Not ideal; main problem is it will reflect more heat back into the floor which could be a problem if there is no insulation under the element. Warm up time will also be around 2 x longer but, equally, the floor will retain heat longer so once it’s up to operating temperature, you shouldn’t really notice much difference; you may even find the temperature is more stable. It rather depends how you use it, for a quick intermittent burst to warm up your tootsies or as continual background heat.