How to Build an ADU, Day 10: Insulation & Drywall

Barak Golan ·

From Skeleton to Skin: Insulation & Drywall

This is the phase where your ADU transforms from an industrial-looking frame into something that actually looks like a home. Insulation goes in to make it comfortable, and drywall covers everything up to create smooth, paintable walls.

Insulation: Your Comfort Layer

Why It Matters Insulation is the barrier between you and the outside world. Good insulation means:

  • Comfortable temperatures year-round
  • Lower energy bills (satisfying Title 24)
  • Sound dampening between rooms and from outside
  • Moisture control

Common ADU Insulation Types

Fiberglass Batts (Most Common)

  • Pink or yellow blankets that fit between studs
  • R-13 for 2x4 walls, R-21 for 2x6 walls
  • Affordable and effective
  • Must be installed carefully — gaps reduce performance

Spray Foam

  • Applied as a liquid that expands to fill cavities
  • Superior air sealing and moisture resistance
  • Higher R-value per inch (R-6 for closed-cell)
  • More expensive but worth it in specific applications

Blown-In (Cellulose or Fiberglass)

  • Great for ceiling/attic insulation
  • Fills irregular spaces completely
  • R-30 to R-38 in ceilings (Title 24 requirement)

Insulation Inspection

After insulation is installed, the city inspector and a HERS (Home Energy Rating System) rater verify:

  • Correct R-values in all assemblies
  • No gaps, voids, or compression
  • Proper vapor barrier installation
  • Air sealing at penetrations (pipes, wires, ducts)

Drywall: Creating Your Walls

The Process

Hanging (1-2 days)

  • 4x8 or 4x12 sheets of gypsum board are screwed to studs
  • Sheets are staggered at joints for strength
  • Special moisture-resistant "green board" is used in bathrooms
  • Fire-rated drywall (Type X) is used where required

Taping (1 day)

  • Paper or fiberglass tape is applied over all joints
  • Joint compound (mud) covers the tape and screw heads

Mudding (2-3 days, multiple coats)

  • Two to three coats of joint compound are applied
  • Each coat is wider than the last, feathering the edges
  • Drying time between coats: 24 hours

Sanding (1 day)

  • All joints and patches are sanded smooth
  • The goal: invisible seams that disappear under paint

Texture (if desired)

  • Smooth finish is most popular for modern ADUs
  • Orange peel or knockdown textures are also options

Timeline & Cost

  • Insulation: 1-2 days installation, $1,500–$4,000 per project
  • Drywall: 5-8 days total (with drying time), $3,000–$8,000 per project
  • Combined, this phase takes about 2 weeks

Coming Up Next

The series continues with finishing trades — paint, flooring, cabinets, fixtures, and all the details that make your ADU beautiful and functional.

Vision ADU delivers smooth, flawless interiors on every project. Get your free estimate today.

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