Comment by vundercind

I’d say -15°F or lower is something you need to design for at least into the light-blue area on this map:

http://4.bp.blogspot.com/-tdK_AMaZ9pg/Vb51WrvPDhI/AAAAAAAAD5...

Which is quite a lot of the country.

Light blue is a typical Winter low point in the -10°F to 0°F, which means you will see -15°F or lower often enough to worry about it.