Sealing Techniques For External Electrical Boxes?

This video explains how to seal the back side of exterior facing electrical boxes in a new construction custom home. Outdoor receptacle boxes are not intended to be sealed against water ingress, and the NEC does not recommend sealing them. However, if you have a plastic junction box and can safely add one, you can air seal the perimeter of the box. Electrical penetrations are often responsible for holes in the most critical locations in your envelope. To air seal electrical boxes, you need a caulk gun, caulk, and aerosol foam.

Gaps around ceiling boxes must be sealed with an intumescent fireblocking caulk or foam. Ordinary spray foam is used for this purpose. The IRC (Internal Residential Code) requires air-sealing electrical box requirements, which are found in Table N1102.4.1.1 (R402.4.1.1). For ceiling-mounted electrical boxes, install the box in the ceiling drywall, then caulk around the base and caulk all holes in the box with fire. Use a tube of silicone caulk on the top and sides to allow water to drain out of the un-caulked bottom.

Check for gaps and seal them up with silicone-based caulk or weather-resistant tape. Fire-rated silicone caulk (not intumescent fire caulk) or orange great stuf foam can also be used. Any connections to such a box should be made with rigid or flexible conduit using watertight connections. Apply paintable silicone acrylic or silicone caulk to the back of the box and around the entry hole before screwing the box to the wall. These boxes were designed to reduce air movement through wall or ceiling cavities by sealing the electrical box to the wall or ceiling air barrier. If outside of a wall or on a post, box it with treated material, foam between the boxing and electrical box, and silicone.