Where do you live? What breed of chickens?
As long as there are no drafts blowing on the chickens, and there is good ventilation in the shed **make sure there is good ventilation in the shed - or that you are able to modify it (i.e. cut metal if need be) to provide good ventilation**, you should not need to heat it.
However, if you feel it is absolutely necessary, the ONLY thing I would use is a
Sweeter Heater above the roost. A Sweeter Heater is a radiant heat panel that warms the birds (NOT the coop).
Chickens are literally wearing a down coat and their internal temperature is about 104°F. Proper ventilation (to rid the coop of moisture and prevent frostbite) and making sure there aren't any drafts are far more important than heat in the winter.
Also, be careful with insulation - it can provide a very cozy place for rodents to make a residence, and if they find a way in, they WILL make a nest in it, causing a much bigger issue than a cold coop.