Insul-bright has directions on the cover sheet that comes with it. It tells which side gets the shiny pieces. I'll quote
from that sheet. "The shinier metal looking side of Insul-Bright should always face the hot or cold item allowing it to reflect the energy back to its source."
Also: "Not for use in the Microwave!"
So you would want the heat to be reflected up to your item being ironed; that means shiny side up.