How do you get proper ventilation - especially of the humid and hot air from respiration as well as warm summer days - with something blocking off the top of the barn? Especially in the winter when all the doors are shut? I'm honestly curious because I've never heard of being able to accomplish that.
Don't shut the doors. Easy.
My hayloft only spans the center aisle of my barn. Over the stalls, it is open all the way to the roof trusses. On the sides of the hay loft are 2 x 4s frames, I just never walled it in. So when you're standing in a stall, you can see right up to the roof. I can also easily drop hay down from the loft (I do have a hay elevator to stack it up there). There are vents on either side of the hay loft and there is still lots of space above the bales so they're not all the way up to the roof.
Finally, I have converted two 10 x 12 stalls into one large run-in which my three horses use. The two four-foot wide stall doors open out onto a paddock. They are never shut, even in winter. I shut them once, I think, for a few hours, when a blizzard was driving the snow sideways, but usually, it isn't a problem even in a storm - and we have our share of those. A big help is the 10 foot long overhang that spans the length of the large, open run-in stall. They often stand under there, or inside the double stall looking out into the paddock when the weather is unpleasant. My barn is not insulated and because it is never closed off, I have never had a drop of condensation. The horses are perfectly fine in very cold weather, it's the humans who don't like it!
Again, I am absolutely not telling the OP or anyone else to build a barn with a hayloft. I'm just explaining how I have good ventilation in mine since you asked. A tightly shut barn might seem more comfortable, but the air quality often suffers when people feel the need to shut in their animals.
Oh, and in the summer, the horses are happy to come in to escape the heat and flies since the barn is nicely shaded. I also have screened windows that are placed up high so the air is always moving. In the summer, it is always much cooler in the barn than it is outside.