As powerful as Garden’s principles are, there are additional factors to consider. When critical building functions depend on effective performance of a single material or assembly (e.g. sealant joints), performance is directly affected by failure to install correctly or maintain that assembly. It is the elegant redundancy of moisture control combined with the passive nature of convective drying that makes ventilated rainscreen wall systems effective. Most water is stopped by the primary rainscreen—moisture entering the cavity between exterior insulation and rainscreen exits by evaporation within the convective loop and an air and moisture barrier behind exterior insulation provides an additional line of defense. There is less reliance on sealants in ventilated rainscreen walls. Where sealants are used, they are often shielded from ultraviolet (UV) exposure. This layered, redundant approach to air, moisture, and thermal management has shown itself to be highly effective.
Introduction of insulation into the wall cavity adds a further consideration regarding air gap size. Air gaps within a wall system open potential pathways for fire propagation that must be carefully evaluated. This is the case whether insulation within the gap is combustible or noncombustible. Given the complexity of flame propagation, large-scale system testing performed per the National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components, or the Underwriters Laboratories of Canada (CAN/ULC) S134, Standard Method of Fire Test of Exterior Wall Assemblies, is needed to establish fire safety. Buildings employing wall systems tested per NFPA 285 have demonstrated good performance in real-world fire situations (consult Jess Beitel’s presentation on “NFPA 285 and Engineering Judgements” at the XXIII Annual Westford Symposium on Building Science). However, it must be noted both NFPA 285 and CAN/ULC S134 are system tests. Any changes in the wall system, including in the size of the air gap, affect fire performance and require expert consideration.
Exterior wall system design has become far more complex than it was in Garden’s day. All components of a wall system must work in concert to manage air, moisture, thermal, mechanical, and fire performance. The system must also have the appropriate aesthetic appeal and perform at a cost that a given project can bear. Further, architects are legally required to produce designs meeting building code requirements, and the general contractor must construct code-compliant structures. This can be a daunting prospect for both the architect and the general contractor given the array of fire, moisture-resistance, and mechanical requirements placed on exterior wall systems. There is an immediately available solution to all of these challenges.
Building product manufacturers are steadily refining approaches to exterior wall design. Specific systems, be they ventilated rainscreens, stucco, EIFS, cavity wall, prefabricated panels, or masonry walls, undergo painstaking engineering review and comprehensive performance testing by leading manufacturers. These manufacturers create extensive detail sets, specifications, and installation instructions that, if followed, result in wall systems with code compliance that is confirmed by third-party documentation. Specification of these engineered wall systems allows architects to focus on attaining desired aesthetic, performance, and cost objectives. It allows general contractors to reduce risk by employing proven installation practices. Additionally, applicators can access focused technical expertise from the building product manufacturer if installation questions arise.
Designing and constructing with engineered, complete wall systems may require changes in construction processes, beginning with an appreciation that line-item value engineering of proprietary, tested, and detailed wall system creates high risk in exchange for minimal returns. For architects, matching the appearance, performance, longevity, and cost of the engineered wall systems they select to the design intent of each project represents a streamlined method of creating design value.
This principle—designing and constructing buildings with tested, code-compliant, complete exterior wall systems—forms a basis for consistent construction of buildings with improved fire safety, durability, longevity, aesthetic appeal, and cost effectiveness.
As Wikipedia noted, the term rainscreen implies a system of building. While at one time that system would have encompassed moisture control alone, the complexity of exterior wall systems now requires a more integrated approach.