Tiny House Cooling: How Many BTUs for a House on Wheels?
Key Takeaway
A 200 sq ft tiny house typically needs 9,000–12,000 BTU — more per square foot than a conventional home due to four-sided wall exposure and loft heat stratification.
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Expert Analysis
Surface-to-Volume Ratio & Loft Heat Stratification in THOWs
Tiny houses on wheels face a thermal challenge that conventional HVAC design does not adequately address: a surface-to-volume ratio approximately 3–4× higher than a conventionally sized home. Every square foot of wall and roof in a THOW is a heat exchange surface; with all four sides exposed and no adjacent conditioned space sharing any wall, the total heat gain per square foot of floor area is dramatically higher than even a well-insulated apartment.
Loft heat stratification is the second critical challenge. Most tiny houses incorporate a sleeping loft above the main living area; hot air rises and accumulates in the loft, where roof proximity further intensifies solar heat gain. The mini-split head located in the main living area below the loft maintains temperature at thermostat level while the sleeping space above may be 10–15°F warmer — a condition the thermostat cannot detect or correct.
The building envelope's integrity is also at risk from road vibration. Travel loosens window seals, trim joints, and door gaskets over time, increasing infiltration rates significantly. Inspect and re-seal all penetrations annually — every air leak bypasses your insulation R-value and directly increases the BTU load on the conditioning system.
Buying Guide
Mini-Split Selection for Tiny Houses: Sizing for Four-Sided Exposure
Must-Have Features
Mini-Split Heat Pump (Ductless)
A ductless mini-split is the correct system for a THOW: no ductwork to route through limited space, operates at high efficiency in both heating and cooling modes, and a single 3-inch penetration through the wall handles refrigerant lines and communication cable. Choose a cold-climate model if you travel or live in regions below 20°F.
Compact Low-Profile Indoor Head
Tiny house ceiling heights are typically 7–9 feet on the main level. A compact indoor head (8–10 inches deep, 24–30 inches wide) fits without dominating the space. Avoid ceiling cassettes in THOWs — the joist structure and minimal attic clearance make installation impractical.
Inverter Variable-Speed Technology
A THOW's load varies enormously with outdoor temperature, solar gain, and occupancy. An inverter compressor modulates down to 20–30% of rated capacity on mild days — preventing overcooling, maintaining humidity control, and using 40–50% less energy than a single-speed unit running short cycles.
Pro Tip
Install a small 4-inch USB ceiling fan in the peak of your loft, wired to run continuously on low speed. This single change can reduce loft-to-main-floor temperature differential from 12–15°F down to 4–6°F by continuously de-stratifying the air column. The fan draws under 5 watts and runs silently, effectively extending your mini-split's reach into the sleeping loft without installing a second head.
Common Mistake
Don't Install a Through-Wall AC in a Tiny House
Through-wall and window AC units require a large penetration in the building envelope — typically 14×24 inches or larger. In a THOW, this penetration runs through the structural wall, potentially cutting load-bearing members and creating a permanent air leakage point that road vibration will widen over time. The unit itself is vulnerable to vibration damage during transport. A properly installed mini-split requires only a 3-inch penetration that can be sealed around the line set, maintaining the building envelope's integrity during transit.
Expert Advice
“Tiny houses present a unique thermal profile: maximum exterior surface area relative to interior volume. Where a conventional home might have 10% of its total wall area exposed on any given wall, a tiny house on wheels has all four sides fully exposed to ambient. A well-insulated THOW (R-23+ walls, R-38+ roof) with a mini-split can maintain comfort to 100°F outdoors; an under-insulated build with a window unit will struggle above 90°F.”
Industry Terminology
Terms You Should Know
- Surface-to-volume ratio
- Total exterior surface area divided by interior volume; THOWs have 3–4× the ratio of conventional homes, driving higher BTU needs per sq ft.
- Loft heat stratification
- Hot air accumulation in the sleeping loft, typically 10–15°F warmer than the main-level thermostat reading.
- Building envelope integrity
- Continuous air and thermal barrier of the structure; compromised in THOWs by road vibration loosening seals and gaskets over time.
- Infiltration load
- Cooling BTU required to condition outdoor air entering through gaps; increases as road vibration degrades seals.
- THOW
- Tiny House on Wheels; a full residential structure built on a trailer chassis, combining fixed-structure thermal loads with vehicle envelope challenges.
- Cold-climate mini-split
- Heat pump rated for full heating capacity at sub-freezing ambient, essential for four-season THOW living.
Quick Reference
BTU Chart by Room Size
| Room Size | BTU Required | Tonnage |
|---|---|---|
| 100 – 150 sq ft | 5,000 BTU | 0.4 ton |
| 150 – 250 sq ft | 6,000 BTU | 0.5 ton |
| 250 – 400 sq ftBest Seller | 8,000 BTU | 0.7 ton |
| 400 – 550 sq ft | 10,000 BTU | 0.8 ton |
| 550 – 700 sq ftMost Popular | 12,000 BTU | 1.0 ton |
| 700 – 1,000 sq ft | 14,000 BTU | 1.2 ton |
| 1,000 – 1,400 sq ft | 18,000 BTU | 1.5 ton |
| 1,400 – 2,000 sq ft | 24,000 BTU | 2.0 ton |
| 2,000 – 2,500 sq ft | 30,000 BTU | 2.5 ton |
Based on ASHRAE Standard 183 guidelines. Assumes 8 ft ceilings, average insulation, and moderate sun exposure. Add 10% for kitchens; subtract 10% for heavily shaded rooms.
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