Damp in a tiny house: ventilating without losing heat

Damp is the first problem reported by people who live in small houses, ahead of cold and ahead of noise. It is not an unavoidable construction defect: it is a matter of design and use. Here is how it appears, how to avoid it, and what an MVHR system actually does.

Why does a tiny house get damp so quickly?

Because the sources of water vapour are the same as in a large house, but spread through a much smaller volume. Breathing, showering, cooking, drying laundry, plants: all of these produce water in the form of vapour, continuously.

In a conventional home, this vapour is diluted in several hundred cubic metres. In a tiny house, the concentration rises much faster. And since the house is well insulated and airtight — which is desirable — the vapour does not escape on its own.

It then moves towards the coldest surfaces, where it condenses: glazing, wall corners, floor-to-wall junctions. ADEME (the French agency for ecological transition) puts it simply: condensation on the windows is a sign of insufficient ventilation.

What are the warning signs?

• in the morning, persistent condensation on the windows that does not clear during the day;
• in corners, behind furniture or at the bottom of walls, black stains;
• a musty smell when you come back after being away;
• your laundry drying poorly and fabrics that feel cold to the touch;
• a humidity level consistently above 60% — a hygrometer costs a few euros and saves guesswork.

These signs usually appear in the very first winter. Dealt with early, they leave no trace; ignored, they damage interior finishes and, in the long run, the frame.

Which ventilation system for a tiny house?

Three types of solution, in increasing order of effectiveness and cost:

1. Manual airing. Open the windows wide for 5 to 10 minutes a day, even in winter, as ADEME recommends. Essential, but not enough on its own in a small space occupied all day.
2. Mechanical extract ventilation (VMC simple flux). It extracts stale air from the wet rooms; fresh air comes in through trickle vents in the windows. Simple and inexpensive, but it expels the heat of the extracted air outside, which you notice in a house with low thermal mass.
3. MVHR (VMC double flux). Two circuits run side by side: one extracts stale air, the other brings in fresh air. A heat exchanger transfers heat from the outgoing air to the incoming air. The air is renewed without throwing away the heat produced inside.

In a well-insulated tiny house, MVHR is the consistent solution: it is precisely because the house is airtight that the air coming in and going out has to be controlled. The Skywood and the Sharewood are fitted with it, along with an air purifier.

Is ventilation compulsory?

For rented accommodation, yes. The decree of 30 January 2002 on the characteristics of decent housing requires openings and ventilation devices that allow air renewal suited to the needs of normal occupation and to the operation of the appliances.

A tiny house on wheels occupied by its owner is not subject to this rule, but the criterion remains a good quality benchmark. If you are thinking of renting it out, the question becomes a contractual one: see our guide on renting out a tiny house.

Ventilation and insulation: why they go together

A poorly insulated house breathes through its defects: it is healthy by accident, and expensive to heat. A well-insulated, airtight house is economical, but it needs organised air renewal. You cannot improve one without dealing with the other.

This is also why a continuous vapour control layer on the inside matters: it stops vapour from moving into the insulation, where it would reduce performance and eventually damage the materials. The subject is developed in our guide on insulating a tiny house.

How do you maintain an MVHR system?

• The filters: clean or replace them as the manufacturer advises, usually several times a year. This is the only truly regular task, and the one most often forgotten.
• The extract and supply vents: dust them, without blocking them or adjusting them yourself.
• The heat exchanger: check it periodically, according to the unit’s instructions.
• The external grilles: make sure no leaves or nests are blocking the air intakes.

A ventilation unit with clogged filters uses more energy and ventilates less: maintenance determines performance. The other maintenance points for the house are brought together in our guide on lifespan and maintenance.

The mistakes that create damp

1. Blocking the air inlets because you feel a draught of cold air: this is the number one cause of mould.
2. Drying laundry indoors without extra ventilation: a load of washing releases several litres of water into the air of the home.
3. Switching off the ventilation at night because of the noise: ventilation only works if it runs continuously.
4. Using a combustion space heater with no flue: combustion produces water vapour as well as pollutants. See our guide on heating.
5. Setting the heating too low while you are away: cold walls attract condensation as soon as you return.

Key points

• A small volume concentrates water vapour: damp is a design issue, not bad luck.
• Persistent condensation on the windows signals insufficient ventilation.
• MVHR renews the air without losing heat: it is the system that goes with good insulation.
• Ventilation only works if it runs continuously, and only if its filters are clean.
• Never block an air inlet, even if it lets in a draught of cold air.