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Cladding constitutes the main facade of a building and the first line of defense against the elements and environmental conditions.
Because of this, a well-resolved cladding design should fulfill both aesthetic and climate control functions.
In this post:
Why using Cladding?
Key Terms
Types of Cladding Systems
Design Considerations
Pro Tips
Resources
Why using cladding?
(Source: BC Housing - Achieving Airtight Buildings)
The main purpose of cladding is to provide environmental protection to the exterior walls to protect the occupants of a building from the weather. This protection constitutes a skin that is attached to the main structure. Such skin has become thinner with the development of new materials, insulation methods and technologies.
Although the cladding of a building doesn’t contribute to the stability of the structure, it does have to resist and transfer loads such as winds, snow and its own weight.
Given its climate control and structural functions, as well as the aesthetic importance that it has; the choice and implementation of systems, materials and elements should be a collaborative effort between architects, engineers and specialists.
As architectural designers, we should understand the basic concepts and considerations that play a role in cladding design to propose the best solution for our projects.
Key terms
The following are key terms and definitions you should know while designing cladding:
Cladding
Non-structural elements erected over or attached to the frame of a building that constitute its external face.
Layer
Each one of the different surfaces put one after the other, necessary to the assembly of any given cladding system.
Panel
Piece of some weather-resistant material that covers the external layer of the cladding.
Fixing / Connections
Components that attach the elements of the cladding (panels, studs, insulation, etc.) to the structure of the building.
Backing Wall
A wall to which the cladding is fixed.
Facade
The face of a building which forms the outer appearance.
Rainscreen
Part of the assembly where the cladding stands off from a moisture resistant layer creating an air cavity that allows drainage, evaporation and air circulation.
Opening
Any hole or space that communicates the exterior of the building with the interior; providing either access, natural ventilation or daylight.
Thermal Insulation
Layer that insulates the outside cladding of a building to improve thermal efficiency.
Types of Cladding Systems
The above diagram shows some of the most common cladding systems used in architecture.
New materials and cladding products are developed constantly as technology advances, giving a plethora of options to choose from.
Other materials include:
- Composite metal panels
- Stone cladding
- ETFE
- Glass-reinforced polyester (GRP)
- Glass-fibre reinforced cement.
The classification of claddings given by the NBS Building libraries can be seen here.
When choosing the type of cladding for a project various factors arise: budget, appearance, materiality, maintenance; and most importantly, fire safety.
As a general recommendation, avoid ACP (Aluminum Composite Panels) and EPS (Expanded Polystyrene), as they are known for increasing the spread of fire.
Check your country’s building and fire codes for more detailed information that may apply locally.
Design considerations
1) Coordination and detailing
From the beginning of the design process facade consultants, contractors and engineers should assist the design team offering feedback in all decisions regarding finishing materials, air and water leakage, insulation, climate control, connections, etc.
Then the design team can proceed to work on the respective details and specifications knowing that the cladding will be buildable and functional.
2) Water Drainage
(SOURCE: Brookes & Mejis, Cladding of Buildings)
When proposing the weather protection for a cladding design, understanding that joints are the most crucial part of the assembly is extremely important.
A substantial film of rainwater is always going to flow down the surface of a cladding (even laterally or upwards in the presence of wind). There are 6 different forces that will act trying to move the water through any available opening (see diagram above).
To combat air currents and pressure differences the most effective approach is to propose a rain screen system, where the outer skin acts as a pressure-equalized wall followed with an air gap and then an insulated airtight backing wall.
For the pressure equalization to be maintained the air gap shouldn't be tightly sealed with the outside.
Minimum width for air gaps should be:
- 50 mm for panels with open joints
- 38 mm for panels with baffled or labyrinth (rebated) joints.
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3) Air Tightness
The cladding assembly should resist any air leakage (also known as infiltration) going either inward or outward.
Benefits of preserving the air tightness of the cladding include:
- Improved comfort.
- Reduced costs on heating due to less heat loss.
- Improved performance of the ventilation systems.
This can be achieved with exterior air barriers systems that limit air leakage through the backing wall.
Some systems include:
- Sheathing Membrane
- Sealed Sheathing
- Liquid Applied Membranes
An air permeability test is required to ensure that the rate of air infiltration is within the permitted amounts. Refer to your local building codes and standards to get the benchmarks for normal air infiltration levels depending on the use of your project.
4) Thermal Insulation
The design should provide continuous insulation around all the facade (covering the entire surface of the backing walls) paying special attention to penetrations through the external panels and being properly cut between the support frame, openings and fixings.
To avoid the performance to be diminished by moisture it should be installed a breather membrane in the outer face of the insulation.
When choosing insulation materials pay special attention to the R-Value (not to be confused with U-Value), which is the metric that indicates the resistance to heat flow for a given material or assembly. The higher the R-Value the better the heat performance and hence, more is saved in energy consumption.
For more information about other thermal performance metrics, visit this post.
5) Maintenance
The design should provide future access for the purpose of cleaning, inspection, maintenance, repair and replacement of any element of the assembly.
This can be achieved with a suspended or elevated working platform in the case of multistory and high-rise buildings. In case of choosing a temporary suspended working platform, anchoring and mechanical equipment should be provided on the roof for the future use operation of the platform.
Other methods include a maintenance access window or maintenance door.
When designing the facades of our buildings we must pay attention to any building recess, projection, cantilever or balcony, so they can enable the temporary erection of scaffolds on the outside.
Contractors and consultants should also create a reparation plan for the staff of the building. This document should include guidelines about frequency of inspection of cladding items, how to check wall anchors for corrosion or moisture damage, correct replacement of panels, etc.
Pro Tips
1) Cladding systems are like your face
... they make the first impression to those visiting the building.
It is also the outer most barrier to the building. So you would want the best protection against the weather elements through the consideration of the various systems in the earlier slides.
You would want them to look good, function well and be protected, right?
2) Don’t Forget About Other Code Requirements (ETTV, Daylight Reflectance, Fire Code)
The cladding system selected affects the thermal performance of the building. If you need guidance on the calculations involved in determining that, do look at the post series on ETTV, RTTV and U-Value.
Daylight reflectance must also fall within the code requirements in your country. This is the measure of light that bounces off the surface of the facade. Architects must ensure a low percentage of reflectance for the visual comfort of passerbys.
References
BC Housing - Achieving Airtight Buildings
