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Old 11-21-2010, 03:37 PM   #1
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Foam board and electrical boxes


I am curious, is it safe/legal/whatever to mount a metal electrical box directly to blue foam board?

Reason I ask is I am working on a box out in my kitchen that has a small shed style roof. In order to gain some R-value to the ceiling I am installing 1" blue foam board to the joists, then I'm going to cover the foam with drywall. The junction box for the light is a 4" diameter 1/2" thick shallow fixture metal box. In order to get the box flush with the drywall, I'm wanting to mount the box directly to the foam board, but I question if that is ok to do. The specs on the foam say protect from high heat.

I am wondering if instead of mounting the box to the foam, I need to go get a thicker box and cut the foam away and mount the thicker box directly to the 2x nailer I put in place to secure the box to.

Any suggestions?? Thanks!!

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Old 11-21-2010, 03:43 PM   #2
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Foam board and electrical boxes


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Originally Posted by Master Brian
I am curious, is it safe/legal/whatever to mount a metal electrical box directly to blue foam board?

Reason I ask is I am working on a box out in my kitchen that has a small shed style roof. In order to gain some R-value to the ceiling I am installing 1" blue foam board to the joists, then I'm going to cover the foam with drywall. The junction box for the light is a 4" diameter 1/2" thick shallow fixture metal box. In order to get the box flush with the drywall, I'm wanting to mount the box directly to the foam board, but I question if that is ok to do. The specs on the foam say protect from high heat.

I am wondering if instead of mounting the box to the foam, I need to go get a thicker box and cut the foam away and mount the thicker box directly to the 2x nailer I put in place to secure the box to.

Any suggestions?? Thanks!!
Nailer

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Old 11-21-2010, 03:55 PM   #3
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Foam board and electrical boxes


Heat won't be your problem. The box is supposed to be securely fastened to the building structure. Foam board isn't very durable for something like this and can easily rip out. You'd better get some solid backing behind it. Also, with foam board, how would you fasten the cable within 12" of the box?
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Old 11-21-2010, 03:59 PM   #4
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Foam board and electrical boxes


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Originally Posted by a7ecorsair View Post
Heat won't be your problem. The box is supposed to be securely fastened to the building structure. Foam board isn't very durable for something like this and can easily rip out. You'd better get some solid backing behind it. Also, with foam board, how would you fasten the cable within 12" of the box?
I placed a 2"x6" between the joists to securely screw the box to. It's 1" thick foam, so as of now the box is secured with 2" long screws through the foam and into the 2x6 support. The wire is secured to that support.
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Old 11-21-2010, 05:13 PM   #5
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Foam board and electrical boxes


It'll work.

A thicker box is better.

You could also get a small 1"-thick block of wood and cut it into the foam...
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Old 11-21-2010, 07:03 PM   #6
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Foam board and electrical boxes


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Originally Posted by emolatur
It'll work.

A thicker box is better.

You could also get a small 1"-thick block of wood and cut it into the foam...
Put in the nailer and fill in after behind the box eith greatstuff expanding foam. Used minimupal expanding foam door and window.

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