A Rain Screen Sealant Joint System.

The importance of good seals in new buildings is noted when walls discolor because of water intrusion; when important papers are soiled, machines get wet and fail, floors and rugs are ruined. Then people think seriously about good sealing practices. Considering the problems and high financial impact that may occur, there is growing attention being paid to good design in sealant joinery.

One of the most reliable ways of eliminating water leakage through the sealant joinery in any building envelope is to apply the rain screen principle within the design, providing pressure equalization, thus preventing the vacuity of water through sealant expansion joints. The basic objective of the Two Staged Sealant System is to provide a major deterrent to water leakage at the outside face of the wall, and seal against air and vapor passage on the interior side of the air space, where the primary seal is exposed to little of the destructive elements of weather and sun.

What is referred to as the Rain Screen Joint is a combination of the exposed exterior sealant bead, backed by an air space and designed so that it shields the primary (or secondary) interior joint from wetting and forbids the vacuity of water through pressure equalization beyond the primary sealant bead to the building’s interior.  

It should be recognized, to begin with, that the term rain screen joints and pressure equalized sealant design, though closely related, and in fact interdependent, are not strictly synonymous. The term rain screen refers to the principle of design which prescribes how penetration of wind driven rain and water may be prevented. Thus the use of the sealant rain screen principle is essential to achieving a pressure-equalized joint method designed to protect the occupants of any building structure against the damaging effects of water alteration and/or intrusion. 

Pressure equalization should not be confused, as some may have the tendency to do, with the more conventional and long accepted 'theory of secondary defense', depending on closed cell backer rods to form a seal. Closed cell backer rods will take a compression-set, thus allowing moisture to bypass its installation. Other “expandable foam tapes” can offer a seal against water intrusion forming a mechanical adhesion to the substrates creating the joint system. In some cases, a single sealant bead is installed over the expandable tape to create a double-line of protection against water intrusion.    

Action of Rain on a Wall Surface

Before examining what causes water to enter through sealant expansion joints, and how the rain screen principle may be applied to prevent it, it would be practical to review briefly how rain-water acts on a wall surface and on what may be the most vulnerable parts of a wall, the joint sealants.

A substantial film of water flows down the wall surface and, if wind is present, as is often the case when it is raining, the water flows laterally. Additionally, on parts of the building fa?ade it may flow upward as well. The taller the building, the greater will be the accumulated flow over the lower parts of its walls. Lateral flow under wind pressure is greatest near the windward corners of the building, and upward flow is greatest at, or near, the top of the building fa?ade facing the wind. Therefore, thought to the design of exterior cladding must be given greater attention.

Picture shows ends of precast panel are sealed to prevent lateral water itrusion.

The Force of Nature and their Control on the Rain Screen Principle

There are various forces which must be considered, and some of them do not result from wind action. Positive and negative forces may be present from the building environmental controls. In a windy rainstorm various forces are likely going to be acting to move the surface water through any available opening in a failed sealant. The installation of a Two Stage Sealant System or Rain Screen Joint System can be one of the most effective ways in providing a protective barrier against water intrusion. 

Two Stage sealant installations in expansion fa?ade joinery (in general) are made more resistant to water penetration by eliminating the pressure differences, or equalizing the pressures occurring on its inner seal. Therefore, the primary fa?ade or envelope joint seals are removed from the outer skin face and installed to the inner part of the exterior cladding, where they are kept dry. Thus, instead of a single joint sealant bead working alone against water intrusion, being subjected to both water and wind pressure, the installation of dual sealant beads adds greater protection. Now, the rain screen seal (outer sealant bead) shields against water penetration as the interior joint sealant seals affectively against both air and water penetration. In the illustration on the right the exterior precast the concrete panel has a Two Stage sealant system installed, designed to prevent vacuity of water into the building interior with the weep located at the bottom drip edge reveal. The weep is thus placed at the bottom to allow gravity to naturally pull any water bypassing the outer seal, as seen in the photo above.

Compartmentalizing the Rain Screen Joinery: Strategic placement of weep tubes and/or vents from floor line to floor line in multi-story buildings can prevent intruding water from running aimlessly through the building envelope before it is discovered at some distance from its entry. 

The benefits of installing a Rain Screen Joint System help fight against:

• Sealant failure that occurs as a result of faulty sealant installation. 
• Kinetic energy force; under the influence of wind, raindrops approach the exterior wall with considerable velocity, and their momentum alone may carry them through openings and sealant failures of insignificant size. 
• Capillary action that may occur through a failed single line of sealant allowing unwanted water intrusion. One way to control flow by capillary action is to introduce a discontinuity, or air gap between two lines of sealant, e.g. a Dual Stage Sealant System. 

Convection and Differential Pressure: These two types of force caused by wind action, are the most critical and most difficult to combat. Air currents may result from differences in pressure over the wall surface, or from convection within wall cavities, and these may carry water through expansion joinery of a single line of sealant. Also, when water is present on one side of the expansion joint, and the air pressure on that side is greater than that on the other side, the water will be moved through any sealant failure, no matter how small, in the direction of the pressure drop. Such pressure differentials may be caused even by gentle winds. It is this latter type of force, differential pressure, which causes most of the leakage at wall joints.

As typically performed, the conventional approach to combating the above types of water intrusion is to install a single sealant bead around curtain wall, window perimeters, and exterior claddings. However, the more effective, and more reliable, approach is to eliminate the pressure differential across the joinery, equalizing the pressures through a primary and secondary sealant installation. It is this approach that is most affective and is known as the Rain Screen Principle.

The ideal leak proof wall employing the Sealant Rain Screen Principle incorporates these essential elements:

• An exterior or rain screen sealant bead barrier to water penetration.
• A confined air space, vented to the exterior compartmentalized to prevent water from wandering aimlessly throughout the building envelope. 
• An interior sealant bead, known as the primary defense against water intrusion.

These elements prevent (for the most part) the passage of air and vapor in joint sealants and are capable of withstanding both positive and negative pressures.

As it is assumed that most water leaks will occur through the building expansion joinery, the rain screen joint system incorporating the air space should always be vented or wept to the outside as seen in these two photos below.

How to Compartmentalize the Rain Screen Joint System

To compartmentalize a Two Stage Joint installation a connecting bead of sealant must be placed between the inner and exterior joint sealant beads, thus allowing any water to exit the cavity created by the two lines of sealant. Before the exterior sealant is completed a weep tube is place in a predetermined location. As shown in the illustration on the right, the weep or vent is located below a sealant intersection to allow any pooling water in the horizontal surfaces to fall and weep to the exterior. →

Below are some examples of how the two beads of sealant are connected. The connecting bead of sealant acts as a trough directing any water to the weep tube. Finally the exterior weather seal is then extruded and tooled for an aesthetic appearance.

This first photo shows how closed-cell backer rod is placed in a downward angle to create a trough:

This next photo shows how the sealant in gunned and tooled over the backer rod to create the trough. The trough must be of proper profile and thickness as any joint per industry standards:

Looking into the joint we see the primary sealant bead in the back with a connecting trough:

Our view of a completed interior trough that will connect with the exterior secondary bead. The downward angle allows for water to weep naturally by gravity:

It is time to place a weep tupe onto the trough we created. But first, to prevent the weep tube from plugging into our freshly installed sealant we will lay inside over the new sealant trough a piece of silicone tape. Now, we will place our polyvinyl tubing on to the silicone tape that will extrude through the exterior secondary bead of sealant:

This final photo shows a finished weep tupe installation at the bottom of a precast panel. This allows for gravity to easily pull any water bypassing the exterior secondary bead of sealant directly to the weep tube and exit: (the joint was sanded for appearance)

Caution: If the silicone tape is not used, then the weep tube will plug and fail. This will capture any water that might enter into the system, causing leaks into the enterior of the building. This author was asked to inspect a new high-rise building with precast design that incorporated a Rain Screen Joint System. First, I noticed that the weep placement was in the wrong location. When I cut into the joinery to inspect I found that the weeps were plugged directly into each sealant trough:

Weep tube is placed too high in the joinery above the window perimeter sealant leading to our inspection of the weep installation.

After cutting open the joint we found a larger problem. Not only was the weep tube plugged, but sealant trough turned out to be a 3 inch thick plug of sealant connecting to the window perimeter sealant. This installation was doomed to failure as a "Construction Defect."

We inspected 8 stories of floors in the precast exterior and found all the weep tubes plugged.

More from the same caulking contractor, different project we were asked to inspect:

This photo comes from our installation of the Two Staged Sealant System used on the US Courthouse in Seattle WA. In every case a piece of silicone tape was used to prevent any of the weep tubes from plugging. The silicone trough was created and silicone tape placed onto the wet sealant.

The prevention of moisture and air penetration begins with joint design that takes all conditions into consideration and joint preparation that is in accordance with recommended procedures. It is the sealant that is specified (and applied) and the workmanship on the job that often determines how long the structure will be protected from the effects of weathering. To engage in this type of sealant installation, where all materials and workmanship shall be subject to stringent test and inspection by the Architect and/or Consultant, requires knowledge, training, and shared experience to do the job right.

Two-Stage Sealing first gained widespread acceptance in Europe and Canada, and has been used minimally by architects in the United States. The effectiveness of two-Stage sealing depends not on completely preventing water penetration, but rather on controlling the forces that act to drive the water inward, while at the some time allowing a way for the unwanted moisture to exit. Its function is to prevent water from reaching the interior seal and thus from reaching the inside of the building. How to actually install a Two-Stage Sealant detail utilizing the “Rain Screen Principle” has been addressed through this article in detail. 

 Since sealants of all kinds are installed by human hands, good sealants can be misapplied. This results in making the applicator ultimately responsible, not for just the sealant installation, but also the condition of the materials being sealed. Architects and specifiers are now beginning to require the sealant applicator to bear some responsibility for the performance of the building envelope joinery engineered and built by others. The character of these requirements is intended to provide a performance-type specification for the design selection and installation of the exterior wall sealants. The Fa?ade Sealants Contractor (often a subcontractor on site) is thereby held responsible for the design selection and installation of the sealants, as well as the performance of the sealant system. 

 Joint sealants are required to establish and maintain airtight and waterproof continuous seals on a permanent basis, with recognized limitations of wear and aging as indicated for each application. Failures of installed sealants to comply with this requirement will be recognized as failures of not just the material, but also workmanship. This is one of the reasons why many architects and contractors are demanding to engage a single firm to assume undivided responsibility for sealing all components of the exterior fa?ade. The sealing conpany chosen should be able to demonstrate many years of successful experience in installation of work─ similar to the work of the project being bid on.      

Ultimately, the Fa?ade Sealants Subcontractor has the obligation to provide a watertight and weather tight seal at all fa?ade systems, surfaces, and related materials required under his contract.

The end.