On adding heat, this is a hotly (pardon the pun) debated topic on here. Chickens grow their own down coat. I'd make your roosts wide, like the wide side of a 2X4, which allows them to rest their body on their whole foot and keep their feet warm. Heating chicken coops tends to prevent them from getting acclimated to the cold, so if there is a cold snap or the power goes out, they can be in trouble. There are people in Alaska who don't provide heat, or only add some if temps get well below zero -- yes, not below freezing, below zero, like 20 below. A pen in a barn should be very well ventilated which should keep frostbite off the combs. Frostbite is mainly caused by trapped humidity, which they release in their breath and poop, which is why the ventilation -- the humid air is also warm so it rises up and out, if there is a place to go. If you get worried about frostbite on some cold night, smear their combs with Vaseline. Plus heat in chicken coops tends to burn them down -- chickens are dusty and dust lights fires easily. If you get a bad cold snap and get worried, you have the space to add a few square bales of hay to make the coop smaller and thus better warmed by their bodies (25 chickens will give off a lot of body heat.) You can also put a temporary cover over, or partly over, the pen to help trap heat. If you have a typical high ceiling barn, and put hardware cloth on the top of their pen to keep predators out, this would hold a tarp easily. Or you could build something temporary. Just don't close off the ventilation.
For hot weather, 100 degrees is considered the danger point for chickens. It gets hotter than that here and I haven't lost any to heat. Mine always have shade and a fan, plus a pretty breezy coop. So for you, 90 degrees with the shade of the barn itself, especially since the barn is also shaded, should be quite comfortable for them. I doubt that even a fan would be needed unless it gets really still and steamy hot in there, and most barns are a lot more airish (breezy) than that! If they start doing a lot of panting and holding their wings away from their sides, they are probably a bit warm.
Is this a dirt floor? Mine is. Bedding is actually optional on dirt, but most people use some kind of bedding because it controls odors and moisture, keeps the floor closer to clean, and makes poop cleanout easier. Also a little warmer in winter. I throw pine shavings and hay on the poop, and stir it if the chickens don't do it for me, and once or twice a year the whole mess goes into the compost bin. I just did that, not because it smelled, because I wanted the compost. Dirt tends to normalize temps, warming the place a bit in winter and cooling it in summer. I have never had anything but dirt and don't want anything else. Thing about bedding is, even without cleaning it out, neither you or the chickens have to do much walking on poop. If you leave it dirt, you're going to be doing a lot of raking and scraping or else a lot of walking in poop.
In my 11X17 coop I probably use 2 square bales of hay and maybe 8 or 9 - $5 bales of pine shavings a year. For startup you'd want maybe 6 bales of the pine. Pine is wonderful for odors and light to handle. Unless you're really strapped financially, I think you'll be glad you invested in the bedding.
If you put down a floor, you will probably want to clean it every week or at least every few weeks, and you most likely will need bedding to keep them from sliding on the floor and injuring their legs or hips. Because, obviously, a floor needs to be linoleum or well painted or something similar, or it's too difficult to clean. The only floor I'd consider is concrete, because it's permanent, never rots, stands up to hose cleaning, and also keeps predators out. Up to you. Where I live, people always had plain dirt floors in their coops. The poop does dry and merge with the dirt, in time. But it does that as it filters through the bedding, too. I'm sure some will argue this isn't clean enough, that you need to be able to clean the floor. If I'd ever had a bout with lice and mites, I might say bad words about my dirt floor. Or I might rake it out, sprinkle Sevin, add clean bedding, and solve the problem. Will cross that bridge if I come to it.
Good luck!
