Matthew Castles, technical manager at Cedral UK, provides some insight into this aspect of facades
Getting the ventilation right on a cladding project is absolutely vital – and could avoid huge amounts of hassle in terms of unplanned expense, additional time (making good) and damaged reputation. Taking time at the outset of a project to ensure you’ve got ventilation covered, could add significantly to the longevity and overall performance of the building.
Wherever possible the facade design should enable natural ventilation. In warm weather, ventilation is necessary to prevent the building from overheating, while in colder, damper weather it will stop the build-up of moisture and condensation. Proper ventilation also minimizes structural movement as temperature fluctuations can cause materials to expand and contract.
Rainscreen cladding works by creating a rear ventilated cavity between the back of the facade panel (or plank) and the outer face of the building. This structure enables air to constantly circulate, forcing out any moisture that penetrates the joints of the cladding, before it reaches the building structure.
Inlet / outlet principle
One of the basic tenants of good cladding ventilation is the inlet / outlet principle. The facade cladding doesn’t sit directly against the insulation. To provide an effective ventilation cavity behind the facade this space must be provided with adequate inlets and outlets. For most structures, we recommend a minimum 30mm continuous gap between the cladding and the building fabric and a minimum 10mm continuous gap at openings.
As part of the Cedral range, we offer a complete set of trims including a perforated closure. This allows the air flow behind the cladding, while sealing off the cavity and protecting against pest infestation and debris.
The use of perforated closures can also have a surprisingly significant impact on the level of ventilation. In cases where airflow is restricted, we recommend a minimum equivalent of 10,000mm2 equivalent to 10mm per 1000mm of facade length. It’s also important not to overlook the importance of proper ventilation around soffits, sills and parapets. Your cladding manufacturer should be able to advice you on this.
Cavities
While different manufacturers may specify slightly different recommended cavity depths, Cedral advises a minimum of 30mm. The aim of the façade design should be to enable natural ventilation of the building wherever possible.
Some problems that can occur without correct ventilation:
- Differential movement of the cladding boards
- Bowing and cracking of the planks
- Temperature inside the cavity affecting the timber or aluminium framing
- Moisture affecting timber battens behind the planks
- Premature coating failure & delamination
Tips to remember
When installing a ventilated rainscreen cladding system, here are a few key tips to consider.
Closure calculations – The importance of perforated ventilation was highlighted earlier on, to avoid insects and infestation. However, be careful when specifying the closer, making sure you calculate how this will affect the effective airflow behind the facade to ensure it stays within the manufacturer’s recommendations.
Mind the air gap – We all understand the importance of an appropriate air gap at the base and other openings within the cladded areas. We sometimes see this vital space being reduced once sand, shingle and vegetation are laid down around the footings, effectively blocking the ventilation. At higher levels the use of sealants to close off air gaps under windows and soffits Similarly, when using membranes ensure they are installed in way that will not restrict airflow to the cavity.
In terms of fixes and remedies, there are grilles and other products that can improve poor ventilation, but they often spoil the look of the cladding. In our experience, it pays to read up carefully before you start a facade project and don’t be worried about calling the manufacturer for guidance.
As well as providing the functional benefits as outlined above, installing a ventilated cladding system can bring design versatility to the home’s exterior. Fibre cement cladding, for example, which is available in a range of colours and finishes can be used to achieve a variety of looks on both new builds and renovated properties.