Cavity wall constructions are a popular choice for self-builders and housing developers alike here in the UK. When building with these constructions, it is important to check whether the walls are likely to be regularly exposed to driving rain. If they are, you’ll need to take this into account when choosing and installing insulation.
What risk does driving rain pose to cavity walls?
Rainwater that is blown into walls at force will seek out weak spots (such as gaps around window and door frames, missing pointing or cracks in brickwork) and eventually work its way into the cavity.
Research by the British Board of Agrément (BBA) has shown that if cavity wall ties are incorrectly installed or the insulation layer is too slim, it can allow this moisture to reach the inner leaf of the wall. The net result is patches of penetrating damp which are unsightly, can allow mould growth and can be expensive to fix. Fortunately, these issues can be easily overcome by taking a little time to check what the wind-driven rain risk is for the property (or properties) you are working on and ensuring the construction is up to standard.
How can you check if a property is at risk from driving rain?
Here in the UK, the worst gales tend to blow in off the Atlantic Ocean. This means that, as a rule of thumb, it is usually properties in Northern Ireland and along the west coast which are more at risk from wind-driven rain.
To make things even simpler, a British Standards Wind Driven Rain Index has been created which separates regions into four categories based on their relative risk: Sheltered, Moderate, Severe and Very Severe. You can view the map within the National House-Building Council’s (NHBC) latest standards.
Can full-fill cavity insulation be used in areas affected by wind-driven rain?
There is a bit of a popular myth that full-fill cavity wall insulation (any application where the cavity void is less than 50 mm) is banned on properties in a Very Severe risk area. In reality, this only applies in Scotland where the Technical Handbooks to the Building Standards recommend that full-fill insulation is used only in areas classed as 'sheltered' whilst the NHBC standards state it is not permitted to use full fill insulation at the time of construction. Additionally, pumped/blown thermal insulation cannot be used to fully full cavities of properties in Northern Ireland or the Isle of Man at the time of construction.
In most other areas, specific full-fill solutions can still be used providing the construction meets local Building Regulations. The NHBC also provides its own guidance for suitable wall constructions for use with full-fill cavity insulation in England, Wales and Northern Ireland (where the external leaf does not exceed a height of 12 m) within table 2 of Chapter 6.1 of the NHBC standards.
The requirements for new homes are shown below:
Exposure category | Wall construction | Minimum built-in insulation thickness (mm) |
Very Severe | Any wall with impervious cladding | 50 |
Fairfaced masonry with impervious cladding to all walls above ground storey | 100 | |
Any wall fully rendered | 75 | |
Fairfaced masonry | Not permitted | |
Severe | Any wall with impervious cladding or render (1) | 50 |
Fairfaced masonry with impervious cladding or render(1) to all walls above ground storey | 50 | |
Fairfaced masonry | 75 | |
Moderate | Any wall with impervious cladding or render | 50 |
Fairfaced masonry with impervious cladding or render to all walls above ground storey | 50 | |
Fairfaced masonry | 50 | |
Sheltered | Any wall with impervious cladding or render | 50 |
Fairfaced masonry with impervious cladding or render to all walls above ground storey | 50 | |
Fairfaced masonry | 50 |
Table notes
(1) Render on an external leaf of clay bricks (F2,S1 or F1,S1 designation bricks to BS EN 771) in severe or very severe exposures is not permitted where the cavity is to be fully filled with insulation.
NB In Northern Ireland and the Isle of Man, it is not permissible to fill the cavity with pumped thermal insulants (for example, UF foam) at the time of construction
How to prevent penetrating damp from wind-driven rain during installation
The quality of the installation is just as important as the choice of materials in preventing penetrating damp and there are a few key points to keep an eye on during installation. All joints between the brick or blockwork should be solidly filled with mortar with any excess on the inside of the outer leaf smoothed off.
- If full-fill insulation is being used, then the mortar should not be recessed.
- Any mortar which drops onto the insulation or wall ties needs to be cleaned off.
- Insulation boards should be tightly butted together with close attention paid around openings and other junctions which are common sources of thermal bridging.
- If you are partially filling the void with rigid insulation, such as PIR boards, then these need to be fixed to the inner leaf with wall ties embedded at least 50 mm into the leaf.
- At least two wall ties should be fitted on the horizontal joint of the first row of insulation boards.
Both EcoTherm Eco-Cavity and EcoTherm Eco-Cavity Full Fill have been designed to provide enhanced protection against wind driven rain and can be picked up from most merchants. In cases where partial fill cavity insulation is required, the low thermal conductivity of Eco-Cavity (0.022 W/mK) allows projects to achieve a compliant U-value with a reduced thickness than is possible with less thermally efficient partial fill insulants, such as EPS.
Eco-Cavity Full Fill matches this thermal efficiency and also features an engineered tongue and groove joint, which helps to ensure improved continuity of insulation within the cavity. Additionally, the boards are fitted with a 10 mm air gap which helps to resist moisture transfer from the outer leaf.
