"The Ministry’s goal of achieving predictability and consistency is laudable. The use of the word “substantial” introduces an element of subjectivity into the assessments that have to be made under the Act. The imposition of an inflexible standard is avoided at the cost of certainty. The use of Guidelines to promote a consistent approach to the judgments enforcement officers are called upon to make is entirely proper provided they do not introduce extraneous considerations which have the effect of unlawfully fettering the exercise of the decisionmaking power. However well intentioned, that is what happened here. The simple factual question of whether or not a space is substantially enclosed is replaced by a calculation intended to assess airflow through the space. That is contrary to the clear scheme and purpose of the statute."