My air handler is located in a closet that has sealed (via expanding foam) OSB sheathing. This closet has the return air plenum directly below it, which is the sole return air duct in the home. The OSB appears to have been installed via construction adhesive as there are no visible screw heads. Two separate internet installers have since run cables into the space with no sealing done to any of the holes added (into attic and stud bay). Currently, the internet coax and an outlet are located on the outside of one of the closet walls (regular sheetrock construction on the closet exterior). I plan to run 8 ethernet lines to various bedrooms and security cameras I am adding. Ideally, I would like to locate these (and relocate the internet coax, by just pulling the cable back through the wall) inside this closet. This will require both adding a low voltage box to terminate the various ethernet cables into jacks as well as adding an outlet to the space (using the line dropping to the existing outlet on the outside of the closet). I am experienced and comfortable with the electrical and ethernet work.

My question is in regards to if I will run into issues with the code (current jurisdiction is IBC 2018) by added penetrations to this space for the wiring and/or housing network equipment in here in general. I can't wrap my head around why the closet needs to be sealed in the first place as the unit appears to be appropriately sealed to both the return plenum and where it exits into the attic. Further, there is a sizable 1.25" gap under the door which would allow airflow between the closet and the conditioned space. I believe that I can add the network equipment (modem, router, switch, UPS, and NVR) without violating working space requirements for the handler. I don't mind sealing any added outlets with expanding foam meant for junction boxes if needed.

The home is energy-star rated and was built in 2006. Not sure if the sealing is going above code for the energy rating.

Is there any reason I can't add a single gang junction box with electrical outlets and a single gang 8-port keystone ethernet box to this area?

Pictures: Closet Closet top Return

  • I see screw heads in your second picture. From my perspective as a network pro, I loathe being jammed in with HVAC, but it's your house, DWYL. From a fire point of view, I'd drywall that (crudely being fine) rather than having exposed OSB and Foam. And fire-caulk (or fire-foam) the penetrations.
    – Ecnerwal
    Sep 11, 2021 at 18:07
  • If that area is considered a plenum you will need plenum rated cables. I am surprised that did not require drywall for a fire rating. But even plenums can have cables if they are rated, a bit more $ but no toxic smoke in in the case of a fire.
    – Ed Beal
    Sep 11, 2021 at 21:00
  • The trouble with > OSB > construction adhesive > expanding foam .... is this... don't put expanding foam inside electrical junction boxes. Sep 12, 2021 at 1:48
  • Type MC cable is permitted in plenum spaces.
    – DrSparks
    Sep 19, 2021 at 4:22

1 Answer 1


I can think of nothing in the national electrical code prohibiting such an installation.

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