What size mini split for a garage?
Garages break the usual rules of thumb. Here is why, and what to size instead.
A garage is the space where square-foot rules of thumb fail hardest, and it is also the most common mini split project there is. Three things make it different from a room of the same floor area.
Why a garage needs more than its floor area suggests
- The ceiling is higher. Garages commonly run 9 to 12 ft, against 8 ft in living space. Load follows volume, so a 10 ft ceiling is 25% more air than an 8 ft one.
- The door is a hole in the envelope. An uninsulated steel garage door has effectively no meaningful insulation value, and the perimeter seal leaks. This is why garage sizing should assume poor insulation unless the door has genuinely been upgraded.
- The slab is uninsulated and the walls often are too. Many attached garages have one insulated wall, the one shared with the house, and nothing else.
Insulating the garage door before buying equipment is almost always the cheaper move. It shifts the calculation from poor to average insulation, which in a two car garage is often a whole unit size.
Worked example: a two car garage in climate zone 4
450 sq ft, 10 ft ceiling, poor insulation, one occupant.
Compare that with the same 450 sq ft as ordinary living space with 8 ft ceilings and average insulation, which comes out at 10,406 BTU. The garage needs substantially more for identical floor area.
Common garage sizes
| Garage | Sq ft | Cooling load | Heating load | Unit |
|---|---|---|---|---|
| 1 car garage | 250 | 7,813 | 12,500 | 9,000 |
| 2 car garage | 450 | 14,063 | 22,500 | 15,000 |
| 3 car garage | 700 | 21,875 | 35,000 | 24,000 |
Heating usually decides a garage unit in zone 4 and north, because a garage loses heat far faster than it gains it. Check both numbers before choosing.