A curtain wall is one of an architects’ favorite building assemblies. Most certainly one of the most popular ways to design glazing on modern buildings. Curtain wall perimeter sealants are always a critical aspect of building design and performance. The perimeter sealant is one of the most important details in the assembly because it is the primary transition between the curtain wall system and the air and water barrier. There are multiple ways of creating this bridge between components. Several factors relate to the design including performance, cost, constructability, and scheduling to name a few.

Creating a complete air and water barrier

Without a robust transition between the air and water barrier and curtain wall system, there is a risk of water, water vapor and air infiltration in or out of the building.  Also, there is a risk of lower thermal performance. Issues with any of these four criteria will negatively impact building performance and have consequences. Therefore, getting the perimeter details right goes a long way in helping achieve a high-performance building.

Multiple perimeter sealant options

All the different curtain wall perimeter sealant strategies need to be evaluated and compared for the project.  After this is completed then a product type and design strategy can be selected for the project.  Based on project experience working with different curtain wall manufacturers and contractors, there are really two major system types for completing the perimeter seal.

The double line of sealant and backer rod

The most common method of providing a perimeter seal is with a traditional sealant and backer rod. The key to getting a high-performance detail relies on several factors.

One of the most common mistakes is getting the line of sealant in the wrong location. The mistake is when the sealant is drawn in front of the curtain wall shoulder. The most reliable way to draw the sealants is to have one drawn at the edge of the curtain wall shoulder and one at the shoulder cover plate. This creates the primary seal where most manufacturers intend it to be. When drawing the sealant over the shoulder plate locations be mindful to not cover the weep hole under the pressure plate cap. At the bottom of this article, there are links for references explaining how curtain wall functions.

After the detail is drawn and the project moves forward into shop drawings it is important to look out for how the curtain wall anchors are detailed for fabrication. Since the sealant and backer rod will be in the same plane as the “F” and “T” anchors going into the vertical mullions of the system the anchors will need to be notched to allow the sealant to be continuous. This is standard in the industry, but good to look out for otherwise the sealants won’t be in the primary location intended by the manufacturer. 

This methodology of providing a continuous air/water barrier transition is very popular and accepted throughout the industry. The great thing about this system is most contractors are very comfortable installing the sealants. Even though the detail is common it is critical from a design standpoint to coordinate the following items to avoid issues.

5 recommendations for detailing curtain wall with two lines of sealant

• Check sealant compatibility with air and water barrier products. Always call both manufacturers to verify. Ideally, your specification is written to take this into account.
• Evaluate the lifespan of the building since the sealants have a service life. Verify sealant service life is acceptable.
• Request color samples for review to make sure the sealant colors jive with the building design. Include this requirement in the specification.
• Make sure the curtain wall fabricator is including “notch” in “F” and “T” anchors.
• Consider having 3^rd party enclosure field testing on the installed product. Sealant adhesion tests are standard in the industry and verify installation and product quality. A spray water test and chamber air/water tests are also common.

Create a physical connection with engineered transition assemblies (ETA)

An alternative method for creating a continuous air and water barrier at the perimeter of curtain walls is to use engineered transition assemblies (ETA). An engineered transition assembly is a physical sheet, usually made of extruded silicone. The sheet is intended to bridge the gap between the curtain wall framing and the adjacent wall construction to form a continuous air and water barrier. The main difference between this system and the sealant and backer rod strategy is the ETA system provides a physical connection rather than a sealant only. The benefit of having a physical connection is that the system doesn’t rely as heavily on trusting a field applied sealant and hoping it has created a full connection by adhesion.

How to create a physical connection with ETA systems

When using an ETA system the silicone sheet can be inserted into the curtain wall pocket creating a solid connection. Then the opposite side of the ETA is connected to the adjacent substrate/air and water barrier with sealant.  If the wall design creates a gap between the primary sealant line of the curtain wall and substrate, then an ETA system can help bridge inherent gaps. It is possible to include a line of sealant and backer rod at the typical line of protection at the curtain wall shoulder if the design allows. This approach creates a belt & suspenders solution because you have primary protection with the ETA sheet, then have a secondary line of protection with the sealant and backer rod.

What ETA system types are best to install and detail?

There are a couple of different manufacturers of ETAs on the market with different system types. It is important to identify which system type is best for your project from a design performance, cost, and installation standpoint. Generally, there are 3 types of ETA connections to curtain wall framing.

• ETA sheet is laid inside the curtain wall shoulder and held in place during construction with sealant. The pressure plate and thermal block hold the ETA in the pocket permanently. Some installers don’t like to rely only on sealant to hold the ETA in place during construction.
• A custom aluminum extrusion clip piece is included to be attached to the curtain wall framing at the edge of the curtain wall shoulder.  This type requires more labor because a separate component from the curtain wall system must be attached to the curtain wall. Then the ETA has a nib that snaps into the aluminum extrusion.
• The third type of ETA has a nib pre-formed on the silicone extrusion that matches the snap-in nib within the curtain wall shoulder. This product type allows the ETA to snap onto the curtain wall framing during construction. While sealant is still required, this method allows for a straight forward installation sequence because the ETA is held in place by a snap nib rather than having to wait for the sealant to dry and hold the product in place.

Since ETA systems are usually more expensive than a traditional double line of sealant it is important to specify a cost-effective system that the curtain wall installer will be comfortable installing. Remember to evaluate the different detail conditions on the project like corners and special conditions. Most manufacturers can provide 3D corners to help complete the air and water barrier at these tricky conditions.

5 recommendations for detailing curtain wall with engineered transition assemblies (ETA)

• If the design aligns the curtain wall shoulder with the adjacent wall assembly. This will allow the ETA system to be flush with the adjacent wall construction. This configuration creates a simpler air and water barrier because all the components are on the same plane. If the adjacent substrates are not in alignment be mindful of the additional coordination that will need to happen.
• Engage the build team early so everyone is on the same page to understand the configuration of the ETA system. Since this is a more belts and suspenders approach it is important to have all stakeholders on board early.
• Make sure to check the compatibility of the adjacent air and water barrier products with the ETA system. For example, if there is a liquid applied air and weather barrier applied to the exterior sheathing then these products need to be verified to be compatible.
• When designing an ETA system, make sure to evaluate all detail conditions. This includes sills, jambs, and head conditions. Also, remember different parts of the building will most likely have varied conditions. All will need to be detailed and coordinated appropriately.
• Straight-forward, clear details have a better chance of being installed correctly.
• The WBDG guide detail shows the ETA strip wrapping back onto the horizontal plane of the knee wall. ETAs should go down the face of the adjacent knee wall construction. Do not wrap these products back into the wall assembly. Make sure the detail is creating a shedding effect so water can lap off the next layer beneath it.

Resources

Highly recommended resources to learn more about curtain wall design and function.

Whole Building Design Guide – Curtain Walls

Whole Building Design Guide – “Perfect Wall”

Shared Learnings: Controlling Storefront and Curtainwall Water Infiltration