Low Energy House - Insulated Cavity Walls - New Houses

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

Insulating Cavity Walls in New Houses

In recent years, because Building Regulations have demanded higher standards of thermal insulation in buildings, the cavity has been used to accommodate thermal insulation. The latest stage in the evolution of the external 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 with thermal insulation, allowing a 50mm wide air gap to remain. On sheltered sites, the cavity 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 to be completely filled and still resist moisture transfer.

Masonry Cavity Walls for New Houses

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 is generally constructed of lightweight concrete blocks.

Concrete Blocks in Cavity Wall Construction

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.

Insulation Materials Commonly Used for Cavity Wall Insulation

Glass Wool Cavity Wall Insulation
Semi rigid batts or slabs of glass wool fully filling the cavity
Rigid batts or slabs of glass wool partially filling the cavity
Loose glass wool fibre injected by blowing fully filling the cavity

Extruded Polystyrene Cavity Wall Insulation
Extruded polystyrene rigid board insulation partially filling the cavity

Rockwool Cavity Wall Insulation
Semi rigid slab of rock wool fully filling cavity

Rigid Urethane Cavity Wall Insulation
Rigid urethane foam insulation boards with aluminium foil facing both sides

Rigid Phenolic Cavity Wall Insulation
Rigid phenolic foam insulation boards with aluminium foil facing both sides

Thermal Insulation of Cavity Walls – 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 wall U-values to be within the range of 0.27 and 0.30 W/m²K

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

Partially filled Cavity Wall

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

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

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