If you're working in Alberta you probably know by now that we cannot install the vapour barriers behind our electrical boxes as we used to, instead we have to use plastic air tight box. The reason for it is the new Alberta Building Code Prescriptive Energy Efficiency Requirements for Houses. Also If the vapour barrier around the electrical boxes on exterior walls is not properly sealed to the side of the box, outdoor air can penetrate through the exterior wall assembly, creating drafts and allowing heat loss. Worse yet, moisture from inside your home can pass into the wall cavity, and if there are additional gaps in the top and bottom plates, then that moist air will circulate freely through the attic, walls, and floors.
As you can see on the page 30 they now require structural support behind the section where two poly sheets get sealed together. In bigger penetrations like recessed lights or exhaust fans its relatively easy to achieve by adding two parallel 2x4 s between the ceiling trusses.
But it's not practical or almost impossible to box around the small device boxes.
The solution is using the plastic air tight box:
These boxes provide an air tight seal to stop the flow of air through the box and receptacle. The box is made of durable PVC so it is able to withstand extreme environments both cold and hot.
You don't need the vapour barrier behind them and the shape of the box will provide the support for the poly attachment.