Working as a residential electrician back in the late 1990’s, I remember learning of a new electrical box that we were asked to use on all exterior walls and in the ceilings of unconditioned attic spaces. At the time, they were called “vapor tight” boxes. 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 and also to seal the electrical wires where they enter the box. Air will contain at least some water vapor, by air sealing the electrical box, we were also reducing the amount of water vapor that could potentially enter a wall or ceiling. The name “vapor tight” was at least partially correct.
A quick search of the electrical code requirements for electrical boxes, Article 314-Outlet, Device, Pull and Junction Boxes in the National Electric Code found no mention of the airtight box requirements. Air sealing electrical box requirements are found in the IRC codes, Table N122.214.171.124 (R402.4.1.1). Under the electrical/phone box on exterior walls section, the code states:
The air barrier shall be installed behind electrical and communication boxes. Alternatively, air -sealed boxes shall be installed.
The code gives us two choices, either install the electrical boxes on the interior side of the air control layer or use airtight boxes.
As I stated earlier, we were using airtight electrical boxes back in the late 1990’s, but I still see errors in these requirements today. Below is a photo from a recent code-compliant blower door test. The home did pass, but I could see obvious issues through thermal imaging during the test.
Both the electrical service panel and the box used for data/communication wires, which was a mud ring used for metal boxes attached directly to a framing member, were air sealed incorrectly. I could reach into the mud ring and touch the fiberglass insulation inside the wall, basically a two-inch by four-inch hole in the air control layer. In years past, it was very common to see electrical boxes without a back used for data/communication cables used on exterior walls. It’s less common today, but I still run into the practice from time to time.
Older box designs
The first two photos of this post show airtight boxes similar to the original designs from the 1990’s. The gasket on the face of the box seals well to drywall and other air barrier products, but the seals where the wiring enter the box leave something to be desired. The insulating contractors of my area try to seal these wires using a canned foam product. These seals are easily broken when the electrician moves and pulls on the wire during the install of the outlet, switch or other electrical device. I often feel at least some air moving through an airtight box during blower door testing because of these older box designs.
A better way
My preference is to have the main air control layer on the exterior of the home, I feel this location provides the best durability and is easier to achieve good continuity for the control layer. My second preference is to install a vapor/air control product on the interior side of the exterior wall, then add a service cavity for any needed MEP systems. Both these options satisfy the code requirement of the air barrier be in front of the electrical box. When neither can be performed, I move to the air sealed box. My preferred box is the Airfoil box. (airfoilinc.com) A shout out to long time GBA member Doug McEvers who created this design. These airtight boxes have a dedicated compartment that can air seal the wires before they enter the main portion of the box. Once the wires are installed and the wire sheathing stripped, this compartment is filled with canned foam creating an airtight seal for the wires. The oversized plastic flange can then be sealed to an interior air control product, I’ve found acoustical caulk or other quality sealants such as Pro Clima’s Contega work best. A smart vapor control product or drywall will complete the air control layer. I can source the Airfoil box at our local electrical distributor, Viking Electric, and online at Small Planet Supply.
When using the typical airtight box, such as the type in the opening photos, air sealing the wires that enter the box with something other than canned foam is preferred. On a recent trip to Kansas City, I saw Travis Brungardt and Joe Cook of Catalyst Construction use a liquid flashing product to seal these connections. Kind of a messy process, but it greatly improves the air sealing of the box.
Another option is to use a putty pad, such as 3M’s Fire Barrier Moldable Putty Bad MPP+. This product is designed to create a fire rated wall assembly when electrical boxes with holes are used in walls. The putty works well to stop air movement through the box.
Do these methods work?
This photo shows a recent project I was involved in. We used the Airfoil box and an air/vapor product (Siga’s Majrex) on the interior side of the wall. This home hit a .76 ACH50 blower door test result, we could not detect any air leakage through any of the electrical penetrations.
The codes for achieving a good air seal around electrical boxes give us a couple choices and manufacturers have come up with a few different designs to help simplify how we can satisfy the code. The products in this post aren’t the only ones available, but in my opinion, they are the simplest to use and create a good continuity when breaks in the air control layer are needed.