at 5 weeks that should be fine. My chicks this year have been off heat at 4 weeks, and our over night temps this summer were going down to 10C to 16C.Welp google tells me it's 23 °C here at 3 AM. I think that's not enough for them ..
If it's possible you should move your brooder and do what you can to keep the ants out. It's not gonna be easy, since you have to have some kind of ventilation but moving the brooder at least should help for a while.