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.
"My philosophy of life is that if we make up our mind what we are going to make of our lives, then work hard toward that goal, we never lose -- somehow we win out." -- Ronald Reagan