Similar to how a winter jacket protects you from the elements, a building envelope protects the contents of a building throughout its lifespan. A successful building enclosure design separates the interior of the building from the exterior while controlling the exchange of heat, air, and moisture between the two. It can extend the life of building components and help keep occupants comfortable year-round.
Since 80% of all construction claims involve issues with the building envelope, it's important to understand the fundamentals of building enclosure design and construction. Here are the key details.
What Is a Building Enclosure?
The building enclosure encompasses all of the exterior elements of a building, from top to bottom and all sides. It should form a continuous, three-dimensional boundary between the interior and exterior environments. The elements of a building enclosure include the following:
- Roof
- Foundation
- Exterior walls (above and below grade)
- Openings (such as windows and doors)
These systems work together as an assembly to mitigate environmental factors that could negatively affect the building and its occupants. Each element of building enclosure design consists of three main components.
1. Structural Support
Resists and/or transfers physical forces—also known as "loads"—from both inside and outside the building. Typical loads include the building elements (dead load), the building occupants (live load), and natural forces that act on the building (wind or seismic load). Examples of structural support include columns, trusses, joists, load-bearing walls, and foundations.
2. Control Layers
Controls the movement of water, air, vapor, and heat into and out of the building. Examples of control layers include roof and waterproofing membranes, flashings, air barriers, vapor retarders, and thermal insulation.
3. Finishes
Installed on the interior and exterior building surfaces and form the aesthetic characteristics of the structure. Finishes are often seen as the first line of defense against water intrusion at the building exterior and heat transfer at the interior. In that sense, they can also function as control layers.
Examples of exterior finishes include brick or stone veneer, stucco, siding, and metal panels on the building. Meanwhile, interior finishes can consist of painted drywall, wood planks, ceramic tile, and vinyl flooring.
Mitigation of Natural Forces
In order for the building enclosure to maintain its integrity, it needs to control the movement of water, air, heat, and vapor.
Water Control
A primary objective of building enclosure design is to prevent or control the movement of water into the building, whether it is "bulk" water in the form of rain and snow, or water that infiltrates the building envelope through capillary action from the ground.
The goal is to create barriers that repel, remove, or channel water away from the building as quickly as possible. This is accomplished by the installation of the following materials:
- Roof membranes
- Roof scuppers and drains
- Exterior building cladding
- Water-resistive barriers within walls
- Flashings at roofs, walls, and all building openings
- Drip edges at roofs
- Waterproofing at below-grade walls
- Foundation drainage
These materials should be installed to shed water in a shingle fashion—from the building's roof to its walls to the foundation. The membrane should be installed in a sequence that maintains a continuous downward water drainage plane onto an acceptable water control layer with an unobstructed path to the exterior of the wall system.
A water control layer should be designed and constructed to prevent the accumulation of water within the enclosure assemblies. Walls should also be designed and constructed to drain water to the exterior, which enters the assembly.
Air Control
Air infiltration through the building enclosure can lead to problems with unwanted moisture, heat, or pollutants entering the building. A well-designed, comprehensive air barrier system can prevent these challenges.
Air barriers prevent outside air from entering and conditioned air from escaping the building, which can help improve energy efficiency, lengthen the life span of building materials, and enhance indoor environmental quality. Accordingly, the International Energy Conservation Code (IECC) requires all new buildings to include air barriers.
Air barriers come in many forms, including the following:
- Self-adhered membranes
- Fluid-applied membranes
- Rigid materials like plywood, OSB, or gypsum board
Testing has shown that a much greater quantity of moisture enters a building due to air leakage through gaps or holes than from water vapor diffusion through building materials. When properly installed, air barriers can block this air-transmitted moisture.
Vapor Control
Vapor retarders help reduce the diffusion of water vapor molecules through building materials. Vapor retarders are ranked Class I, II, or III based on their permeability or how much water vapor passes through them over time. A Class I vapor retarder has the lowest "perm" rating, which means it allows the least amount of vapor diffusion, while Class III allows a larger amount. Generally, a product that is greater than a Class III is not considered a vapor retarder and is considered vapor permeable. Since other building materials also have perm ratings, the ratings of adjacent materials should be considered when choosing a vapor retarder.
The choice of vapor retarder is also influenced by the building's climate zone, location of the insulation within the enclosure assembly, and moisture durability of the materials.
Also, some building assemblies need to "dry out" after construction is complete. If vapor is trapped within a building enclosure with no way to escape, it could lead to air quality or corrosion issues. Be sure to consult with a commercial building expert to determine the locations and types of vapor control the project may need.
Thermal Control
Insulation is a crucial part of the building envelope, helping to control heat transfer between the interior and exterior. By minimizing heat loss in cold weather and heat gain in warm weather, insulation can make a building more energy efficient and more comfortable for occupants.
The IECC requires the installation of continuous building insulation in most climate zones to increase energy efficiency and reduce thermal bridging. Interruptions of more conductive material, like framing members or gaps, cause thermal bridging, which allows heat to pass more easily through the insulation control layer. Most continuous insulation is in the form of rigid boards installed toward the outside of the building envelope, usually made of polyiso (polyisocyanurate), XPS (extruded polystyrene), EPS (expanded polystyrene), or mineral wool. Of these four types, polyiso has the highest R-value per inch compared to other insulation types.
Securing Building Enclosure Systems with Siplast
Siplast provides a range of durable waterproofing systems for building enclosures that can help meet environmental goals while offering practical protection and aesthetic appeal. Curious to learn more? Reach out to a Siplast associate to get any questions answered by a knowledgeable roofing professional, or find a sales representative near you.
