HVAC is not a detail decision on an ADU — it’s a core design choice that affects construction cost, operating cost, comfort, and maintenance for the life of the structure. In Wisconsin’s Climate Zone 6, where heating demand is significant and cooling demand is real but shorter-season, the system choice deserves a deliberate analysis.
Mini-Split Heat Pump: The Default for Good Reasons
A ductless mini-split — one outdoor compressor connected to one or more indoor air handlers — has become the standard HVAC choice for new ADU construction in Wisconsin. Modern cold-climate mini-splits (Mitsubishi Hyper Heat, Daikin Fit, and others) operate efficiently down to -13°F ambient temperatures. At typical Wisconsin winter operating temperatures, a high-efficiency mini-split achieves a COP of 2.5–3.5 — meaning it delivers 2.5–3.5 units of heat for every unit of electrical energy consumed. Resistance heat delivers 1.0. The operating cost difference is significant over a full heating season. A single-zone mini-split for an ADU in the 600–800 sq ft range typically costs $3,500–$6,500 installed. No ductwork required. The same system provides air conditioning in summer.
Radiant Hydronic Heat: Comfort-Optimized, Cost-Intensive
Radiant floor heating — tubing embedded in the slab circulating hot water from a boiler or heat pump water heater — provides what many occupants describe as the most comfortable heating experience available. Heat rises from the floor evenly without drafts. For an ADU where comfort is the primary goal — a parent’s in-law unit — radiant has real appeal. The tradeoffs: installation cost significantly higher than mini-split ($8,000–$15,000+ added to project cost); radiant alone provides no cooling, requiring a separate cooling solution; and thermal mass makes it slow to respond to setpoint changes. Most cost-effective when embedded in a concrete slab during original construction.
Forced Air: Familiar but Wrong for New ADUs
Forced air — a furnace and air conditioner connected through ductwork — is the most common HVAC approach in existing Wisconsin homes. In a new ADU, it has significant disadvantages: ductwork consumes ceiling or wall space in an already-small structure; a gas furnace requires a gas line, a flue, and a combustion appliance in a small structure; and a high-efficiency gas furnace is less cost-effective per BTU delivered than a modern mini-split when electric rates are considered. The main scenario where forced air makes sense: sharing the existing HVAC system from the primary residence, which is sometimes applicable to attached ADUs but rarely to detached.
What We Specify
On nearly every ADU we build in Dane County, we specify a cold-climate mini-split — typically Mitsubishi Hyper Heat or equivalent. The efficiency, installed cost, and combined heating/cooling capability make it the right answer for the majority of ADU applications. For clients who specifically want radiant and can accommodate the cost, we build it and coordinate the supplemental cooling solution. Forced air is essentially never our recommendation for a new detached ADU. Start with a feasibility check to discuss the right HVAC for your specific situation.