You sealed every seam, taped every joint, and foamed every gap. Congratulations: you just built a terrarium. Two people breathing, cooking, and showering inside 250 square feet produce gallons of water vapor a day, and without ventilation it all condenses on the coldest surface it can find. That is how mold starts, quietly, behind the walls.
Fresh air has a price in heat, and this article shows how to pay as little as possible for it. You will learn how HRV and ERV systems differ, which one suits humid versus dry climates, how much air a small space actually needs, and how to ventilate without throwing your heating budget out the window. Tight houses need lungs.
The Tight House Paradox
Modern tiny houses are built tight — sealed envelopes, spray foam, taped seams. Tight is efficient. But tight without ventilation is a terrarium: cooking steam, shower vapor, and human breath have nowhere to go, and the result is condensation, then mold, then rot. HRV and ERV systems solve the paradox: fresh air without throwing away your heat. This guide explains why they are not optional.
1. Why Tiny Houses Need Mechanical Ventilation
The physics: a family of two generates 2–3 gallons of water vapor daily (breathing, cooking, showering). In 2,000 sq ft, that disperses; in 250 sq ft, relative humidity spikes past 70% — the mold threshold — within hours. Cracking a window works in mild weather but fails in winter (you lose all your heat), in summer (humidity pours in), and during rain. Bathroom/kitchen exhaust fans help but create negative pressure (pulling cold air through every crack — and backdrafting combustion appliances). The answer: balanced mechanical ventilation — exhaust stale air and supply fresh air in equal measure, recovering the heat in between. That is exactly what HRVs and ERVs do.
2. HRV vs ERV: The Difference
HRV (Heat Recovery Ventilator): transfers heat between outgoing and incoming air streams (70–90% efficiency) without mixing them. Best in cold, dry climates — keeps heat in, dumps humidity out. ERV (Energy Recovery Ventilator): transfers heat AND moisture — in summer it pre-dries incoming humid air; in winter it retains some indoor humidity. Best in hot-humid climates or mixed climates where winter air gets painfully dry. The rule: cold/dry climate → HRV; hot/humid or mixed → ERV. Most tiny houses in temperate zones do well with either; the ERV is the safer default.
3. Sizing: How Much Airflow
ASHRAE 62.2 (the ventilation standard): 7.5 CFM per person + 3 CFM per 100 sq ft. For two people in 300 sq ft: 15 + 9 = 24 CFM continuous — call it 30 CFM with margin. Tiny house units: the Panasonic Intelli-Balance 100/200 (30–100 CFM, ~$700–1,000), the Lunos e2 (through-wall, no ducting, ~$500/pair), or the Broan AI series. Do not oversize: a 150 CFM unit in a tiny house short-cycles and wastes energy — match the unit to the 30–60 CFM range. Power draw: 15–40W continuous — trivial for solar (360–960Wh/day, and much of it can run on a timer).
4. Ducting in a Tiny House: The Layout
The ideal: supply fresh air to living areas and bedrooms (the loft!), exhaust from bathrooms and kitchen (the moisture sources). In practice, tiny houses use simplified layouts: a single-point ERV (one unit, short duct runs — supply to the main room, exhaust from the bathroom), or through-wall units (Lunos e2 pairs — no ducting at all; two units alternate push/pull through the wall — the simplest retrofit). Duct rules: keep runs short and straight (every bend costs airflow), insulate ducts in unconditioned spaces (cold ducts condense), and put the loft on the supply side (the sleeping loft is the stuffiest spot — it needs fresh air most).
5. Installation: What It Takes
New build: plan duct chases during framing (a 4″ duct needs a real path — design it in, don’t discover it later), mount the unit in a conditioned space (closet, utility nook — not the unheated underfloor), and wire a dedicated circuit or plug (most units are plug-in 120V). Retrofit: through-wall units (Lunos) need only two 6″ wall penetrations — a weekend project; ducted units in a finished house are much harder (opened walls). Cost: $500–1,500 for the unit, $200–500 for ducting and install in new construction, $1,500–3,000 retrofitted. Install during the build — the retrofit penalty is steep.
6. Controls: Set It and Forget It (Mostly)
Continuous low speed is the baseline (the ASHRAE rate, 24/7 — ventilation is not an event, it is a condition). Boost mode for cooking and showering (wired to a timer switch or humidity sensor — 20 minutes of high speed clears the moisture spike). Smart controls: humidity-sensing units ramp automatically (set and forget — the unit responds to actual conditions). Winter consideration: in extreme cold, some units need defrost cycles (the core ices — quality units handle it automatically; verify the low-temperature rating matches your climate).
7. Maintenance: The Filter Ritual
The system is only as good as its maintenance: clean/replace filters every 3 months (the #1 neglected task — dirty filters strangle airflow and the unit might as well be off), clean the core annually (rinse the heat-exchange core per the manual — dust and pollen accumulate), check exterior hoods (insects, snow, and lint block the intake/exhaust — seasonal check), and verify airflow yearly (hold tissue at the supply grille — it should flutter firmly; weak flow means filters or blockage). Total maintenance: 2 hours a year. Total cost of skipping it: mold remediation.
8. The Mold You Are Preventing
What balanced ventilation prevents: surface mold (on walls, ceilings, and window frames — the visible kind), hidden mold (inside wall cavities where condensation forms on cold sheathing — the dangerous kind, discovered during demolition), window condensation rot (sills and frames rotting from constant wetness), and health effects (mold exposure in 250 sq ft is concentrated — respiratory issues, allergies, the sick-building syndrome of small spaces). The HRV/ERV is not a comfort upgrade; in a tight tiny house, it is structural and health protection.
Final Considerations
Build tight, ventilate right. The HRV or ERV — 30 CFM continuous, supply to living spaces, exhaust from wet rooms, filters cleaned quarterly — is the lung of the tiny house. It costs $1,000–2,000 installed and protects a $60,000+ house from its own moisture. In the hierarchy of tiny house systems, ventilation sits beside the roof: invisible when working, catastrophic when absent.