20 C = 68 F. That might help some people with this.
You can use any heat source you wish. People will often tell you many reasons why you can’t do something while others do it quite successfully. There are a lot of different ways you can provide heat, not all of them involve bulbs. You might want to check out this thread to see an alternative that is becoming popular with many people. A hover, an inverted shallow box a couple of inches off the floor to trap heat, is another option.
Mama heating pad
You are not trying to warm their entire universe to some perfect temperature. That perfect temperature doesn’t exist anyway. Just like people, different chicks prefer different temperatures but can operate in a fairly wide range. To me the perfect brooder is one that is big enough that you can heat one area warm enough yet let the rest cool off quite a bit so he chicks can self-regulate. That way they will find the temperature they want to be.
Let the chicks tell you how you are doing. If they all crowd in the heat you need to increase the heat. If they are as far away from the heat source as they can get, it is too hot. If they are scattered a bit and wandering from one zone to another, you are doing great. They tend to sleep in a group, often pretty close to the heat source. Don’t let that fool you, they are not cold, they just like company when they sleep. If they are cold you will hear a plaintive peeping. Just listening to it you know something is wrong, it’s that sad.
How are you set up for outside? They will grow really fast, they will create a lot of dust, can be loud, and if you don’t keep the brooder pretty dry and cleaned out, they can stink. You might want them out of your house a lot sooner than you think. Most chicks feather out at four to five weeks and can keep themselves warm after that in fairly cold temperatures, but I don’t know how cold your temps will be then.
I’ve had chicks less than six weeks old go through nights at -5C with no supplemental heat with no problems, but mine are raised outside in a large brooder where one end is kept toasty while the other is allowed to cool off as it will. I’ve put chicks in that brooder straight from the incubator. While one end has to remain warm the far end sometimes has ice in it. Mine are acclimated to the cold. Once they get a little age on them they wander all around, including the cold end. Then coop they are in has good ventilation up high so good air is exchanged for bad, but they have good wind protection down on the floor where they are.
If you have electricity in your coop you might want to consider rigging up a brooder out there. They do need an area they can go to so they can warm up when they need to, but they will spend a surprising amount of time in cooler areas.
Good luck with it.