During the extreme cold weather it's best to leave the underfloor heating system on because it can take 3 to 4 hours for the underfloor heating to heat back up and would be wasting more energy by turning it off.
During extreme cold when it gets 32 F or below I leave the underfloor heating on all the time but when it gets to 45 F or above during the day I turn the underfloor heating off to save money and energy but turn it back on after I get home.