Louise, Any of the Sempervivum and Jovibarba that I've run into should be safe down to the temperatures you mentioned. These are outdoor plants that survive to below 0 temps for most of them.
I would, however, put them on the ground unless you are afraid something will eat them or they will get flooded then frozen or stay excessively wet. The ground helps moderate the extremes of temperature and moisture changes. The plants can handle being frozen. During those years where there is no snow cover, you could see some losses due to excessive drying (from winter winds, etc), so if you typically do not get snow cover, you may want to protect from drying winds. Please update this thread and tell me what your winter weather is like. I need to know moisture type and levels, in addition to temperatures. What are the extremes?
Most collectors keep their plants outside, although some do use rain shield or wind shields to protect them. Professional growers will use unheated greenhouses or alpine houses for them.
