i would just buy a couple 2x4s and frame up a room real fast. even if you don't know how to do it you can youtube it and it'll be pretty easy. you can add your insulation and then put that black and white poly. now you have a durable, strong, easy little room that can handle the weight of the equipment you'll have, plus it'll be insulated against the unstable conditions that you'll likely face in a barn. i'm telling ya, you won't know what can go wrong until it does...
i had the nail heads inside my building with droplets of water on them once i got my humidity up. so then i though i needed more airflow in the attic, so i cut a gable vent, even though i already have a slit vent down the apex of the roof and i have metal corrugated roofing. the night that i cut the gable vent it was really windy, and cold wind ended up getting down my walls and was blowing through my electrical sockets. here is a lesson for you, orenative. when an object is colder than the dewpoint of the air in which it resides, water vapor will condense onto the object. so, when i had cold air blowing through my walls and through the electrical sockets, the air cooled down the sockets, and the sockets reside in my grow room which is nice and warm and humid. the sockets were sweating like crazy!!! that's also why the nail heads were sweating, because the tip of the nail is closer towards the outside where it is cold. that cold is conducted up the body of the nail to the head, which is sitting in my grow room, which again, is nice and warm and humid.
i'm telling you these things because i'm imagining that at some point your grow room will be nice and warm and humid, and the temperature outside will be cold and you will remember my warnings about moisture and dew point, temp and humidity.
believe me, it is was cheap and easy, i'd have over thirty stalls filled and operating. but it really is worth it to do it right, that way when the construction is complete you can truly enjoy the gardening and not have the worry about environmental parameters. but at the same time, don't spend all your time and money planning the grow room and never actually get to growing. remember, you can always make something better the next grow, but you also don't want to spend this whole grow regretting that you didn't run that power line, or put that shelf in, or put a waterproof liner on the floor...
take it from experience, especially with barn growing, you're going to want to do it right, but do it quick and easy. hell, i'm sure you know this already, quick, easy, and right are the barn way