He paid them for insulation, but it didn't work! We figured out why.
A blower-door test revealed leaky top-plates. A top-plate is the 2x4 top of a framed wall, visible here from the attic. The seam where the drywall is attached was leaking warm air up into the attic.
This flue pipe is round, but it goes up through a square hole. That's pretty far from air-tight. Lets seal it up with some metal flashing, fire-rated caulk, and insulating rock-wool.
See those electrical wires going down from the attic into a wall cavity? When the house was built, the electrician drilled a large hole that lets heat escape up into the attic. Today, fire-code requires those to be sealed, but they rarely are, even in newer homes. Regardless, they are a big contributor to heat-loss.