mm (2 inches). Cavity walls with a space less than 45mm should not normally be insulated; care should be taken to inspect the cavity with a boroscope to determine the exact width.

Materials that can be used - Mineral fibre, blown fibre

Also known as Rockwool or Glass wool, made from spun volcanic rock or glass, it is an entirely dry system which uses granulated fibre blown into the cavity to a predetermined density. The mineral fibre is treated with a water repellent during the manufacturing process and the material is blown into the cavity through a series of 22mm holes. This form of insulation has been used widely throughout the British Isles since the 1960s 1970s and has a proven track record of performance.

Materials that can be used - Expanded polystyrene beads

Thousands of small polystyrene beads are blown into the cavity space and normally bonded using a binding agent or PVA adhesive. Generally there are less holes required to fill the cavity with this system.

Materials that can be used - UF Foam

Urea Formaldehyde cavity wall insulation was introduced into the UK in the late 1960's. Injected foam is mixed on site in accordance to BS 5617 (1978) and installed in accordance with BS 5618 (1978). The UF foam expands and cures in the cavity, filling space and trapping air in tiny pockets, as the material cures the water content evaporates and dries out through the outer wall.

The holes are generally smaller than with other systems but in all other respects the completion work is carried out in the same way.

The Installation Process - site survey

Typically cavity wall insulation would take around half a day to install for a normal semi-detached house or a small detached property, but longer for larger properties. The installer / technician will do a preliminary survey to check for access around the site and look for any obstructions, for example sheds against house walls or conservatories that may restrict access. (Note a competent surveyor will identify these matters before an installer arrives on site).

All necessary air vents are checked - for example those providing under floor ventilation and combustion air. Air bricks are generally “sleeved” to allow air to pass unimpeded from outside to inside.

In all cases flues are carefully checked on completion of the installation process by means of an appropriate test to ensure they have not been obstructed by the insulation material.

The Installation Process - the drilling procedure

A series of holes are drilled in the outer walls, normally in the mortar joint between the bricks. Typically with the mineral fibre system the hole diameter is 22mm. Fewer holes are drilled with the polystyrene bead system, but with all insulation systems enough holes are drilled to create a pattern that allows the walls to fill evenly.

The system manufacturers will train and provide installation manuals that must be complied with by the installing company. The installation must be carried out in accordance with the relevant BBA Certificate, and CIGA guidelines.

The Installation Process - filling the cavity walls

The insulation material is blown or injected into the cavity wall working from the bottom upwards in a specified sequence to provide an even and uniform fill. Where a semi–detached or terraced property is to be insulated, the insulation material is contained by inserting a cavity barrier at the dividing line of the properties. This prevents material spreading into your neighbour’s cavity.

When the filling process is complete, the holes are filled with matching mortar. The technician will mix this to blend in with the original mortar colour, and any new air bricks are fitted and re-pointed in the same way.

The Installation Process - post installation inspection

On completion of the work the technician will do a post installation survey, this includes an inspection of the flues, air bricks, and the interior of the building to check for the presence of any surplus material. If this has occurred in inhabited parts of the building, it must be removed.

Customer service from THIAS

For more information on the various materials available and any technical advice please contact our technical team. Our technical director Andrew Spence has over 25 years experience in the home insulation industry and will provide help and assistance and answer any questions.

Andrew can be contacted on at 01937 573367 during normal office hours.

We regularly monitor the work carried out by approved contractors and customer feedback is requested on a regular basis. As part of our clients for life philosophy we actively encourage customers to share their experiences of working with our service and the contractors we have recommended.