The short answer

Blown-in insulation should be evaluated as a coverage map, not a single ruler reading. Fiberglass or cellulose can settle, shift near eaves, compress around stored items, or leave low strips where installers could not reach. Before adding material, record depth across the attic, locate bypasses, and confirm that the material is dry and compatible with the assembly.

Build a repeatable survey

Location Record Why it matters
Center of each roof bay Several depth readings Finds broad under-coverage.
Eaves and edges Depth and wind-washed areas Air movement can displace material.
Around penetrations Clearance and missing insulation Gaps can be air leaks or required clearances.
Attic hatch Lid and surrounding depth The access boundary is often a weak point.
Moisture locations Dry, stained, compressed, or clumped Wet material should not be buried.

Use a marked survey map and photograph the measuring stick in place. Do not crawl across ceiling drywall or place weight on recessed fixtures, ducts, or wiring. If the attic contains vermiculite, broad contamination, or uncertain electrical work, stop rather than disturbing it.

What a topping-up proposal should say

Ask for the insulation type, target coverage, air-sealing locations, baffle or ventilation treatment, fixture clearances, access-path protection, and cleanup. A contractor should say whether existing material remains, is redistributed, or is removed. “Add insulation” is not a complete scope when the cause of the low area is a leak, an open bypass, or wind movement.

Limits

This guide does not set a universal R-value or approve a material for a climate zone. It is a coverage-survey aid, not an energy audit or electrical inspection.