How hot is too hot for winter rugs.......(please help)?

some of my horses are normally stabled at night. i wont be able to stable them for the next week and i wont be there to take there rugs on and off. they will be out in the paddock. its winter here and the lowest it gets a night is about -5 degrees and it frosts but the hoteset it gets up to during the day is about 15 degrees. the only rugs i have are really heavy weather beater rugs so should i leave their rugs on for the whole time ( will they get to hot) or should i leave them off ( will they get to cold) ??????????????