Curtain walling and cladding

6.9

6.9.18Rainscreen cladding

Rainscreen cladding systems shall ensure adequate in-service performance. Issues to be taken into account include:

  1. acoustic performance
  2. weather resistance
  3. thermal bridging and condensation
  4. air infiltration
  5. compartmentation
  6. certification
  7. site testing.

Acoustic performance

Noise from the rainscreen cladding system caused by rain striking the outer surface of panels should be accommodated without being intrusive through the use of material that is:

  • noise absorbing, or
  • anti-drumming.

Weather resistance

To ensure moisture is directed to the outside, DPC/DPM arrangements should be correctly formed with suitable upstands and stop ends, including at the junction between the rainscreen cladding and any other component or system. External and internal air and water seals and a drained cavity should be provided at all interfaces.

The air gap between the face of the insulation and the back of the panels should be of sufficient width and have suitably sized drainage, allowing any water passing the joints to:

  • run down the back of the rainscreen panels
  • be discharged externally without wetting the insulation or the backing wall.

Free drainage

Air gaps should be adequately ventilated and the following minimum widths maintained behind all rainscreen panels:

  • 50mm for panels with open joints, or
  • 38mm for panels with baffled or labyrinth (rebated) joints.

Open, baffled or labyrinth (rebated) joints should have a minimum 10mm opening, unless specified otherwise.

Thermal bridging and condensation

The system should:

  • be designed to minimise the risk of thermal bridging, surface and interstitial condensation
  • be assessed using a BS 5250 condensation risk analysis
  • generally include a vapour control layer fixed to the warm side of the wall insulation.

Air infiltration

Before installation of the system, the backing wall should be reasonably airtight with:

  • masonry walls jointed to a high standard, i.e. each joint filled
  • framed walls, including a rigid sheathing on the cavity face, with each joint taped or sealed.

Where reasonable airtightness cannot be achieved:

  • a separate continuous vapour permeable air barrier should be provided on the outer face of the backing wall
  • joints should be taped or sealed.

Compartmentation

Rainscreen cladding systems that have open joints between the panels should be designed to be pressure equalised. The cavity should be compartmented by:

  • a horizontal cavity closer at each floor level
  • vertical cavity closers at centres not exceeding 6m
  • vertical cavity closers at centres not exceeding 1.5m within 6m of an internal or external corner
  • a vertical cavity closer as close as possible to an external corner, generally within 300mm.

The NHBC Standard for compartmentation is in addition to building regulations (to control the spread of smoke and fire), but may be used for the same purpose.

Cavity closers should:

  • be rigid and installed in accordance with the manufacturer’s recommendations
  • enable ventilation and drainage to be maintained in accordance with the design.

Certification

Rainscreen cladding systems, including panels, should have current certification confirming satisfactory assessment by an appropriate independent technical approvals authority accepted by NHBC.

Site testing

On-site hose or sparge bar testing should be carried out with emphasis on interfaces that are designed to be permanently closed and watertight.

The building should remain watertight during and after the test.