Ventilate the pole barn. Add a fan or fans depending on the size of your structure. Attach a heavy hook to a wooden support beam at roof level. Drill a hole in the beam and screw in the hook. Hang a fan from the hook, either by the handle or by threading a rope through the top of the fan cage and tying it to the hook if there is no handle. If your barn does not have electricity you need to run an extension cord to the nearest outlet to power the fan. Start with one fan at each end of the roof and add another in the middle section if necessary.
Add vents to the building. Purchase a vent of your choice. Cut a hole in the roof near the bottom of the roofline. Install the vent according to the directions.
Add an insulating lining to the underside of the roof. There are a couple different types of quick insulation. One is a self-adhesive felt. Calculate the square footage of your roof. Purchase the correct amount of felt to cover the underside of the roof. The felt comes in rolls. Install the felt according to the directions. The other type is a foam spray. Purchase enough spray to cover the underside of the roof. Spray on a layer of foam 1 inch thick, starting at the edge of the roof and moving toward the center until you have covered the whole roof.
Install a vapor barrier. Hang a reflective laminate under the roof. Attach the laminate with adhesive tape along the edges of the roof, letting the material drape between the support beams. The laminate should not make contact with the metal roof.