I have a fairly small barn, and in the coldest part of the winter, everything is closed up pretty tight, (its well insulated) resulting in very high humidity in the barn, ranging sometimes from 75-85%. We do have an exhaust fan, but it freezes up in winter from frost building up on it and also its way too strong for my small barn. Would an overhead fan like those casa blanca (sp???) fans work. Or what else could I do. I don't think the high humidity is good for the horses. The temp in the barn ranges from 45 to 50, and I don't want it much colder than this.
There is nothing more importent than good air circulation in a barn interior. Keeping a barn shut up tight is bad for your horses airways. Sorry, I'm guessing that at this point you may have quit reading but why would you be more concerned about keeping the interior temp up at 45-50 degrees in the winter, than keeping your horses healthy. That high humidity does mean your barn is holding moisture from horse pee, that's ammonia in the air they breath. Let the temp drop a bit. Put on a warmer coat or blanket, crack the door open or windows open and give your horses lungs a break.
Moving the air in a closed barn with a fan will not reduce the humidity. I am in chicamuxen's camp here. Leave a window or door open a little bit.
My metal barn is not air tight and was not designed to be that way. FWIW, if I stall both of my horses in my very open 40' by 60' barn the inside barn temp is 10 degrees warmer than outside. So if it's 20 degrees outside then it's 30 degrees in the barn.