Building Envelope Water & Moisture Management
Overcoming two common threats to building performance, structural integrity
Uncontrolled rainwater penetration and moisture ingress are two of the most common enemies of the building envelopes, threatening structural integrity and performance. While authoritative data is not available, it is estimated that up to 80% of construction related lawsuits are related to the building envelope.
Three conditions are required to move water through the building envelope:
- a water source
- an opening or path for the water to follow
- and a force to drive the water through the opening.
Far too often building design professionals and contractors choose to rely on controlling rainwater penetration by sealing building envelope openings. Dependence on face-sealed exterior wall systems to control rain-water penetration can have devastating building performance results. Water and moisture management strategies must consider the forces that drive water into the building envelope and the building envelope system components required to combat and regulate these forces.
The most significant forces that drive rain into buildings are gravity, capillary action and wind pressure differences. These forces can come alone or in pairs or all at one time during a windy rainstorm. Understanding how these forces impact rainwater penetration is critical in the design of building envelope water management.
The force of gravity can be both a friend and a foe when it comes to managing rainwater. Using gravity to control rainwater by means of weep and flashing systems is fairly well understood, but the importance of well-defined clear path drainage, vertical to horizontal plane changes, end dams, ponding water and continuous water tight membrane detailing that these systems are so critically dependent upon is often neglected.
Capillary action – the movement of a liquid along the surface of a solid caused by the attraction of molecules of the liquid to the molecules of the solid – varies greatly within façade systems depending on the porosity of the materials within the wall and the openings that allow a path for water movement inward. Capillary forces can defy gravity and can generate deep moisture saturation into building envelope systems. For example, grade-level masonry walls can wick surface ground water vertically at a surprising rate depending on the porosity and openings of the masonry material. Understanding how certain wall systems react to capillary forces allows for detailing capillary breaks or small gaps within the wall system. A simple drip edge metal detail is a good example of a capillary break.
Pressure differences across the building envelope can result from wind, mechanical systems and stack effect. Mechanical and stack effect pressures are generally considered static or constant pressures that impact building envelope moisture infiltration and exfiltration. Wind pressures can transport active rainwater and vary greatly depending on seasonal climatic conditions and can impact the building envelope differently depending on building height and location (windward or leeward facing). High-rise buildings on the windward side can receive tremendous wind-driven rain pressures and prolonged wetting of materials.
Basic exterior wall types
There are primarily three basic exterior wall types used for commercial building enclosure design and construction in the United States:
Cavity wall (screen or drainage walls)
Today the cavity wall is the most common exterior wall construction in the United States. The cavity wall relies on a fully or partially concealed open air space and drainage plane used to resist rainwater penetration and, depending upon the design, the cavity is used to improve the thermal performance of the building enclosure.
An exterior wall system that relies on the outer wall surface to act as the primary defense against rainwater and moisture penetration into the assembly. The barrier wall is to perform as a single line that provides a weather-shield seal to prevent moisture and rainwater ingress.
Mass walls rely on wall thickness, storage capacity and drying to prevent rainwater penetration. Mass walls are labor intensive and expensive to build, and for the most part considered economically impractical in today’s economy.
A qualified building envelope water and moisture management system must include rain and bulk water movement, airflow and diffusive vapor flow. The storage capacity (ability of materials to absorb and retain water) and drying rates of materials must be understood prior to system design and detail placement of drainage planes, vapor barriers/retarders and air barriers within the wall assembly. Improper material selection and/or improper placement of moisture management systems within the wall assembly can result in poor performance of the complete wall assembly.
Provided the design team has selected the proper materials for the wall assembly design, considered the geographic climate zone impact on the wall assembly, and carefully considered the placement and function of drainage and thermal planes, and vapor and air barrier performance requirements a good foundation for a building envelope moisture management system should be in place. The ultimate success of the moisture management system, however, will greatly depend on the ability of the design professional to map out the continuity of the moisture management components at critical wall assembly details.
As noted in the Construction Waterproofing Handbook (by M. Kubal), “as much as 90%of all water intrusion problems occur within 1% of the total building or structure exterior surface area.” The following questions must be successfully answered for a qualified water/moisture management system to be in place:
- Are all exterior rain screen interruptions properly considered and detailed?
- Is the exterior wall system dependent on exterior seals as the only line of rain penetration defense?
- Are drainage planes maintained and open at all details, and is the bulk rainwater directed to the outside?
- Is there a clear separation between dry and wet zones within the wall system, and are these zones maintained throughout the building?
- Is the insulation continuous and without unplanned thermal breaks?
- Is the vapor/moisture barrier continuous, and are all flashing systems properly detailed to the vapor/moisture barrier?
- Is the air barrier continuous? Have all wall system penetrations been properly detailed for a continuous air barrier seal?
- What functional tests will be required during construction to ensure success?
Capillary Creek: Grade-level masonry walls can wick ground water vertically at a surprising rate, causing damage.
Originally printed in Properties Magazine.