First, someone in the company must have encountered these temperatures before. Ask them.
Also Environment Canada should have historical records that will help you decide the level of protection you will need and a reasonable temperature value.
You, and your customer will have to come to an agreement on the level of protection required and how to achieve it.
"Not what we say about our blessings, but how we use them, is the true measure of our thanksgiving." -- W.T. Purkiser