I've made umpteen dozen casserole carriers and cast iron handle covers now using insul-bright. I can definitely see a difference in sides. Cotton fabric, thin fusible batting, insul-bright, thin fusible batting, cotton fabric. Shiniest side toward the item you want to keep hot/cold. So for a table runner to protect your table I would guess shiniest side up, leaving more batting/protection between the hot item and table. I'm not quite sure how the muslin would help with condensation as it's not going to 'waterproof' anything. I can't imagine you would have that much condensation on a table runner anyway.