Cavity and joints in rainscreen systems

The cavity

The cavity in rainscreen facades emerged as a practical solution to hardships that Norwegian farmers faced; the double-leaf facades kept moisture out of barns, protecting their grain from molding. This approach they came up with is called the "open-jointed barn technique”.

How does it work? An air barrier behind the outer wall protects the inner wall from atmospheric influence and improves the indoor microclimate.

Importance of the rainscreen system cavity

A continuous cavity is essential for the effectiveness of the rainscreen system:

  1. It is the air barrier between the insulation or the WRB layer (vapour-permeable membrane) and the elements. Prevents water penetration in the building layers and structure, and drains the accidental moisture;
  2. Creates pressure equalization that serves as a capillary break and drainage for liquid water transport;
  3. It is responsible for air circulation behind the cladding. Prevents condensation on the back of the cladding and dries the cavity;
Rainscreen system 2
Rainscreen system
Rainscreen system 1
Rainscreen system - Non-Vented

Image by: Scott D. Wood

From non-vented to ventilated wall systems

According to the air barrier, three types of walls exist:

  1. Non-vented

In the past, double brick walls were used with an air barrier between them. Sometimes the cavity was filled with styrofoam insulation, which led to reduced energy consumption but was more conducive to vapour condensation. Styrofoam is hygroscopic and therefore becomes wet, blocking the transfer of moisture from the inside to the outside and vice versa, a thriving environment for mould growth.

Nowadays due to code requirements for commercial buildings, the inner part of the cladding ventilated walls is finished with drywall – metal studs and gypsum-supported continuous insulation. In this case, again there’s an internal vapour barrier behind the drywall which in the long-term presence of moisture leads to construction components’ degradation.

  1. Vented

The vented wall only has vent openings at the bottom of the wall - for drainage. There is no air exchange and in conditions of high humidity the inner side of the cladding can become wet.

  1. Ventilated

The ventilated wall is a cladding wall with a ventilated and drained air cavity behind  it, with openings at least at the top and at the bottom of the wall. In the last decades, this has been the preferred option for rainscreen facades.

Volume shaping via the rainscreen system cavity

Rainscreen system cavity widths vary depending on different reasons.

The minimum width of the cavity is 20mm, locally can be 5mm due to unevenness of the concrete or supporting construction. The maximum width is limited by the rainscreen system capabilities and the possible addition of custom structural elements. According to the NHBC Standards 2011, Ch.6.9, D7b, the minimum cavity width for open joints is 50mm and for baffled/rebated joints – 38mm.

The main factors that determine the rainscreen system cavity width are:

Width of the insulation layer

Thermal insulation requirements keep increasing, and with them the insulation width. It’s important to leave the necessary minimum cavity width between the insulation and cladding layers.

Regulations and standards

In most countries, the rainscreen façade's design is local rgulations control and ensure the long life of the façade, as well as user safety and comfort.

Architectural considerations

At the intersection of different cladding materials or façade structures architects can specify different depths, further increasing the cavity widths. Alternatively, sometimes the ventilated façade has to follow the interior wall relief because of structural, acoustic, sunlight requirements or other technical reasons.

Ventilated wall cladding also means design freedom – architects can design the cladding to follow the primary structure’s relief, if any, or create a completely different cladding profile.

Design inspiration and imagination

Rainscreen systems give architects, designers and engineers great possibilities to create. Even if the primary wall structure is a plane- because of concrete usually being poured between formwork sheets - the façade outer leaf can have a curved, or otherwise completely different section.

That is why a rainscreen system is one of the most convenient and practical solutions for complex, versatile façade design with many pilasters, cornices, cantilevers, etc. For a façade professional the mastery comes in crafting the interactions between different materials with varying cavity widths.

Joints

Rainscreen system facades are ventilated through the joints or openings in the cladding material. They are responsible for the pressure equalisation, allowing instantaneous the pressure inside the cavity to be equal to the pressure outside. For this purpose, the facade design should ensure that no excessive moisture can penetrate and be trapped behind the cladding.

Some of the countries have already structured façade standards such as NHBC Standards 2011, Ch.6.9 - Curtain walling and cladding and the DNV BTI 1.5 Fassadenbekleidung 2016.

Generally, according to the joint two types of façades are distinguished - with closed-joint or open-joint rainscreen system.

The closed-joint (opaque) rainscreen system works with openings only in the lowest (inlets) and highest (outlets) façade area that both have to be at least 50 cm2/m to secure the proper ventilation. This kind of façade protects the inner construction and other metal infrastructure from corrosion better and has less chance of insects entering.

rainscreen systems 3
Image by Krzysztof Schabowicz & Łukasz Zawiślak

Closed or sealed joints are mostly used with fibre cement panels, insulated metal panels, stone, brick, stucco, etc. The sealants must be flexible and able to move within 20-25% of the joint width.

The open-joint rainscreen system has been more popular lately for aesthetic reasons. Why? It makes façades seem more structured and less flat. In this case, the most important issue is the insulation behind, its type and width which need to be carefully selected and calculated. Where insulation forms part of the rainscreen cladding system it should cover all exposed areas of the backing wall, be neatly cut around fixings and brackets and fixed according the product manufacturer’s recommendations.

Several types of open joints are applied: straight open, baffled or rebated (labyrinth). The minimum width of the joint is determined by the thickness of the anchors, deviations in the size of the panels and an additional 2 mm advance, which means 8mm, preferably 10mm especially at joints with other components.

The pressed (zero) joint is a corner joint which tends to stay invisible.

The last façade’s joint is the blind (false). The blind joint is used when there is adjacent end panels with different height, but the joint has to continue in the larger panel. It hads equal depth and width, not exceeding 2 cm. When panel thickness doesn’t allow for a deep joint, it can be coloured black. The blind joint isn’t positioned in the middle of the panel because that’s where the largest deformations happen.

Last, but not least, an essential element is the screen. It prevents birds and animals from entering the cavity at the top and bottom of the rainscreen.

Rainscreen system 4
Image by BILDA team

References

  1. DNV BTI 1.5 Fassadenbekleidung. (2016, July). https://www.din.de/en/wdc-beuth:din21:269404726
  2. John Straube. (2009, September 15). RR-0907: Ventilated Wall Claddings: Review, Field Performance, and Hygrothermal Modeling. Building Science Corporation. https://www.buildingscience.com/documents/reports/rr-0907-ventilated-wall-claddings-review-performance-modeling/view
  3. John Straube, E. F. P. Burnett. (1998, December). Drainage, Ventilation Drying, and Enclosure Performance. https://www.researchgate.net/publication/237673354_Drainage_Ventilation_Drying_and_Enclosure_Performance
  4. Krzysztof Schabowicz, Łukasz Zawiś (2020, May).Numerical Comparison of Thermal Behaviour Between Ventilated Facades. Sciendo. https://doi.org/10.2478/sgem-2019-0044
  5. (2011). NHBC Standards 2011. https://www.newbuildinspections.com/wp-content/uploads/2016/07/Chapter6_9_2013.pdf
  6. Scott D. Wood, CBST. (2018, March). Vented cladding assemblies prevent reverse vapor drive and allow vapor-permeable water-resistive and air barrier (WRB/AB). https://vaproshield.com/images/pdf/2018-cts-wood.pdf
  7. Upendra Walinjkar. (2018). Rainscreen Ventilated Façade. WFM Media. https://wfmmedia.com/rainscreen-ventilated-facade/