Tiny House Ventilation: Why HRV and ERV Systems are Crucial to Prevent Mold

You sealed every seam, taped every joint and foamed every gap — and built yourself a terrarium. Two people breathing, cooking and showering inside 250 square feet release gallons of water vapour every day, and in a house with no leaks left, that moisture has nowhere to go. It condenses on the coldest surface it can find, and that is where mold quietly begins.

Tiny House Ventilation: At a Glance

Fresh air always has a price in a heated house: every cubic foot of warm air you push out must be replaced by cold air you pay to warm up. The whole craft of tiny house ventilation is paying as little of that price as possible — changing the air completely while keeping the heat, which is exactly what heat and energy recovery ventilators are built to do. A modern tiny house is deliberately built tight, with sealed envelopes, spray foam and taped seams of the kind compared in our guide to how to insulate a tiny house. Tight is efficient, but tight without ventilation is a moisture trap. The ten sections below work through the whole system in the order an owner meets it: the paradox first, then why the air must move mechanically, the HRV-versus-ERV decision, the airflow arithmetic, duct layouts that fit small floor plans, what installation really involves, controls, the small maintenance ritual that keeps it all working, and finally the mold — visible and hidden — that the system exists to prevent.

The system by the numbers: 2–3 gallons of moisture a day, a 70% humidity mold threshold, roughly 30 CFM of continuous airflow, 70–90% heat recovery and a $1,000–2,000 installed cost.

1. The Tight House Paradox

The paradox sits at the centre of modern small construction. Everything builders do to make a tiny house efficient — sealing the envelope, foaming the cavities, taping the seams — also makes it airtight, and an airtight box full of people is a moisture generator with no exit. Cooking steam, shower vapour and human breath all stay inside, raising the humidity hour by hour until the air can hold no more and the excess condenses on whatever is coldest: a window first, then the inside face of an exterior wall.

That is why ventilation is not an optional extra bolted onto a tight house; it is the second half of the tightness decision itself. Build tight and ventilate right are one instruction, not two. The recovery ventilator resolves the paradox because it separates the two things a cracked window mixes together: the air changes, but most of the heat stays. Everything in this guide follows from that single trade — fresh air is non-negotiable, so the only real question is how little heat you can pay for it.

2. Why Tiny Houses Need Mechanical Ventilation

Start with the physics. A household of two generates roughly 2–3 gallons of water vapour daily through breathing, cooking and showering. Spread through a 2,000 sq ft house, that moisture disperses and the building absorbs it without drama. Concentrated in 250 square feet, the same output pushes relative humidity past 70% — the mold threshold — within hours. Small volume is the whole problem: there is simply very little air to dilute what two lives produce.

The folk remedies each fail in a predictable way. Cracking a window works in mild weather, but in winter it throws away the heat the house was sealed to keep, in summer it lets humidity pour in, and in rain it is not an option at all. Bathroom and kitchen exhaust fans help, but a fan that only exhausts puts the house under negative pressure, pulling cold air in through every remaining crack — and it can backdraft combustion appliances, which is a safety problem, not just an efficiency one. The heat those fans discard is the same heat budgeted so carefully in our guide to how to heat a tiny house. The answer is balanced mechanical ventilation: stale air exhausted and fresh air supplied in equal measure, with the heat recovered in between. That is, precisely, what HRVs and ERVs do.

3. HRV vs ERV: The Difference

An HRV — a heat recovery ventilator — transfers heat between the outgoing and incoming air streams without mixing them, at 70–90% efficiency. The stale air leaves warm and arrives cold; the fresh air enters pre-warmed. An HRV moves heat only, so humidity leaves with the exhaust and stays out. That makes it the natural fit for cold, dry climates: keep the heat in, dump the humidity out.

An ERV — an energy recovery ventilator — transfers heat and moisture together. In summer it pre-dries the incoming humid air before it reaches the room; in winter it holds on to some of the indoor humidity instead of stripping it all away. That makes the ERV the better fit for hot-humid climates and for mixed climates where winter air gets painfully dry. The working rule is short: a cold, dry climate points to an HRV; a hot, humid or mixed climate points to an ERV. Most tiny houses in temperate zones do well with either — and where the climate sits between the clean cases, the ERV is the safer default, because it forgives both a humid July and a desiccating January. Choose by the moisture behaviour of your winters and summers, not by which box is on sale.

4. Sizing: How Much Airflow a Tiny House Needs

The sizing standard is ASHRAE 62.2: 7.5 CFM per person plus 3 CFM per 100 square feet. Run it for the typical tiny household — two people in 300 square feet — and the arithmetic is 15 + 9 = 24 CFM of continuous ventilation. Call it 30 CFM with margin, and you have the number the whole system is designed around. It is a smaller figure than most people expect, and that smallness is the point: a tiny house needs a gentle, constant exchange, not a powerful occasional blast.

The units built for this scale match the arithmetic. The original survey names the Panasonic Intelli-Balance 100/200, adjustable across roughly 30–100 CFM at around $700–1,000; the Lunos e2, a through-wall unit at about $500 a pair that needs no ducting at all; and the Broan AI series. The warning that matters most is against oversizing: a 150 CFM unit in a tiny house short-cycles and wastes energy, so match the unit to the 30–60 CFM range instead of buying capacity you will never use. The running cost is almost reassuring — 15–40 watts drawn continuously, some 360–960 watt-hours a day, much of it manageable on a timer. That is a trivial load in the power budget of an off-grid build, as the consumption figures in our guide to how to live off grid in a tiny house make clear, and it is easily covered by the arrays sized in our roundup of solar panel kits for tiny homes. Ventilation, in energy terms, is one of the cheapest systems in the house to run — which removes the last excuse for switching it off.

5. Ducting in a Tiny House: The Layout

The ideal layout follows the moisture. Supply fresh air to the living areas and bedrooms — the loft above all — and exhaust from the bathrooms and kitchen, where the vapour is made. Air then travels through the home in one deliberate direction: in at the clean, dry rooms, out at the wet ones, carrying the household’s moisture with it. The wet rooms themselves, and why they dominate the moisture budget, are the subject of our roundup of tiny house bathroom designs.

Tiny houses usually simplify that ideal into one of two practical layouts. The first is the single-point system: one compact unit with short duct runs, supplying the main room and exhausting from the bathroom. The second removes ducting entirely: through-wall units such as the Lunos e2 work in pairs, two units alternating push and pull through the wall — the simplest retrofit there is. Whichever layout you choose, the duct rules are the same. Keep runs short and straight, because every bend costs airflow. Insulate ducts that pass through unconditioned spaces, because a cold duct condenses the very moisture you are moving. And put the loft on the supply side: the sleeping loft is the stuffiest spot in the house, where warm moist air collects overnight, and it needs fresh air most.

6. Installation: What It Takes

In a new build, ventilation is a framing-stage decision. Plan the duct chases while the walls are open — a 4-inch duct needs a real path, and it must be designed in rather than discovered later. Mount the unit in a conditioned space, a closet or utility nook, never in the unheated underfloor where cold and access both work against it. Most units are plug-in 120V, so the electrical ask is modest — a dedicated circuit or simply a well-placed outlet, of the kind planned in our guide to tiny house electrical wiring.

Retrofitting a finished house is a different scale of job. Through-wall units are the gentle path: a Lunos pair needs only two 6-inch wall penetrations and is a weekend project. A ducted unit in a finished house means opened walls, and the bill shows it. The original’s cost ladder runs: $500–1,500 for the unit itself; $200–500 for ducting and installation in new construction; and $1,500–3,000 retrofitted. The conclusion writes itself — install during the build, because the retrofit penalty is steep, and it is paid entirely for access that was free when the walls were open.

7. Controls: Set It and Forget It (Mostly)

The baseline setting is continuous low speed at the ASHRAE rate, running 24/7. Ventilation is not an event; it is a condition, like a roof. On top of that baseline sits boost mode for the moments moisture spikes — cooking and showering — wired to a timer switch or a humidity sensor, with 20 minutes of high speed clearing the spike before it can condense anywhere. Humidity-sensing units take the decision away entirely, ramping automatically as conditions change: set, forget, and let the unit respond to the house’s actual state.

Winter adds one consideration. In extreme cold, the core of a recovery ventilator can ice as warm moist exhaust meets freezing intake air, and units handle this with defrost cycles — quality units manage it automatically, but verify before buying that the unit’s low-temperature rating matches your climate. The cold-weather behaviour of the whole house, ventilator included, is part of the picture drawn in our guide to tiny house living in cold climates. A unit rated for the climate runs all winter without attention; a unit that is not becomes one more system to nurse through January.

8. Maintenance: The Filter Ritual

A recovery ventilator asks for very little, but it does ask. Clean or replace the filters every 3 months — the single most neglected task in the system, and the one that matters most, because dirty filters strangle airflow until the unit might as well be switched off. Clean the core annually, rinsing the heat-exchange core per the manual as dust and pollen accumulate. Check the exterior hoods seasonally, since insects, snow and lint all block intakes and exhausts. And verify the airflow once a year with the simplest instrument in the house: hold a tissue at the supply grille and watch it flutter — firmly means healthy flow; a weak flutter means filters or a blockage somewhere upstream.

Add it up and the total maintenance burden is about 2 hours a year. Set that against the cost of skipping it — mold remediation, the subject of the next two sections — and the filter ritual is the cheapest insurance in the entire build. The owners who fail at ventilation rarely fail at choosing or installing the unit; they fail in month four, when the first filter change comes due and nobody does it. Put the quarterly date in the calendar when the unit goes in, and the ritual takes care of itself.

9. The Mold You Are Preventing

Everything above exists to prevent what follows. The first category is surface mold: the visible growth on walls, ceilings and window frames, unsightly and announcing itself early. The second is the dangerous kind — hidden mold inside wall cavities, where condensation forms on cold sheathing and colonies grow for months behind finished surfaces, typically discovered only during demolition or a deep renovation. The third is slower and structural: window condensation rot, sills and frames softening from constant wetness until the wood itself fails.

Notice what the three categories share: none of them is caused by a leak, a flood or a failure of the building. All three are caused by ordinary life — breathing, cooking, showering — in a box too tight to clear its own air. That is why balanced ventilation is not a comfort upgrade in a tight tiny house. It is structural protection, defending the sheathing, the framing and the window joinery against the house’s own occupants, and it works silently for as long as the filters are clean and the unit is running.

10. The Health Stakes in a Very Small Space

The last category of harm is not to the house but to the people in it. Mold exposure in 250 square feet is concentrated exposure: there is no spare bedroom to retreat to, no far end of the house where the air is cleaner, because the whole dwelling is one small volume of shared air. The effects the original lists are the familiar ones — respiratory issues, allergies, the low-grade sick-building syndrome of small spaces — but their weight is different at this scale, where the source and the sleeper are never more than a few feet apart.

That concentration cuts both ways, and the hopeful direction deserves the last word in this section. The same smallness that concentrates moisture also makes the cure unusually complete: one modest unit, moving some 30 CFM continuously, genuinely exchanges the air of the entire home. There are no under-ventilated wings, no forgotten back rooms. A tiny house with a running HRV or ERV is, in air terms, one of the healthiest small dwellings it is possible to live in — provided the small ritual of the previous section is kept.

Final Considerations

Build tight, ventilate right. The HRV or ERV — around 30 CFM continuous, supply to the living spaces, exhaust from the 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 it is working, catastrophic when it is absent. Of everything this guide to tiny house ventilation has covered, that is the sentence to build by.