Most definitely cold slows their growth. They can handle 50 but it will affect your yield. I don't pump in any air when I grow in the closet during the winter but then I check on them at least twice a day which has worked for me. You could go to your local home depot or other big box hardware store and buy yourself a couple of sheets of styrofoam insulation get the thin 1/2 inch comes in 4'x8' sheets and line your closet (its easy to cut with a box cutter or knife) your t5s should generate enough heat all by themselves to warm them up some.