TL;DR: Closed-cell backer rod resists moisture and holds its shape well in outdoor, high-movement joints, but doesn’t compress into irregular gaps as easily. Open-cell backer rod compresses more readily into uneven or oversized joints and is often used indoors, but is more prone to water absorption. The right choice depends on your sealant type, expected joint movement, and moisture exposure, and ASTM C1330 gives you a standardized way to specify it. Skip to the comparison chart below if you just need the quick answer.

Why Backer Rod Type Matters More Than It Seems

Backer rod looks like a simple product — a foam cylinder pushed into a joint before the sealant goes in. But the cell structure of that foam directly affects three things every installer cares about: how well the rod controls sealant depth, whether it creates a bond-breaker so the sealant can move freely, and how it holds up over the life of the joint. Get the cell structure wrong for the application, and you’re looking at bubbling, adhesion failure, or a joint that can’t handle the movement it was designed for.

The ASTM C1330 Classification System

Before comparing performance, it helps to know how the industry classifies backer rod. ASTM C1330 classifies cylindrical sealant backings into three types: Type C, Type O, and Type B, made predominantly of closed-cell material, open-cell material, and bi-cellular material, respectively. The standard tests backer rod for water absorption, density, outgassing, compression deflection and recovery, and tensile strength.

Closed-Cell Backer Rod: Where It Performs Best

Closed-cell rod is built from foam with sealed air pockets, which gives it two defining characteristics. The sealed cells block moisture and air, and the rod doesn’t allow sealant to bond to it, which helps prevent three-sided adhesion and lets the sealant stretch and move as the joint moves. Closed-cell rod also holds its shape well and performs reliably through weather changes, which makes it a common choice for outdoor sealing and joints that see significant movement.

Closed Cell Foam (not Soft Rod Type B): If a closed-cell backer rod is significantly compressed to the point that the cell structure is ruptured, it may not achieve the same 95% compression recovery. Open-cell materials will consistently recover because they do not rely on intact closed cells for resilience.

Best suited for:

• Exterior expansion and control joints exposed to weather
• Below-grade or high-moisture applications
• Joints with higher expected movement cycles
• Projects where sealant manufacturers require a non-bonding backing to prevent three-sided adhesion

See our Titan Foam^™, Mile High Foam^®, and Mile High Foam XL^®

Open-Cell Backer Rod: Where It Performs Best

Open-cell rod has a porous, sponge-like structure. That structure makes it more prone to water absorption, which can actually be an advantage in applications where moisture needs to be absorbed or allowed to pass through rather than blocked. Because it’s more flexible and compresses more easily, open-cell rod works well in joints that aren’t perfectly uniform in shape, and its texture can improve contact between the sealant and the backing.

Silicone and other air-cured sealants need airflow to cure properly. Open-cell backer rod’s porous structure allows air to move through the material, which supports the curing process from behind the sealant bead, making open-cell the preferred backer rod for silicone joint applications.

Best suited for:

• Interior joints where moisture exposure is minimal
• Irregular or inconsistent joint widths
• Applications where a degree of sealant/backing contact is acceptable or preferred
• Situations calling for maximum compressibility during installation

See our Denver Foam^®

Compression Recovery: The Number That Often Gets Overlooked

Cell structure isn’t the only variable. Compression recovery matters just as much for long-term joint performance. Industry testing on backer rod formulations has shown open-cell and hybrid open/closed-cell rods recovering roughly 92–95% of their original diameter after extended compression, which is a useful benchmark to compare against a manufacturer’s published spec sheet when reviewing product options. A rod that doesn’t recover close to its original diameter will lose contact with the joint walls over time, undermining the tooling surface the sealant depends on for proper adhesion.

However, if a closed-cell backer rod (not applicable to Soft Rod Type B) is compressed to the point that its cell structure ruptures, it may not achieve that same ~95% recovery. The resilience of closed-cell foam depends on the cells staying intact. Open-cell foam doesn’t have this vulnerability. Because it doesn’t rely on intact closed cells for its resilience, it recovers consistently even after significant compression.

How to Choose: A Quick Decision Framework

The Bottom Line

There’s no universal “better” option between open-cell and closed-cell backer rod. The right choice comes down to matching the rod’s characteristics to the sealant manufacturer’s requirements, the joint’s movement profile, and the environment it will live in. When in doubt, confirm compatibility using ASTM C1087 testing data and check the sealant manufacturer’s spec sheet before finalizing a joint design.