The wording on CAPIC's website is all you need in your contract to retain copyright when being commissioned.
"Do Canadian creators own the copyright in their works?
Yes. All Canadian creators of original works automatically own the copyright in their original (personally created) works.
EXCEPTION:
In commissioned (assigned) photographic works, if the creator does not have an agreement to the contrary with the commissioner of the work, the commissioner of the work shall own the copyright in the work, once the work has been paid for.
See laws.justice.gc.ca - rid-38969 Ownership of Copyright, Section 13(2).
As a photographer, how do I ensure that I retain copyright in my assignment work?
To retain copyright in assigned photographic works, you must have an agreement between you and the commissioner of the work (your client) stating "Copyright in this work shall be owned by the photographer." This statement should be in the normal Assignment Terms & Conditions that would accompany your Estimate or Assignment Confirmation."
I touched base with a Cdn IP lawyer (employed by a leading worldwide CPG company) a few years ago and he stated a simple sentence "The photographer (studio) retains copyright to all works created. Any usage other than stated in the usage license requires a separately negotiated usage license".
He also suggested the other, sometimes more important, point is to include a line about payment & usage. "Usage license takes effect once full payment has been received. Any usage of unpaid work is an infringement of copyright (add whatever threatening action you feel like...).
I've never had any client question my retaining of copyright. I think many Canadians in the general public assume copyright is the photographers, though agencies, corporations and marketing professionals know better.