Building envelope of enclosure is created with the main function of protection the indoor spaces and occupants from external barriers. The idea of ‘shelter’ has been around since the history of mankind and the same concept applies to a building envelope. However, our needs have evolved a lot and with introduction of advanced and improved technologies, building designers and engineers are able to integrate complex materials into building enclosure. The need for an improved and sturdy building envelope comes into the picture when external factors like natural calamities or terrorism are considered.
Currently, the 2 most common threats for the integrity of a building envelope are rainwater penetration and moisture infestation. The aim of this article is to familiarize you with the basic principles of building envelope design along with their definition.
External Wall System: Principles and Definition
- Exterior Cladding
It is the protective layer attached to the exterior of building envelope. Exterior cladding, in most cases, is the first defence barrier against rainwater penetration. The term “cladding” is often used to describe many building envelope materials, components and systems.
- Wet Zone
This region of exterior wall system is designed to be exposed to short and long term effects of rainwater and moisture. The wet zone is located on the exterior side of the innermost drainage plane of assembly.
- Dry Zone
Unlike wet zone, this region of the exterior wall system is not designed to be exposed to moisture and rainwater as the materials used have low tolerance to moisture. Dry zone is located on the interior side of the innermost drainage plane of assembly.
- Drainage Plane
It is designed to shed rainwater and condensation downward and outward for preventing uncontrolled water infestation. The drainage plane describes any element that is exposed to the weather or is located between the wet zone and dry zone. Based on the wall specifications, the drainage plane can be specified as waterproof and vapour-impermeable or water resistant and vapour-permeable.
- Air Barrier
It consists of elements that are designed to control the air movements across an exterior wall system. For optimal performance, some key aspects of the air barrier are:
- Must reside within the wall, and be continuous from roof to wall to foundation.
- Should have high quality materials that provide high resistance to air pressure without compromising their performance.
- Must be durable with ability to maintain the structural integrity throughout its life cycle.
- Air Retarder and Vapour Retarder
The element that resists and limits the airflow across the exterior walls system is called an air retarder. The element that controls and limits vapour flow across the exterior wall systems is called an vapour retarder.
A vapour retarder must meet the building codes, and the designers must consider factors like material selection, occupancy, local climate and building location.
- Insulating Element
An insulating element regulates and controls the heat transfer across the exterior wall system. These elements are designed to minimize the thermal bridging in the dry zones of the assembly.
- Structural Element
These elements are specifically designed to withstand any live and dead loads on the structure. The loads present must be distributed effectively onto the structural frame.
- Interface Condition
It is the line of separation between individual façade elements of the exterior wall system. For example, the perimeter lines of window openings, plumbing lines, mechanical ventilation, etc.
These elements collect, contain and divert the rainwater to the exterior sides of the building envelope. They are responsible for keeping the water away from interior spaces and the dry zone.
- Moisture Management System
A combination of elements that control and manage the adequate distribution of moisture through an exterior wall system. The moisture management system deals with precipitation, air flow and diffusive vapour flow.
- Storage Capacity
The ability of a material or element to safely absorb and hold moisture. Many materials used in exterior wall design and construction can safely hold a significant moisture level. However, some materials suffer moisture deterioration and failure over time, when exposed to repeated and prolonged wetting.
- Diffusive Vapour Flow
This term describes vapour transfer through the layers of an exterior wall system. The rate and direction of diffuse vapour flow are affected by the interior and exterior ambient temperatures and relative humidity. The differences in vapour pressure and vapour permeability between layers also affect flow.
- Capillary Action
This refers to the absorption of moisture into small voids in a material, until it reaches maximum capacity. Capillary action in wood products is a common issue that causes deterioration of exterior wall systems.
- Wind-Driven Rain
The process by which rainwater is driven through an exterior wall system when exposed to wind loads. Water can flow through existing voids in the system, or voids that are created by deflection under external loads. Under extreme weather conditions, the wind itself can cause deflection.