I agree that 70 degrees is WAY too hot. I don't turn on the supplemental heat until it gets down around -20 real temp. and turn it off when the temps get above that. My coop does not get warm. The poop still freezes on the poop board. It just gives them a way to get some relief, if they choose, and they do, to get warmed up a tad, by standing under the lamp.
I do use a heat lamp, BUT...I have it wired all up to keep it from falling apart, hang it from a roof rafter with a chain and locking hooks. I still worry though as heat lamps can and do cause many fires. Please, if you are using a heat lamp, reinforce it, best you can and attach it securely also.
Also remember that humidity is more harmful to them, than the cold. Getting a good air exchange going is a must in these frigid temps, but also remember that what ever the outside humidity is, so will it be on your coop. If your outside humidity is higher than inside the coop, than it becomes a challenge to get it right. I know, as I have been fighting the high humidity this winter and saying it's a challenge is a tad understated.
Turn the heat down for the sake of your birds! They will be much happier and will thank you and be back in their coop!
ETA...wet cold is different than dry cold also...here in the north east, we get more of the wet snow, high humidity stuff than just about anywhere else. I have to check our weather daily to check the temps and humidity, so I can adjust ventilation and decide on turning on the heat lamp or not. AS a rule, I do not advocate adding heat, but there IS a limit on how cold the birds can comfortably survive and stay healthy. I wouldn't worry until the temps hit bout -20 with humidity at or near 100%.