Which item helps prevent water intrusion in insulated assemblies when fastened to sheet metal and lagging?

• A. Duct tape
• B. Neoprene washers
• C. Foam sealant
• D. Zip ties

Answer: (B)

When a fastener passes through sheet metal and lagging, the goal is to stop water from sneaking into the assembly along that hole. A seal that stays flexible and compresses evenly around the fastener does this best, especially where there are temperature changes and vibration.

Neoprene washers act like tiny gaskets. They are elastic and compressible, so once the bolt is tightened, the washer deforms to fill gaps between the metal, lagging, and the fastener. This creates a watertight barrier and helps prevent moisture from wicking into the insulation, which can cause corrosion, mold, or insulation performance loss. Neoprene is also resistant to water and many chemicals and can handle the temperature swings common in duct and pipe installations, so the seal remains effective over time.

Duct tape isn’t reliable for long-term sealing in damp environments; it tends to peel, dry out, or lose adhesion under heat and moisture. Foam sealant can fill gaps, but it’s not ideal around moving or mechanically loaded penetrations and may trap moisture or degrade with temperature cycling. Zip ties only secure components and do not provide a water seal, so they won’t prevent intrusion.

So the neoprene washer is the best choice because it provides a durable, compression-type seal that blocks water at the fastening point while tolerating the conditions typical of insulated assemblies.