Rainscreen – The Science
Whilst many of us will be jetting off to sunnier climes over the coming month, our buildings are not so lucky.
They have to tough out whatever the British summer has in store and, for many, the first line of defence is rainscreen cladding. But beneath the attractive façade, how do these systems help to keep out the elements?
In its most basic form, a rainscreen consists of three components: an outer layer / barrier, a drainage cavity, and an inner layer which acts as a moisture and air barrier. The outer layer keeps out the majority of moisture, and any that does penetrate is dried or carried away by air circulating through the cavity, protecting and extending the life of the internal building structure.
As mentioned in our recent blog covering the history of rainscreens, modern rainscreen systems are generally split into one of two categories: drained and ventilated or pressure equalised.
Drained and ventilated rainscreen
These systems feature openings at the top and bottom of the cladding allowing a current of air to flow through the rainscreen cavity. Moisture which ingresses past the outer façade panels either drains to the bottom of the cavity or is dried by the air current.
Pressure equalised rainscreen
Pressure differentials are one of the leading causes of moisture entering the building envelope. If the pressure within the rainscreen cavity is lower than in the outside environment, air and moisture is sucked into the cavity. The convection ventilation approach used on drained and back ventilated systems can help to equalise pressure on low-rise projects. It is, however, less effective on taller buildings which are frequently exposed to dynamic wind loading.
To combat this dynamic loading, the cavity within pressure equalised rainscreen systems is compartmentalised. Each compartment features a vent, allowing wind pressure to flow into the cavity, equalising pressure with the outside environment and preventing further moisture entering the cavity.
In practice, there is always some lag in the pressure equalisation process. The degree to which this affects the rainscreen is determined by the size of the vent and the size and airtightness of the compartment. Effective drainage is also required at the bottom of each cavity to expel any moisture which does enter the system.
In addition to addressing moisture ingress, the compartmentation of the rainscreen system is also an essential requirement under fire regulations, particularly above 18 metres. Pressure equalised rainscreens are therefore the standard choice for high rise applications.
In the next blog in this series we’ll explore some examples of modern rainscreen systems in-situ.
In the meantime, why not catch up with last week’s post?
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