The Construction regulations state that construction washrooms shall be heated 'if that is possible'.
The term 'if that is possible' leaves some contractors to believe it is optional, but not so fast... The MOL will take into consideration if you have a heated site trailer or availability of electricity in order to determine if heat is possible.
Washroom trailers are really the way to go to have happier workers who will likely produce better work. As a worker, if I have to sit in a freezing cold plastic box I will drive to the nearest public washroom at my employers expense. Cold in the kidneys is no fun health matter for any worker to deal with.
The MOL has indicated they use the OASIS Ontario guideline of 10 degrees Celsius as considered at minimum an adequate amount heat.