Wind-driven rain penetration (WDRP) on high-rise building facades may lead to water penetration damage to the existing cladding, corrosion of wall-ties and mechanical fixings securing the cladding to the building, structural cracking, etc, which not only affect the durability of the claddings, their structural integrity and consequently increase the risk of the cladding becoming detached but also will reduce the thermal benefit of any standard insulation materials (mineral fibre, polystyrene bead, open cellular foam, etc) incorporated within the facade.
The amount of water deposited on any tall building envelope by driving rain is significantly greater than in any other building types. The fact that wind speed increases rapidly with height means that the driving rain exposure of tall buildings is much higher than for low-rise buildings. The top elevations and upper corners of high rise buildings are particularly prone to ingress of wind driven rain. The response of the wall to WDRP is also determined by a diversity of other parameters including material characteristics such as moisture permeability and moisture retention, construction details, cracks and joints, etc. Both the deposited rain absorbed by the cladding and water penetration into and behind the cladding can be significant sources of moisture for many deterioration mechanisms.
Despite the importance of wind driven rain to building performance, relatively little is known about the magnitude, duration, and frequency of driving rain penetration on buildings. A major programme of measurements was undertaken in the UK in the 1950’s and 60’s to collect the data that formed the basis of the British Standard Code of Practice for assessing the exposure of walls to wind driven rain, published in 1992 (1). This standard used a methodology developed by Lacey and other to estimate the amounts of rain passing through a vertical area on an exposed ‘airfield site’ in a year and in continuous ‘spells’ of driving rain. This gave the ‘airfield’ annual and spell indices. Wind speeds increase with height above ground and are accelerated close to the ground on hilltops, and will be higher in exposed conditions (open country) than for buildings protected by other houses, trees or by hills. Several state-of-the-art rain penetration testing facilities have been developed and applied to simulate the effect of wind driven rain penetration based on this data which may be suited to buildings below 10m (33ft) in height but this does not adequately take into account WDRP of tall building envelopes above 10m in height.
Data from wind driven rain penetration tests should be used with great care, their accuracy and reliability should not be relied upon alone.
For high-rise buildings, to avoid rainwater penetration into the inner leaf the wind driven rain exposure conditions for the building should always be classified one grade more severe than would appear to be required. See also Hard-to-Treat-Homes. Certain parts of the building may also require a more severe grading, for example, the tops of parapet walls, tops of walls unprotected by roof overhangs and areas of walls below damp proof courses (DPC) adjacent to ground level.
The assessment should include rainwater penetration mechanisms including both hydrostatic pressure and wind pressure, and expert professional judgment should also be sought and be supported by data from more onerous Building Research Establishment (BRE) flood resilience tests.
Technitherm® is suitable for use in all high-rise buildings in any wind driven rain exposure category and is the only cavity wall insulation system to meets both the BBA wind driven rain penetration test requirements for resistance to penetration of liquid water and effect on the water resistance of a cavity wall, and the Building Research Establishment (BRE) flood resilience test requirements.
Ref: (1) BS8104:1992: Code of practice for assessing exposure to wind driven rain.