OP, I think the first thing to ask yourself is this: what do you intend to do with the footage? Will you use it as evidence to evict your roommate? Will you make public the footage on youtube? Or, will you use it simply for your own purposes to confirm your suspicions?
In Ontario, as is the case with other provinces, there is a reasonable expectation of privacy. And the level of privacy depends on the context. For instance, making a recording of your roommate in his bedroom or bathroom is far different than making a recording in a living room (where the expectation of privacy is less).
If you want to look at laws, you'll have to look at three levels: (1) judge-made laws (common-law), in terms of invasion of privacy in tort law; (2) criminal law; and (3) maybe even statutes (PIPEDA).
But, with that said, I think you may be over-thinking things. If your concern is getting sued for invasion of privacy, then consider this: litigation is expensive, and the amounts awarded for damages (assuming your roommate is successful in pleading his case) is likely not worth his time.
If you are really concerned about getting sued, then you can get some free legal advice at either York or UofT's Community Legal Clinics.
