Converting a garage to an ADU, JADU, home office, or gym means treating it like a habitable space — and that triggers Title 24. Insulation specs are part of plan check. Get them wrong and the permit comes back with a redline.

Here’s what code actually requires in San Diego, what the real costs look like, and where homeowners commonly trip up.

What Title 24 requires (typical garage in CZ7 or CZ10)

For a habitable conversion in the most common San Diego climate zones:

  • Walls (2x4 framing): R-13 cavity insulation minimum. R-15 in CZ10.
  • Walls (2x6 framing): R-19 to R-21.
  • Ceiling/roof (vented attic above): R-30 minimum, R-38 better.
  • Ceiling/roof (unvented or cathedral): R-30 closed-cell spray foam at the underside of the roof deck.
  • Floors (slab or over crawl): Slab edge R-5 to R-10 continuous; over crawl R-19.
  • Garage door: Insulated panel kit, U-factor and R-value per assembly.
  • Air-sealing: Continuous air barrier, gasketed openings, weatherstripped doors.

Climate Zone 10 (inland — Escondido, El Cajon, Santee) is slightly stricter than Zone 7 (coastal/central). The numbers above cover both.

What the work actually looks like

A typical 400 sq ft garage conversion includes:

  1. Wall insulation: R-13 fiberglass or mineral wool batts in the wall cavities. About 250–300 sq ft of wall area at $1.20–$2.00 per sq ft.

  2. Ceiling insulation: R-38 batts in joists if there’s an attic above, or R-30 closed-cell foam on the underside of the roof deck if the assembly is unvented or low-pitch.

  3. Slab edge insulation: Rigid foam at the perimeter of the slab. Usually 1–2 inches of XPS or polyiso. Optional but recommended for inland zones.

  4. Garage door panel kit: Pre-fabricated insulation panels that retrofit into the existing door. R-value 6 to 13 depending on product.

  5. Air-sealing: Top plates, electrical penetrations, plumbing chases, threshold gaskets.

  6. Mechanical ventilation: Required by code for habitable space. Usually a bath fan on a timer or an ERV.

Not insulation but related: a mini split for heating and cooling, and door and window weatherstripping.

What it costs

Range for the insulation-only scope on a typical 400 sq ft garage:

  • Walls + ceiling, batt method: $1,200 to $2,500.
  • Walls + ceiling, mixed (foam at deck + batts at walls): $2,500 to $5,500.
  • Garage door insulation kit: $400 to $1,200 installed.
  • Slab edge rigid foam: $400 to $900.
  • Air-sealing scope: $300 to $800.

Total insulation-only: $2,000 to $9,000 depending on scope. The full ADU build (framing, drywall, electrical, plumbing, finishes, mini split) is typically $40,000 to $80,000 — insulation is one line item among many.

Where homeowners trip up

Skipping the ceiling. Walls feel obvious. Ceilings get forgotten. The garage roof is usually 50 percent of the heat-loss surface — skip it and the space won’t hold temperature no matter how well the walls perform.

Insulating a still-vented assembly. If the existing garage roof is vented at the eaves and ridge, you can keep that assembly and add R-38 batts to the joists below. If you can’t vent it, you need to convert it to an unvented assembly with closed-cell foam at the deck. Mixing them — venting at the eaves but no insulation at the deck — guarantees moisture problems.

Forgetting the garage door. Code requires the door to perform thermally. A bare metal panel door with no insulation kit will fail plan check. Pick a kit before you submit.

Ignoring the threshold. Garage door thresholds typically have a half-inch gap to grade for water drainage. That gap is also a giant air leak. Threshold seals fix it.

Not coordinating with other trades. Insulation can’t go in until rough electrical, plumbing, and HVAC are done. We schedule with the framer and electrician so we’re not redoing work.

ADU-specific considerations

If you’re converting to a rentable ADU under California law (AB 68, AB 881, etc.), there are some additional considerations:

  • Sound walls: If the ADU shares a wall with the main house, code may require a fire- and sound-rated assembly. Mineral wool batts plus a layer of damping compound do both.

  • Mechanical ventilation: Required. Bath fan on a timer is the cheapest path; an ERV is better for indoor air quality.

  • Energy code path: Performance path (HERS rater) vs. prescriptive path (specific R-values). Performance path lets you trade off — better windows in exchange for lower R-value in walls, for example. Prescriptive is simpler but stricter on each component.

We can produce the insulation specs that match either path. Most plan checks accept prescriptive without HERS testing if all components meet minimum.

The bottom line

Garage conversion insulation isn’t complicated, but it has to be done right. The penalty for getting it wrong is failed plan check, redo work, and an ADU that’s miserable to live in.

For most homeowners, the right move is letting an insulation contractor handle the full envelope — walls, ceiling, garage door, slab edge, and air-sealing — as one scope. We coordinate with the framer and the GC, document the work for the inspector, and hand the completed envelope off ready for drywall.

Free in-home estimate gets you the spec and the price for your specific garage.