I have a deep 3 gang box on and exterior wall. There is cold air that pours
out of it since I am guessing not much insulation could be put behind it.
Is it safe to fill the gang box with fiberglass insulation?
I don't know the regulations, but my gut feeling says it wouldn't be safe
to fill the box with insulation. Instead, I would remove the
outlets/switches, and break away a few tabs on the back of the box, then
spray minimally expanding foam behind the box. After it has finished
expanding, cut or break away any insulation that is in the box, and caulk
around the openings and where cables enter the box. I'd also caulk around
the outside of the box where it meets the sheetrock/wall covering.
Reinstall devices, put on the cover plate, and you should be good to go.
And then place a foam gasket under the cover. This won't block all of
the air, but it will block most of it. I don't know of a safety issue
with regard to fiberglass insulation inside the box, but it likely won't
be very effective and will really annoy the next electrician who works
on that outlet.
No, you need free air circulation. Insulation would violate code, and
(properly) be a hazard. Plus, were I an electrican and I found loose
fiberglass in a box, I'd charge double to take it out!
Get some squirt foam in a can, and squirt that behind the box (not
inside! Nothing but wires in a box, please...) Tha may help. Also
gasket the cover plate which may help some as well. 'Caulk' between
the box and the sheetrock (or plaster) as well if possible.
The best solution that no one seems to have addressed, it why is there air
movement in the outside wall cavity? Check at the header and sill plates
and find out why there is air infiltrating the space. Fix the problem, not
Fiberglass insulation will do nothing to stop air infiltration and it is
useless as long as air is leaking through or around it. The best
solution is to find where the air is leaking into the wall cavity and
seal it there. If you can get above and/or below the wall to where the
electrical goes through the header and sill plates and shoot some
expanding foam in all the holes the electrician created it will probably
solve your problem.
If you can't do that then use a switch plate gasket. Stopping the air
from exiting the wall cavity should also stop the air from entering it.
That will allow any insulation that is in the cavity to do it's job.
Never put anything inside an electrical box except for approved
components. Foam and fiberglass are not approved components.
Lets get the myth that fiberglass will STOP air flow. It can only do
this if jammed extremely tight into a confined space. When it is this
tight its insulation value is worthless. The insulation value of a
product is dependent on how many and how well air pockets are
trapped. In the case of installing fiberglass tight enough to STOP
air flow the amount of air space is greatly reduced and the amount of
glass is greatly increased. Glass is great are transferring heat.
I agree with the answer to turn off the power to the outlet, remove
the outlet/switch (should not need to unwire). Use non or low
expanding foam behind and beside the box. Let set and trim any foam
that ends up in the box. Use a good grade of caulking to seal all
holes in the box and around the wires. Hint (tape about 4 inches of a
straw to the end of the caulking tube to reach the back). Let
caulking and foam setup good before reinserting outlet/switch (this
could take several days).
If done properly the air leakage should be near zero, this would mean
the foam gasket will do little or no good for air flow. It will add a
small amount of insulation to a very weak area so put it on anyway.
As for the answer "Fix the problem, not the symptom". Good luck, if
one looks at how a house is put together and the endless gaps between
the studs and plates, 2 by's and drywall, holes drilled by plumbers,
HVAC technicians, cable, phone and electricians etc. Not that I
disagree in sealing plates, most of the top plate can be sealed
providing one wants to move all the insulation in their attic over
every interior and exterior wall (yes it is recommended). Use
caulking or expanding foam, if the gap has a heat producing device
such as recessed lights (non IC, non air tight) do not seal. If it is
a fireplace, vent for furnace or water heater the can normally be
sealed with metal and fire rated caulk and or fire rated foam (check
with local codes).
Then there is the leaks of the siding, yes it is supposed to leak
air. The drain plane is next, it can leak air (should not leak
water). After the drain plane there may or may not be sheathing and if
one has ever looked at it installed it has lots of gaps.
So if the house was built correctly in the first place we would not be
hiving this dialog.... But then I'd be out of business.
Remember to do the inside wall outlets and switches as most homes have
interior wall that a technically out side (yes that is correct).
I hope this helps
Studying residential energy since 1981
Performed over 7,500 residential energy analysis
Presently doing Whole House Performance diagnostics.
We use Building Science to diagnose houses
The most common place that the air is coming from is almost always oversized
holes drilled for the wiring to pass through. This is usually in the attic
and usually difficult to find. The suggestion for using foam is the best
idea, just remove any that enters the box (under special circumstances it
has been known to change into an extreme fire hazard).
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