Short answer
To improve energy efficiency and stop condensation, air-seal the garage ceiling first, maintain roof ventilation with baffles, then insulate the joist cavities to the highest R-value that fits (often R-30 for 2x8 or R-38C for 2x10 using high-density/cathedral batts). Add a smart or kraft-faced vapor retarder on the warm-in-winter side, and finish with code-required gypsum board—5/8 in. Type X if there’s living space above. Avoid blocking soffit vents, use IC-rated airtight recessed fixtures, and consider adding continuous rigid foam under the joists for a thermal break if headroom allows.
Why garages get condensation
Condensation forms when warm, moist air hits a cold surface. In attached garages, this often happens because:
- Air leaks from the house into the garage ceiling cavities.
- Vented roof bays above the garage pull cold air through unsealed gaps.
- Wet cars spike garage humidity.
The solution is a continuous air barrier at the ceiling, correct insulation, controlled ventilation in the roof, and the right vapor retarder for your climate.
Recommended R-values and vapor control
Targets (United States):
- Ceiling/attic over garage: R-38 to R-49 where space allows.
- Common joist depths: 2x8 ~7.25 in (R-30 high-density mineral wool); 2x10 ~9.25 in (R-38C fiberglass/mineral wool).
Vapor retarder (warm-in-winter side):
- Cold/mixed climates (CZ 4–8): Class II (kraft-faced) or a smart vapor retarder membrane.
- Hot-humid (CZ 1A–3A): Avoid interior polyethylene; use smart membrane or unfaced batts + painted drywall (Class III).
Materials and tools
Materials:
- High-density batts: mineral wool R-30 (2x8), R-38C (2x10), or dense fiberglass equivalents
- Cardboard/plastic rafter baffles and ventilation chutes
- Smart vapor retarder membrane or kraft-faced batts (climate-dependent)
- Fire-rated sealants: fire-block foam and fire-rated acoustic/latex caulk
- 5/8 in. Type X drywall (if living space above) and tape/compound
- Optional: 1/2–1 in. continuous rigid foam (polyiso) under joists + compatible tape
- Insulation supports (wire rods) or strapping
Tools/PPE:
- Respirator (P100 or N95), safety glasses, gloves, long sleeves
- Utility knife, insulation saw, straightedge
- Staple gun and 1/2 in. staples
- Caulk gun, can foam gun
- Drill/driver, drywall screw gun, drywall lift (rental), ladder(s)
- Headlamp and inspection mirror
Step-by-step
1) Inspect and plan
- Identify joist depth and ventilation path (soffit to ridge or gable vents). If attic is accessible above the garage, note existing baffles and airflow.
- Check codes: ceilings under habitable rooms typically require 5/8 in. Type X drywall on the garage side. Foam needs an ignition barrier.
- Address any roof leaks before insulating.
2) Air-seal the ceiling plane
- Seal gaps around electrical boxes, plumbing, and wire penetrations using fire-rated caulk/foam. Seal the top plate at the common wall to the house.
- Replace recessed lights with IC-rated, airtight (ICAT) fixtures or build code-compliant fire-safe enclosures.
- Seal duct/pipe chases thoroughly; do not foam within clearance zones to flues or chimneys.
3) Maintain roof ventilation
- Install rafter baffles at each bay from soffit upward to preserve a 1–2 in. air channel. Extend chutes a few feet up to prevent wind-wash.
- Keep soffit vents clear; add vent screens if debris-prone.
4) Install insulation
- Cut batts to a friction fit—no compression, gaps, or voids. Split batts around wires and pipes rather than crushing them.
- For shallow joists, use high-density/cathedral batts sized for the cavity (e.g., mineral wool R-30 for 2x8). For deeper joists, R-38C.
- Optional performance boost: add 1/2–1 in. rigid foam across the underside of joists for a thermal break, then strap and drywall. Tape seams to form a continuous air/vapor layer where appropriate.
5) Vapor retarder
- Cold/mixed climates: install kraft-faced batts with the facing toward the garage interior, or add a smart membrane over unfaced batts before drywall.
- Hot-humid climates: avoid interior poly. Prefer smart membranes or rely on painted drywall as Class III with unfaced batts.
6) Drywall and finish
- Hang 5/8 in. Type X drywall if living space above; 1/2 in. gypsum may be acceptable otherwise—verify local code. Seal seams and perimeter with caulk after taping for a tighter air barrier.
Safety considerations
- Wear PPE; mineral wool and fiberglass are irritants.
- Maintain required clearances to flues, B-vent, and recessed lights. Use fire-rated sealants where specified.
- Ladder safety and panel handling: use a drywall lift for ceilings.
- Do not block combustion air to fuel-burning appliances in the garage.
Tips for best results
- Add weatherstripping at the overhead door and seal the garage-to-house door. This reduces humid air exchange that drives condensation.
- If you regularly park wet vehicles, consider a small exhaust fan on a timer/humidity control to purge moisture.
- Mark joist locations before hanging drywall to speed fastening.
- If headroom is tight but you need higher R, dense-pack cellulose or blown mineral wool behind netting is a good option (rents a blower; plan for 4–6 hours for a 2-car garage).
Common mistakes
- Blocking soffit vents with insulation.
- Compressing batts or leaving voids around wiring and boxes.
- Installing poly sheeting on the interior in mixed/hot-humid climates.
- Using non-ICAT recessed fixtures under insulation.
- Leaving foam or faced insulation exposed to the garage without the required gypsum barrier.
Costs and time
- DIY batts + baffles + drywall: roughly $2.00–$4.00 per sq ft in materials. A typical 2-car garage (400–500 sq ft) is a solid weekend for two people.
- Blown-in with netting: $2.50–$4.50 per sq ft materials; blower rental ~$50–$100/day.
When to call a pro
- History of ice dams, leaks, or persistent moisture staining.
- Complex framing, limited ventilation, or you’re considering an unvented “spray-foamed roof deck” assembly (closed-cell foam requires specific thickness for condensation control and an ignition barrier).
- Knob-and-tube wiring, asbestos, or unknown insulation.
- You’re unsure about code requirements for the garage/living space separation.
With careful air sealing, correct R-values, proper vapor control, and a code-compliant gypsum finish, an attached garage ceiling can be significantly warmer and drier, protecting the rooms above and reducing energy bills.