Moisture penetration

The type and method of wall construction will influence its ability to resist wind-driven rain. For both single-leaf and cavity walls, total resistance to rain penetration can only be achieved by cladding. Whilst rendering can substantially enhance the rain resistance of the wall, it is essential to select the right type of mix, thickness and number of coats.

The use of masonry paint systems and other proprietary external finishes may increase the resistance to rain penetration, but may also reduce the rate of evaporation of any water from the wall.

Joint protection
It is essential to fill all the joints to minimise the risk of rain penetration. Tooled mortar joints are more resistant to rain penetration than joints which have not been tooled. Recessed joints increase the risk of water penetration.

Rain resistance
The rain resistance of rendered and un-rendered single leaf concrete masonry walls is dependent on the thickness as shown below.

Exposure category	Rendered dense concrete blocks	Rendered lightweight concrete blocks	Unrendered dense concrete blocks	Unrendered lightweight concrete blocks
Very severe	Not recommended	Cladding should be used	-	-
Severe	250mm	215mm	Not recommended	-
Moderate	215mm	190mm	440mm	440mm
Sheltered	90mm	90mm	328mm	328mm

Use of hollow blocks
Where hollow blocks are used in external walls, the use of shell bedding may reduce rain penetration to the inner surface and so give some of the advantages of cavity wall construction. In unfilled cavity walls, a 50mm air space is considered satisfactory in most situations to prevent water passing from the outer leaf to the inner leaf. Partial filling of cavity walls does not affect the resistance to wind-driven rain of the wall, providing the width of the remaining air space is not less than 50mm.

When fully filling the cavity with insulation, the manufacturers of the insulant should be consulted to obtain the limiting exposure condition for the particular insulation type. The risk of rain penetration will be reduced by using a cavity, clear or filled, wider than 50mm.