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ECS Lessons Learned


After more than 30 years in the industry, we’ve amassed some great takeaways pertaining to the work we do. We felt this key information was important to share, so in 1996, Lessons Learned was created as a mailed piece sent out to our peers. Now, we are pleased to offer Lessons Learned electronically once a month, accessible on this page or delivered directly to your inbox when you sign up.

What is the Building Envelope?

Construction Engineering Services

What is the Building Envelope

The Building Envelope includes all materials, assemblies, and systems that separate the interior environment from the exterior environment, the “inside” from the “outside.” Physically, the typical building envelope consists of the roof system, the above-grade wall system including windows and doors, the below-grade wall system, and the base floor system. These systems, and the interface conditions between them, work to separate the inside from the outside and comprise the building envelope. Interior partition assemblies can also serve as parts of the Building Envelope when they separate interior zones with differing temperature, humidity, or pressurization requirements or as contaminant or odor control. Interior Building Envelope examples may include crawl spaces, attics, separation of an indoor pool (natatorium) from the rest of a hotel; interior wall of a loading dock; laboratory space; hospital rooms for infection control; and datacenters especially where evaporative cooling is used adjacent to traditionally cooled space. In the simplest terms the Building Envelope must completely contain a volume of space and consider all “six-sides” including the top (roof), sides (four walls), and the bottom (floor).

When carefully considered, the Building Envelope can significantly impact not only the proper load calculations and “right-sizing” of HVAC systems but also the building’s ability to provide occupant comfort which leads to reduced complaints, call backs, and claims.

Envelope v. Enclosure. The industry recognizes both “envelope” and “enclosure” as interchangeable or selectively used in specific standards with the same meaning. For example, ASHRAE 90.1 and the International Energy Conservation Code (IECC) use “Envelope” where ASTM E2813 and E2947 use “Enclosure”. Building Envelope is utilized by ECS.

Control Layers. Building Envelope performance is separated into multiple control layers, including the following in order of highest risk to least: water, air, thermal, and vapor control. Each control layer must be considered both separately in its ability to protect the structure and occupants and in combination with adjacent control layers to understand their combined effect. Additional control layers may also be considered such as acoustic, fire, daylighting depending on the goals of the project. The following is a description of the four primary control layers:

Water Control. Water is the most significant source of building failures and claims in the form of moisture accumulation, condensation, and leaks leading to corrosion, mold, rot, and other damage. Three-dimensional interface conditions and the overall system design needs mitigate bulk water penetration through the assemblies and direct any water off the building as quickly as possible.

Air Control. The goal of the air control layer is to manage the air flow within or through the building envelope. This requires properly identifying the critical air control layer in three dimensions and to maintain the location of the air control layer sufficiently to the interior of the thermal control to mitigate moisture accumulation or condensation. It is important that the three-dimensional interface conditions and the overall system design mitigate air flow through or around assemblies.

Thermal Control. The primary role of thermal control, including insulation, is to reduce the amount of conductive heat loss through assemblies through the use and proper placement of insulating materials. The challenge is created when we consider the differences in thermal resistance, air permeance, and vapor resistance of the variety of available insulating materials. It is necessary to work with the mechanical designer and energy analyst so that they understand and can utilize the correct effective R-value or U-factor for the building envelope assembly when developing thermal loads and energy analysis.

Vapor Diffusion Control. Strategically reducing vapor control and allowing diffusion of water vapor to both the interior and the exterior is critical to allow for drying which is critical for long term performance and durability of assemblies. The overall assembly requires the ability to dry via diffusion and internal air flow to address initial construction moisture loads and potential incidental moisture impacts on the assembly performance overtime.

The design and construction of the Building Envelope is important to overall building performance and energy efficiency. 

We hope this Lessons Learned has increased your understanding of the Building Envelope. For more information and support, contact the ECS office nearest you. 

ECS Group of Companies

The ECS Group of Companies (ECS) is an employee-owned engineering consulting firm with more than 2,000 employees providing geotechnical, construction materials, environmental and facilities consulting services. ECS has grown to over 65 locations and five subsidiaries spread across the Mid-Atlantic, Midwest, Southeast and Southwest. The firm is ranked 69 in Engineering News-Record’s Top 500 Design Firms (April 2020) and 52 in Zweig Group’s 2020 Hot Firms (June 2020).