My Dominique chicks are usually about 1/2 feathered at 2 weeks. At that point, if the overnight temps are above 20F, I put them outside in one of the brooder coops with the door closed and a heat lamp (yes, there is ventilation, and I have remote digital thermometers to monitor temperatures). This heat lamp is connected to a thermostat. The thermostat is variable. Right now they are in the house at 75F, with no problems. They will go outside on Wednesday afternoon into the brooder coop. Each week I slightly lower the temperature and by the time they are 4-5 weeks they hav eno extra heat, unless the temps drop dramatically, at which point the lamp will turn on automatically.
Different breeds feather out at different rates, so watch your chicks. They will indicate whether they are too cold or too warm. The "guidelines" are great, but remember that they are just guidelines, and they need to be adjusted for your particular chickens. Normally, if the chicks are fully feathered and healthy they shouldn't need supplimental heat.