Insulating behind/around electrical boxes


Almost ready to start the rewiring of my house, just need to finish up my plans. I am probably going to go with plastic old work boxes that are 3.5" inches deep in most situations.
This is going to leave me with 3/8" at most between the back of the box and the sheathing on my outside walls(3/8 drywall).
Any reccomendations on how to insulate behind the boxes and around the sides of them?
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Use a latex spray foam like that recommended for window installation at the back of the box. Expansion won't bother the box installation. Then cover the remainder with fiberglass bats, whatever. Do you really have 3/8" drywall exterior sheathing?
Joe
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No. I have 3/8" drywall on the interior, guess I should have been a little clearer. Although these houses were built so cheaply I wouldn't doubt they used 3/8" plywood for sheathing.
I did some work on a Levittown house years ago and the sheathing was some type of fiberboard that disentegrated after getting wet. Not sure what it was but seemed like it was the same stuff that gets layed under a rubber roof.
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On Mon, 15 Mar 2010 11:00:33 -0700, Limp Arbor wrote:

We have something very like that in our garage (which, going by the bakelite electrical fittings I pulled out of it, must have a few years on the clock). It does actually say 'sheathing' on it in big letters, but the smaller text beneath is all but unreadable after so many years. I know exactly what you mean about damp - it's rather horrible stuff if it ever absorbs moisture.
(I'm aiming to get another 4 or 5 years out of that garage, then I'll pull it down and build something less crappy :-)
cheers
Jules
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*One way is to use a vapor box around the electrical box: http://www.energyfederation.org/consumer/default.php/cPath/21_1272_62
Another method that I have used with outdoor receptacles that have recessed boxes is to wrap the electrical box with a putty pad. They can be a little expensive though.
Others may suggest spray foam. I have never tried that.
You could also use 4"x4"x1.5" square electrical boxes. That will give you room to put insulation behind.
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Why not use spray foam, and shallower boxes on all exterior walls???
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On 03/15/2010 08:06 PM, hr(bob) snipped-for-privacy@att.net wrote:

see the other thread going re: boxes, code doesn't let you use shallower boxes if there's more than one cable.
nate
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I would pack the space with the highest R-rated foam sheet or spray or any other insulation that will give you the most insulating and vapor barrier, that you can obtain. If you have to buy a sheet, cut it into squares about 8 to 12 inches and fit behind each box even if the ground and clamp screws penetrate into the insulation. While 3 1/2 inch deep boxes are great for wiring, you may be better in locating some 2 1/2" deep boxes for the outside walls, this will give you much better insulation and prevent condensation from forming inside the backs of the boxes, which is never good.
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Spray foam behind them and fibergalss around them...HTH...
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wrote:

first. Latex foam is water based - and if it expands and hits the terminals on the outlet it can (and usually will) sizzle and heat. The propellant in that stuff is basically propanol, so you need to take care.
After it has set, shave the excess out of the box and turn the power back on.
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