Context
User is constructing a two-story, 24’x36’ garage with heated radiant slab on the main floor (gas boiler), and plans to use a Mitsubishi mini split system to heat and cool the upstairs apartment and cool the garage. Garage will be spray-foam and fiberglass insulated, and finished with drywall.
Key Points
- R-47 wall insulation using 2x8s may not provide good ROI if garage doors are only R-9.
- Proper outlet wiring in drywall-finished garage does not require conduit, but must follow NEC (Romex at least 1.25” from stud face or protected by nail plates).
- In-floor heating design requires at least 6” of 5000 psi concrete with rebar and rigid insulation (R-10+); spray foam does not increase cracking risk if properly installed.
- Radiant slab must be poured and insulated before wall framing in most builds.
- Mitsubishi multi-zone mini split system is ideal for this setup:
- Ceiling cassette (or ducted) head in garage for cooling only.
- Wall-mounted or small ducted head upstairs (apartment).
- Outdoor unit: Mitsubishi MXZ series (e.g., MXZ-3C30NAHZ2).
- Hyper-Heat model recommended for low temperatures (-25°F).
- Additional electrical tips:
- GFCI protection required on all 120V outlets in garages.
- Multiple outlet heights (12–18” standard, 48” for workbench).
- Consider extra circuits during construction.
Actions
- Finalize insulation and electrical layout prior to slab pour. — stage:: 2
- Coordinate mini split lineset and drain placement before drywall. — stage:: 3
- Perform Manual J load calc for garage and upstairs zones. — stage:: 3