The building envelope concepts
The building envelope consists of all exterior components of a building - roof, walls, below-grade waterproofing, windows, skylights, and so on. When looking at these components from a weatherproofing perspective, it is important that each component be taken into consideration to prevent moisture or air from migrating into the building.
The basic function of the envelope or enclosure of a building or structure is to protect the covered or otherwise conditioned interior spaces from the surrounding environment. This fundamental need for shelter is a concept that is as old as the recorded history of mankind. However, as our needs have evolved and technologies have advanced, the demand placed on designers to both understand, and integrate, a wide range of increasingly complex materials, components, and systems into the building enclosure has grown in equal proportion. This is particularly true when one considers the emerging threat of terrorism and the impact of that threat on the design and construction of the building enclosure. However, despite the recent emphasis on blast-resistant wall systems and hardening of the building enclosure (see the Blast Resistance section for additional information on this topic), uncontrolled rainwater penetration and moisture ingress remain two of the most common threats to the structural integrity and performance of the building enclosure.
This guide, and the additional resources referenced herein, is intended to facilitate a better understanding of the basic principles behind heat, air, and moisture transfer (including bulk rain water penetration and precipitation management) through the exterior walls of a building or structure. Specifically, it focuses on six (6) commonly specified exterior wall systems in the United States, and illustrates how proper selection, use and integration of the various materials, components and systems that comprise those wall systems is critical to the long-term durability and performance of the building enclosure.
COMMON ELEMENTS OF AN EXTERIOR WALL
Each of the above wall types, or combination thereof, generally consists of the following basic elements, or layers:
Exterior Cladding (Natural or Synthetic)
Air Barrier System(s)
Several of these layers may, at the discretion of the design professional, serve multiple purposes. For example, in barrier wall design and construction, the exterior cladding material may be designed to function both as the primary drainage plane and principal air barrier for a building or structure. Similarly, in cavity wall construction, rigid insulation placed inside the exterior wall cavity, if properly detailed, may also function as the air barrier and a drainage plane for a given exterior wall system or assembly.
Decisions such as these are typically made during the Schematic Design phase of a project, when basic programmatic requirements such as building use, orientation, environmental exposure, and overall response to the surrounding climate (micro and macro) are first given consideration by the design team. These decisions are then further refined during the Design Development and Construction Document phases of a project, when a more detailed and specific response to issues and concerns related to air/moisture transfer through the building enclosure, material selection, coordination of drainage planes, interface detailing, and the long-term durability and performance of exterior wall systems and assemblies is required. This process can be defined as a building enclosure assessment process. Throughout this process, it is both useful and advantageous for the design professional to consider the building enclosure as a series of layers that, with proper material selection and the effective coordination of drainage planes at interface conditions, should result in a fully integrated, thermally-efficient and weather-tight building enclosure. The ability of the material to meet the design intent at each layer, whether it is intended to be the drainage plane product, a vapor retarder, an air barrier or part of the rainwater management system, must be analyzed. Careful review of the material properties at each layer, including the air and vapour permeance, resistance to bulk water entry (depending on where the material is located within the assembly), structural rigidity, and bulk-water storage capacity needs to be completed. Once completed, the building enclosure system as a whole need to be critically examined to understand how the materials and components are interacting.
The durability, and the limitations and appropriateness of the materials used for the building enclosure, especially for critical layers such as the drainage plane and air barrier, also require close examination during the building enclosure assessment process. This review is critical as the use of inappropriate or less durable building enclosure products may result in unplanned premature failure of the enclosure and higher long-term maintenance costs that are greater than initial costs of the superior material or component.
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