Exactly as TonyK said.
Not to sure on the heat output of 20watt heat cable, if your ambient Temperature in the room is to cold then the heating cable will lose heat and not be Efficient heat source to reach the target goal you wont.
Also depends what your racks are made out of.
Depending how your racking system is set up, you can always try box it closed, cover it with a thick blanket, use polystyrene to clad it or the easiest and less fuss, heat your room if it is not to big.
Consider getting yourself one or two panel heaters, they are low on power usage and will get the ambient temperature up enough for the heat cable to work at the right temps you wont.
You can even mount the heating panels behind your racks as I'm sure your heating cables are running at the back of your racks, that will also help.
Here is a link to the Econo heating panels information.
http://econo-heat.com/za/products/eheat ... el-heater/