Secure a vapor barrier to the bottom of the pole barn's ceiling joists with insulation staples, using a staple gun to place a staple every 6 inches along each ceiling joist. Work from the lowest part of the pole barn toward the ceiling's peak. Cut the vapor barrier to size with scissors. The vapor barrier looks like a foil-backed wax paper.
Hold a 1-inch-thick foam board insulation panel against the bottom of the pole barn's ceiling joists. Secure the foam panel to the bottom of the joists with 1-1/2-inch cap nails, using a hammer to drive a cap nail every 8 to 12 inches along each ceiling joist.
Secure each piece of foam insulation board to the bottom of the ceiling joists, using the same method used to hold the first piece in place. Cut the last piece in each row to size with a drywall saw.
Fill the gaps between the foam panels and the pole barn's walls with an expanding foam. The foam prevents condensation-causing drafts between the walls and the ceiling.
Seal the seams between each foam insulation panel with mastic, using a disposable paintbrush to push the mastic into the seam. Mastic, a paste-like sealant, creates an airtight insulation joint.
Cover each of the cap nails holding the foam insulation to the pole barn's ceiling with the mastic, using the paintbrush to spread the mastic.