Garages are tough envelopes
Standard uninsulated garage doors leak air and radiate heat. Block walls may have no insulation. Calculators assume poor insulation unless you have upgraded the shell.
A 400 sq ft two-car garage workshop may calculate 12,000–18,000 BTU with poor insulation — 18,000 BTU is common before you insulate. After insulating doors and walls, recalculate — you may drop to 12,000 BTU.
Workshop heat gains beyond sq ft
Size for peak use when machines run and doors stay closed — not empty garage volume alone.
- Welding, compressors, and dust collection motors add heat
- South-facing garage doors act like radiators in afternoon sun
- Epoxy curing and paint booths need ventilation — exhaust air replaces cooled air
- Occasional use vs daily use: modulating inverter helps part-time shops
Install and electrical notes
Mount indoor head away from dust-heavy zones — filters clog faster in workshops. Mount outdoor condenser where lawn equipment will not hit it.
Many 12k–18k BTU units need 220V. Run dedicated circuit before closing drywall if finishing the garage. Permit and code rules apply — this is planning info, not electrical advice.
Garage upgrade path
Many owners oversize because they skip steps 1–2. See DIY install basics for electrical planning before cutting drywall.
- Step 1: Insulate garage door panels and add bottom seal
- Step 2: Insulate walls (batts or foam) if finishing interior
- Step 3: Rerun BTU calculator with improved insulation
- Step 4: Install mini-split — often drop one head size after steps 1–2
- Step 5: Add shop air filtration separately from HVAC cooling