by Tiffany Coppock, AIA, NCARB, CDT, ASTM, RCI, EDAC, LEED AP
Hundreds of years ago, buildings were thought of much more simply—as shelter, meant to protect people from the elements. Today, that purpose holds, but the technology, practices, and materials that go into high-performing building enclosures have evolved to do more. Assembly components like continuous insulation (CI) and air- or water-resistive barriers (WRBs), once considered progressive practices, are now a requirement across the country.
There are still challenges associated with continuous insulation and water-resistive barriers, including maintaining continuity. Anticipating and addressing this challenge when designing details requires constant application of building science fundamentals and an in-depth understanding of a wall system’s materials, layers, and performance. It is the junctures, or transitions, in building detail design that matter most. Improper design of these transition details can lead to some of the most common, detrimental, and expensive issues in wall assemblies: leaks and thermal bridges.
Building science fundamentals
Before examining complete wall system design, it is important to review foundational concepts in building science and how they are addressed at each of the layers within a wall. Additionally, it is necessary to evaluate the performance of these layers by testing wall assembly components separately and together. While essentially a review, these aspects are important to reconsider each time details are designed.
A wall system (Figure 1) is essentially an assembly of multiple components or layers. Forming the base of the wall is a structural system, which could be any of a variety of construction types, such as steel stud with gypsum sheathing, wood stud with wood sheathing, or concrete masonry unit (CMU). The next layer is a cavity containing multiple products, followed by cladding, which can be brick, CMU, stone, metal, or aluminum composite materials, to name a few. These components ultimately create the cavity wall.
Within this cavity wall, the most important aspects of building science and performance are addressed: air and water management, thermal comfort, vapor control, fire resistance, acoustic isolation, and structural considerations (such as how insulation and cladding are attached to the wall).
Moisture as a liquid
The most obvious performance layer to consider initially is liquid moisture control. An easy way to understand this layer is to ‘think like a raindrop.’ A raindrop flows from the top of the building, down past penetrations and openings, and along the wall surface itself, without resting on a seam or finding
a pathway into the building. This is described in ASTM E2112, Standard Practice for Installation of Exterior Windows, Doors, and Skylights, and demonstrated in ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference, which is often referenced in well-crafted specification documents.
The next performance layer to consider in a wall assembly involves heat or thermal comfort (i.e. the transfer of thermal energy from outside in or inside out). Heat or thermal efficiency is denoted by total assembly thermal characteristics like R-values and U-factors. Those building code requirements are often based on the benchmarks set by American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and the International Energy Conservation Code (IECC).
The air layer is addressed next. Air leakage through wall assemblies is a significant source of heat loss/gain and undesirable moisture accumulation. In the past, air leakage was tolerated to some extent, and perhaps even expected. It was said a building needs to ‘breathe.’ That notion begs the question, what are the ‘lungs’ of the building?
A building does not breathe through its walls, but via the air exchange that occurs through an HVAC system and other controlled ventilation, such as operable windows. The HVAC system is in place to control the amount of air going in and out, and to filter it based on the needs of the building with regard to energy efficiency and indoor air quality (IAQ). When an air barrier is not present in a wall assembly, the HVAC system is breached. Installing an air barrier as tested per ASTM E283, Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen, ASTM E2178, Standard Test Method for Air Permeance of Building Materials, or—most commonly—as an entire assembly per ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies, creates an assembly allowing the HVAC system to do its job.
Moisture as vapor
After air is controlled, one can turn to vapor moisture. An easy way to understand this concept is to consider the ‘sweating’ that happens on an ice-cold glass soda bottle in a hot, humid climate—along with refreshment, the drinker can be left with a handful of water. This is caused by a high vapor content in the air reaching the cold temperature of the glass, causing a phase change where the vapor gas condenses to a liquid.
This phenomenon is exactly what must be avoided within a wall. If the issue does occur, as much drying as possible should be allowed. According to the International Building Code (IBC), vapor-permeable materials and vapor retarders are measured and classified in accordance with ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials, Method A, and should be described in this manner in the specifications.
The next building science fundamental to consider—often overlooked until it is too late—is fire. Due to fire rating requirements, both the components of an assembly and the system as a whole must be tested for compliance. There is fire resistance, fire propagation, and fire stopping/containment to consider. In multi-story buildings, floor-to-floor firestopping and fire containment at the perimeter should be designed to prevent fire and gas spread between rooms through voids at the intersection of a fire-resistance-rated floor assembly and an exterior wall, per ASTM E2307, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate-scale, Multi-story Test Apparatus.
In wall systems, fire resistance is often referenced in relation to National Fire Protection Association (NFPA) fire propagation test NFPA 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Nonloadbearing Wall Assemblies Containing Combustible Components, or structural fire resistance per ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials. In NFPA 285, fire propagation via an exterior wall system is evaluated by creating a lower-story flame plume exiting a window opening, then monitoring flame spread and heat development throughout the wall. A passing test is when the flame has very limited propagation vertically or horizontally away from the point of origin. ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials, is used to evaluate and compare the basic fire characteristics of specific components.
Lastly, sound attenuation (i.e. the extent to which transfer of noise or sound energy can occur) is a consideration. There are three main ratings:
- sound transmission class (STC), which rates sound energy passed through the assembly;
- noise reduction coefficient (NRC), which measures sound energy reflected/absorbed in components; and
- articulation class, an assessment of privacy (i.e. the extent to which one can understand what is being said on the other side of a wall).
All three ratings are important to remember when reviewing acoustic performance in details and transitions, and may be required in specific local building codes and zones.