By Simon Lloyd – Technical Project Manager – Kiwa Building Products
A CWI bead system improves the thermal insulation of external masonry cavity walls. It is typically used in walls up to 12 m in height, with cavity widths not less than 40 mm in existing or not less than 50 mm in new domestic and non-domestic buildings. Walls up to 18 m in height can be treated with CWI where this has been specifically assessed as suitable by the System Designer for a project specific design.
A CWI bead system typically consists of small, spherical, closed cell, expanded polystyrene (EPS) beads, in accordance with draft BS EN 16809-1. Typically, an air drying water-based polyvinyl acetate (PVA) bonding agent is simultaneously injected with the EPS beads to give a homogeneous finished insulation.
EPS bead used as CWI is water vapour permeable and can reduce the risk of condensation in or on a masonry wall when a project specific design takes into consideration the provisions of BS 5250. Gaps between EPS beads when installed allow any wind-driven rain water that penetrates a cavity void to drain down to ground, rather than cross the cavity void. A CWI project specific design can allow for the bridging of a damp proof course (DPC) in masonry walls as the EPS bead will resist the transfer of ground moisture to the inner leaf at DPC level.
EPS bead itself is combustible but unlikely to become ignited within a cavity void due to the low oxygen levels present. If fire does penetrate a cavity void, the amount of air present will be insufficient to support combustion.
The thermal resistance of a CWI bead system depends on the density of the EPS bead used and the thermal resistance will be affected any bonding agent and the installed density of a CWI bead system.
A CWI bead system is injected through a nozzle into a wall cavity through a series of holes, drilled in a pre-determined pattern. Horizontal layers of EPS beads are built up until a cavity void is fully filled in accordance with BS EN 16809-2.
To determine whether a building is suitable to receive CWI, an installer shall undertake a pre-installation survey to assess the suitability of a cavity void by using a borescope and tape measure.
When assessing a given building for its suitability to receive CWI treatment, due consideration shall be given to the local wind-driven rain index and the site exposure zone according to BS 8104. The NHBC does not allow CWI to be used in areas of very severe exposure to wind-driven rain where fair faced masonry is the external leaf of a cavity wall.
Typically, 22 mm diameter holes according to the System Designer’s standard drilling pattern are drilled into mortar joints at the point where the vertical mortar joint meets the horizontal mortar bed, in the main wall area. Closer drill hole spacing is usually required around doors, windows, cavity closures, chimneys/flues and gables.
When installation commences, the entire cavity void from ground level up should be fully filled to the highest point in each elevation, or to a level determined by any horizontal cavity barrier (whichever is the lower).
Difficult to fill areas of a cavity void such as above a conservatory or car port where no drilling or injection is easily facilitated, can be filled by using a lance system, from holes drilled at the corners of a building (to allow lateral injection of EPS bead).
To finish the installation, injection drill holes are fully filled and sealed with mortar (of a similar type, colour and texture to that of the existing wall). Combustion air vents and flues are checked, and any obstructions cleared.