Waterproof and thermal hutch covers are a must for outdoor piggies. They need protection from the elements. Having a thermometer on the hutch is important
Their temperature comfort range is around 18 degrees, with 25 degrees and over being dangerous (heat stroke), ensuring they can be somewhere very shaded and sheltered, and anything less than 15 degrees being cold for them and requiring extra protection using incremental efforts the colder it gets as necessary - hutch covers (with blankets underneath as well when it is cold) extra hay, houses to snuggle into stuffed with hay and heat pads (ideally moving into a shed/garage/in the house in winter).