[whalesalad]

R15 feels a little low? Here in Michigan, for example, the ceiling requires R49. Walls require at least R20.

[patrickg_zill]

For one thing it is a smaller space, easily heated and cooled. Second, closed cell foam means no air infiltration to suck away your heat.

Walls in houses, assuming stud framed, are insulated to R20 but effective R value is less due to thermal bridging, air infiltration.