@Diannastarr, when a mother hen broods her chicks they do not get to eat or drink while warm. They do not need to stand under a heat pad, they can easily sit or lay instead. The heat mat mimics nature very effectively. Like I said, the chicks use it as needed. If it's too warm they move to the entrance of the dome to get radiant heat instead. At some summer temperatures a mother hen's body heat is too much for chicks as well. They just go under their mother less.
Personally I found that with the heat mat they were actually MORE active, not less. This was the most active and rambunctious group of chicks I have ever raised. They turned into the most rambunctious group of adults I've ever had, too. I was just impressed by how much more robust they were under the heat mat instead of the hot lamp.
To answer some other questions;
I was brooding my chicks indoors in the spring. I would say for the first couple weeks it was in the low 60's. We keep our house a 62*F most of the time.
The heat mat SAYS it will raise the temperature "10*F"-ish, but that's not quite true.
What actually happens is it puts out a certain amount of energy that, when layed flat on a surface, it raises the air temperature immediately above the mat by around 10 degrees. The energy output is constant, so when you turn it upside down, make a dome, and insulate it, the actual temperature increase is theoretically very high. With perfect insulation you could theoretically make an enclosed area as hot as touching the open heating element. But the heating element is a surprisingly low temperature. I think somewhere around 120-150*F or less, it is hot to the touch only if you press your hand firmly against it for a long time, so the risk of fire or injury is very low. It's cool enough to place plastic seed flats directly on it and they will never melt, but hot enough that if you actually sat there with your hand pressed on it for a long time it would start to hurt.
As such, I would say a heat mat like this COULD be used outside OR inside, depending entirely upon how well your dome is insulated. My dome was fairly open. I did not even have a towel over it. But if you brooded in colder temps, you could theoretically insulate the dome better by placing layers of towels or other insulation, over the mat to help retain the heat. Because the max temperature can never exceed the output, it's safe to use even with layers of things on top of it; it will STILL never get hot enough to catch fire.
(Boiling temp is about 200*F and paper catches fire at about 450*F, so the mat could hit 200*F and not be a fire risk. A heat lamp produces 480*F near the surface of the bulb, hot enough to instantly ignite bedding or dander that gets to close, creating a hot ember that can fall into bedding. This is true of ALL heat lamps and bulbs! This will NEVER happen with even the hottest heat mat!)
As for "is it too hot/hot enough for chicks" there's an easy way to tell and we do this with heat lamps too. It's not like a heat lamp puts out exactly 98*F radiant heat at all times, in reality a heat lamp is 480*F and we just move it far enough away that the radiant heat is 98*F or so in the hottest spot. So the way we tell if it's too hot or not without a direct measurement is we watch our chicks behavior. If the chicks huddle under the lamp it's too cold, and if they refuse to go under the lamp, hold out their wings, or pant it's too hot.
The same principle is true of heat mat brooding. You have a radiant heat source (the mat/dome) that the chicks will either crowd under, shiver, and refuse to move from if it's too cold, or they will refuse to go under if it's too hot.
This is no different from brooding with a heat lamp, except the heated area is much smaller. If you are concerned about extremely bitter temperatures (lets say, below freezing), I don't know if this would work, but I would also NOT suggest hatching or raising chicks outdoors in that weather unless you have to. But if you're over about 40*F, I think you could easily create a warm enough environment for the chicks with a few towels or other insulation. I wouldn't know having not experimented with it much, but I plan on giving it a shot in the future.