Choosing Sealants for Curtain Wall Expansion Joints

Because Building Movement Doesn’t Pause for a Weather Forecast

Curtain walls are sleek, high-performance envelopes that define modern architecture—but behind the glass and aluminum, movement is constant. Wind load, thermal expansion, and structural shifts all act on a curtain wall system daily. The expansion joints within those walls need sealants that move with the building—without cracking, shrinking, or leaking.

For façade contractors, glaziers, and commercial GC teams, selecting the right sealant isn’t just about gap-filling. It’s about ensuring water-tightness, thermal performance, and long-term flexibility across every movement joint in the system.

Here’s what to look for when choosing sealants specifically engineered for curtain wall expansion joints.

Why Expansion Joints Matter in Curtain Wall Design

Curtain walls are non-load-bearing, but they’re far from static. Movement comes from:

Thermal expansion and contraction of aluminum framing

Story drift during wind or seismic activity

Interfloor movement from HVAC, occupancy load, or elevator shafts

Air pressure differences that can stress perimeter seals

If the expansion joint sealant fails, the result is water intrusion, loss of thermal efficiency, and potentially compromised structural integrity. That’s why curtain wall joints demand high-performance, flexible sealants that are tested for movement, adhesion, and durability.

Keywords: curtain wall joint sealant, expansion joint waterproofing, high-movement façade sealant

Performance Traits to Prioritize

1. Movement Capability

Choose sealants rated for at least ±25% joint movement under ASTM C920. Many silicone-based sealants designed for curtain wall systems are rated for ±50% movement or higher, critical in taller buildings or climates with wide thermal swings.

2. UV and Weather Resistance

Curtain wall joints are often fully exposed. Sealants must resist UV degradation, rain, snow, and freeze-thaw cycles for 10–20 years or longer without hardening, chalking, or shrinking.

3. Adhesion to Non-Porous Surfaces

Most curtain wall framing is aluminum, glass, or coated metal—surfaces where not all sealants bond effectively. Use primers if required, and confirm adhesion with test joints before committing to a product.

4. Compatibility with Adjacent Materials

Sealants must be compatible with pressure-equalized glazing gaskets, foam backers, aluminum frame coatings, and waterproofing membranes. Incompatible materials can cause adhesion failure or chemical breakdown.

Keywords: ASTM C920 sealant, UV-resistant joint filler, aluminum-to-glass joint sealant

Best Sealant Types for Curtain Wall Expansion Joints

Silicone Sealants

Silicone is the gold standard for curtain wall joints, thanks to its flexibility, weather resistance, and adhesion to glass and metal. Neutral-cure silicones are especially useful in minimizing compatibility issues and are available in both one-part and two-part formulations.

Ideal for:

Vertical joints between curtain wall panels

Mullion splices and perimeter seals

Areas exposed to direct sunlight and wind

Hybrid Sealants (MS Polymer or STPU)

These offer better paintability and lower odor than silicone, with good movement ratings and adhesion. Not as UV resistant, so better for interior joints or partially protected conditions.

Foam Expansion Joint Sealants (Pre-Compressed)

Used in deeper or irregular gaps, these pre-compressed foam seals expand to fill joints and are often impregnated with water-repellent agents. Ideal for retrofit or hard-to-access vertical expansion joints.

Keywords: silicone curtain wall sealant, pre-compressed foam joint, hybrid polymer sealant façade

Installation Guidelines

Use a closed-cell backer rod to control joint depth and prevent three-sided adhesion. This ensures the sealant flexes as intended.

Tool the sealant properly—smooth it out with light pressure to ensure bonding and avoid air pockets.

Follow joint dimension rules—ideally, width-to-depth ratio of 2:1 for optimal performance.

Check temperature and humidity at application time—some silicones require cure temperature ? 40°F.

Distributors Should Stock

Distributors like Buldix should offer:

ASTM C920-compliant silicone sealants in black, gray, bronze, and aluminum shades

Primers and cleaners for aluminum, anodized metal, and glass substrates

Backer rod in multiple diameters, including closed-cell for exterior use

Pre-compressed joint fillers for deep or irregular vertical joints

Application tooling kits, including joint knives and sealant guns with precise flow control

Offer project kits by system type: unitized curtain wall, stick-built, or punched window systems.

Avoid These Common Mistakes

Choosing the wrong modulus—low-modulus silicone is more flexible and better for curtain walls than high-modulus, which can crack under movement

Skipping the primer on anodized or coated metal

Overfilling the joint—which leads to excess stress and early failure

Ignoring compatibility with waterproofing membranes or glazing gaskets

Always review the curtain wall manufacturer’s installation guidelines for approved sealants—many will list specific brands or test standards.

Conclusion: Flexibility is the Standard—Not the Upgrade

In curtain wall systems, movement is constant. Sealants that can flex, adhere, and weather the elements are your building’s first line of defense. Choosing the right product ensures long-term performance, eliminates costly remediation, and upholds the building envelope’s integrity.

Distributors who lead with technical expertise and offer trusted, tested systems earn loyalty from façade installers and project managers alike.