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. The reason for it is the new Alberta Building Code Prescriptive Energy Efficiency Requirements for Houses.
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 imposible to box around the small device boxes.
The solution is using the plastic air-tight boxes:
You don't need the vapour barrier behind them and the shape of the box will provide the support for the poly attachment.