Cavity Wall Insulation

Cavity wall insulation (CWI)  has been a standalone measure for over 30 years. The majority of cavity constructed homes were either built including  insulation in the cavity or insulation has been retrofitted, although research confirms that there are still around 5 million homes left uninsulated. Some of these may not be suitable for insulating and some are classed as hard to treat, which we’ll get to later.

Cavity wall construction started to become a common wall built from the 1930's. Walls are built with two leaves: an external wall and an internal wall, with  a cavity between the two.  The main reason for this is to reduce water penetration. In driving or heavy rain, the external wall gets wet, and the cavity acts as a barrier for either water run-off or just to break the moisture travel to the inner leaf.  This form of construction dates back to late Victorian times, but in the early days were mainly in coastal areas where buildings are more exposed to driving rain.  In the early 80s, new builds often included cavity walls with partially insulated cavities, understood to maintain the benefit of the cavity but improving it.

Cavity wall insulation is a very cost-effective method of insulating external walls (if suitable). It has a relatively low install cost with a  good payback period and it is fitted externally so is not very disruptive. It has normally attracted some form of grant funding over the years and importantly,  it’s not necessary to carry out other measures for CWI to perform effectively.

First we need to ascertain that the cavity is a true cavity. It needs to be made of two masonry walls tied together with metal wall ties. Earlier cavities sometimes used bricks to bridge the gap and tie the two walls together, a form called 'rat trap' construction–not a true cavity wall. A borescope will be used to look in the cavity. The inner leaf must be masonry (brick, block or stone) and be clear of obstructions and bridges like debris and fallen bricks. The cavity needs to have at least a uniform depth of 40mm (some bead insulations can be fitted from 40mm) The wall should be in good condition and weather tight, so failed pointing, frost damage or spalled bricks would need to be rectified prior to install. Rendered walls may be covering up defects and there must be a working DPC and ideally a clearance of 150mm between the DPC (damp proof course) and the ground (although it is acceptable to have 75mm above ground level if the ground falls away from the building). We will check internally for signs of rising and penetrating damp–all these issues would also have to be addressed and rectified prior to planning any further work.

There are a number of different cavity insulation materials on the market.

Blown mineral wool (white wool) is a loose fill fibre blown into the cavity with air. This material needs to be kept completely dry otherwise it will lose its thermal capability.  It also has an acoustic value. This system has lower thermal performance compared to other materials out there, but the installation cost is lower.

Polybead (EPS)--this was historically installed loose with the same method as blown wool but now is blown in with a pva glue so it stays lightly bonded in the cavity. This can be fitted into narrower cavities and has minimal risk of creating damp or bridging.  It can cause issues if fitted loose as the beads escape through the tiniest of cracks and crevices.

Polyurethane Foam is an expanding foam fill with a very good thermal value. It can be used in uneven cavities such as random stone and in properties without damp proof courses.  It can have a structural value too and be used to bond walls together in areas where wall ties have failed.  The only real issue is if for any reason it needs to be removed as once installed it forms part of the structure and would need to be cut out!

Hard to treat cavities are cavities with a depth of less than 50mm, partial filled cavities, and properties in the areas of generally high exposure (a BRE table will verify whether or not this is the case) or if positioned specifically such that it is in high exposure. PU foam can be used to treat most of them, but also beads can be used for narrow and partially- filled cavities. (partial fill insulation needs to be fixed to the inner leaf with  a residual empty cavity of 40mm, though some exceptions apply where existing materials have been installed and can’t be changed)

Finally, properties with access issues can be called hard to treat. Three-storey buildings, walls obstructed by conservatories or lean-tos, boundary issues–most can be overcome by internally drilling the walls or using poly bead systems that use a lance method ( thermabead being one of them). 

In general cavity wall insulation is a very effective form of insulation, definitely worth investigating if your home is cavity constructed to ensure it is fully insulated and if not, enquire with an installer who is a member of the National Insulation Association (NIA) and the Cavity Insulation Guarantee Agency (CIGA) to arrange a survey.