Karnak 323 K-NRG Gap Seal FR™ is a fire-resistant building envelope sealant designed to prevent air passage by sealing joints, penetrations and transitions on above-grade wall air barrier assemblies. Karnak 323 K-NRG Gap Seal FR™ is a single-component, water-based acrylic mastic that cures to a tough monolithic elastomeric-like membrane. This sealant is fully compatible with Karnak 321 K-NRG Seal VP™.