I agree with Julia totally. I keep terrestris and fastuosa in an outside enclosure. No heating provided. As Julia says, allow the animals to get underground away from the surface. Mine have little cavities made with small logs and thick bark overlaid with ten inches of responsibly sourced leaf mould over which a thick layer of semi decomposed oak, beech and sweet chestnut leaves are placed. Terrestris certainly is active even at 7C as can be seen when they come to the surface whereas my fastuosa seems to prefer it a little milder. If I expect a very heavy frost or snow, I place a folded hessian sack over the lid just in case. My big concern would be rapid swings in temperatures if the enclosure rceived a lot of sun light as a -6C night could easily become +something very quickly and I'd not want my animals dormant one moment and active a few hours later only to be faced with another cold night.