Attic

The attic is where insulation money goes furthest, for a reason worth understanding: it is the only assembly in a house where you can add depth almost without limit, and where heat loss is driven hardest by the temperature difference.

But the first inches do nearly all the work

Savings follow the difference between two reciprocals, not two R-values. Going from an uninsulated attic to R-30 removes about three quarters of the heat loss through it. Going from R-38 to R-49 removes about six percent more.

That is why "more R is always better" is bad advice, and why the right question is never "what is the maximum" but "where does the return stop paying".

Air sealing comes first, and it is not close

Insulation slows heat moving by conduction. It does nothing about air moving bodily through gaps around can lights, top plates, plumbing stacks, wiring penetrations and the attic hatch.

Blowing insulation over unsealed penetrations buries the problem somewhere you can no longer reach, and the return per dollar on sealing those gaps is several times higher than the return on the last R-10. Do it in that order or do not bother.

Two things that ruin a correct job

Blocked soffits. Baffles in every rafter bay holding the material back from the vents. A roof that cannot breathe grows ice dams in cold climates and cooks shingles in hot ones.

Buried can lights. Older recessed fixtures are not rated for insulation contact and covering them is a fire risk. Either fit IC-rated fixtures or build a clearance box — and while you are up there, seal around them, because they are among the leakiest holes in a ceiling.

Depth, not bag count, is the check

Staple depth rulers to the trusses before blowing. It is the only way to know what you actually laid down.