Low Energy House

Low Energy House - What Is An Insulated Cavity Wall?

Traditionally, the external walls of houses were constructed of solid brickwork. In the 1920s, builders began to construct air gaps in external walls, creating a physical barrier to driving rain, and preventing moisture from passing through the walls and staining the plaster finishes inside

Glass Wool Batts - Insulated Cavity Wall - Image Provided by Knauf Insulation Insulated Cavity Wall - New Build

In recent years, because Building Regulations have demanded higher standards of thermal insulation in buildings, the external cavity wall has been used to accommodate thermal insulation. The latest stage in the evolution of the cavity wall has been to increase the filled cavity width to 100mm wide, to accommodate more thermal insulation.

On exposed sites, with high levels of driving rain, it is often advantageous to partially fill the cavity wall with thermal insulation, allowing a 50mm wide air gap to remain. On sheltered sites, the cavity wall is often completely filled with thermal insulation so that there is no cavity left at all. Proprietary insulation systems are available which allow the cavity wall to be completely filled and still resist moisture transfer.

Insulated Cavity Wall - Masonry Construction

Brickwork and blockwork outer leaves can be finished externally with a sand and cement render but, in the UK, it is more usual for the outer leaf of masonry to be constructed of facing brickwork. For speed and ease of construction, the inner leaf of the cavity wall is generally constructed of lightweight concrete blocks.

Concrete Blocks and Insulated Cavity Wall

A wide variety of concrete blocks is available, from dense to lightweight, offering a range of load-bearing strengths and thermal insulating properties. Lightweight blockwork has considerable economic advantages over brickwork because of its speed of construction.

New External Wall with Partly Insulated Cavity Wall
New External Wall with Fully Insulated Cavity

Insulation Materials Commonly Used for Insulated Cavity Wall

Insulated Cavity Wall - Glass Wool Insulation
Glass Fibre Insulated Cavity Wall Batts - Image Provided by Knauf InsulationSemi rigid batts or slabs of Glass Wool fully filling the cavity wall
Rigid batts or slabs of glass wool partially filling the cavity wall
Loose glass wool fibre injected by blowing fully filling the cavity wall

Insulated Cavity Wall - Polystyrene Insulation
Extruded polystyrene board insulation partially filling the cavity wall

Insulated Cavity Wall - Rock Wool Insulation
Semi rigid slab of Rock Wool fully filling the cavity wall

Insulated Cavity Wall - Urethane Insulation
Urethane foam insulation boards with aluminium foil facing both sides

Insulated Cavity Wall - Phenolic Insulation
Phenolic foam insulation boards with aluminium foil facing both sides

Insulated Cavity Wall - Target U-values

Current Building Regulations require new dwellings to be compliant with an overall energy and carbon performance with the Target Emissions Rate (TER) based on the whole building envelope. Thermal U-values for individual elements are not, therefore, set for new houses. In order to achieve the overall Target Emission Rate, most dwellings will require external cavity wall U-values to be within the range of 0.27 and 0.30 W/m²K

Insulated Cavity Wall Construction - the following examples of material combinations should achieve a U-value of 0.27 W/m²K

Partially Filled Cavity Wall Insulation

102.5mm fair faced brickwork outer leaf, 50mm clear cavity, 50mm foil faced polyurethane foam, 100mm lightweight blocks and 12.5mm plasterboards on dabs.

Fully Filled Cavity Wall Insulation

102.5mm fair faced brickwork outer leaf, 100mm fully filled cavity of blown mineral wool, 100mm lightweight blocks and 13mm dense plaster.

All insulation materials must be fitted in accordance with the manufacturer’s instructions

About Insulated Cavity Walls - House Insulation - Home Insulation