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I am working to tighten my 1920s house. With respect to junction boxes (receptacles and switches) the common approach seems to be sealing the gap between the box and the wall with spray foam and/or caulking and optionally adding a gasket.

However, there are still holes into the junction box: Knockout through which the cables enter and other small holes. Various sources explicitly say not to put spray foam into junction boxes. But since there is still a small gap between a switch paddle and the case as well as receptacles (even if only the holes) I don't understand what's the point.

Is it really not necessary to seal the gaps/knockouts in a junction box itself? If no, why not? If yes, how? Is a bit of spray foam permissible? For example a bit spray around the gaps and where romex wires enter the box?

To clarify, I mean all the gaps and small holes in the junction box itself. They are far from airtight. As an example, this is a metal junction box in my attic (but plastic boxes have the integrated clamps that let air through):

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Of course, the extreme is a low voltage ring because that goes straight into the wall cavity. This shows an example of a low voltage ring (installed CAT6 cabling) next to a receptacle:

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What is the right way to seal these?

divB
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Generic 1-part spray foam out of a can is basically napalm, so I wouldn't recommend its use anywhere near an electrical box or anywhere fire safety is a consideration, e.g. North America lol.

They do make firestop putty/ caulk that are used e.g. inside conduit to turn the conduit into a fire stop. That would be fine for small screw holes.

They also make firestop foam, but I would be careful with that stuff and give it an outdoor "flame test" to satisfy yourself as to its non-flammability. It's important to note that conduit fire blocking foam is designed to stop air movement carrying fumes and fire, NOT designed to be a fire-stop material all by itself. But give it a flame test and see its performance with your own eyes.

You could also try foil tape.

For "wish boxes" (boxes you wish had a back), commonly used for data cables, feel free to replace with proper boxes with cable clamps.

If a whole knockout is missing, they make knockout seals in every size that cost a few pennies.

The cables going into a box will be protected by the UL listed cable clamp which is approved for the cable they were used with.

They make retrofit cable clamps designed to be installed after the wire.

Harper - Reinstate Monica
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