kermit are outside cat

kermit who lives outside can it get to cold for cats? we have a big box he's sleeps in with a wool rug inside the box and a tarp on the outside and and night we bring him up on the porch which is all covered with a heavy platic for the winter

the past few nites the temps have got to the lowers 30s and the other nite it got down to 21 degrees we are worried about kermit i wish we could bring him in but i have 3 cats and 2 dogs that live in the house [kermit]is the cat my husband found about two months ago which he was skin and bones but is doing very well now and looks good next month we are taking him to the vet to get all his shots and to be nerted we just don't want anything to happen to this little guy can someone tell me how cold is to cold on cats?

Can you insulate the box with a thick layer of straw under the tarp? Tie it on round the sides and put plenty on top.That would help a lot. A layer of straw inside as a bed is good too - cats like to make a nest in straw and it is warm.