45–55% — While stored; every surface is a cold surface in these
An RV or a boat concentrates every condition this site warns about into a few hundred square feet: thin uninsulated walls, single-pane windows, no heating for months, soft furnishings everywhere, and a sealed door.
They also fail expensively and quietly. Nobody opens them in February, so the first anyone knows is a smell in spring and mildew on every cushion.
Why they fail so reliably
The envelope is the problem. Walls that are an inch thick with minimal insulation mean the interior surface is close to outdoor temperature, so every wall is effectively a cold wall. Aluminum framing and single-pane glass make it worse — these are structures with thermal bridges everywhere rather than in a few corners.
Then there is what is inside. Upholstered cushions, mattresses, carpet, curtains and stored fabric all hold moisture and sit in still air. In a house those items are in a heated, ventilated room; here they are in an unheated sealed box.
And the moisture has sources even when nobody is there. Damp bilges on a boat, a full fresh-water tank, residual moisture in soft furnishings, and ground moisture rising into an RV parked on grass or dirt all keep the interior humid without a single person breathing in it.
Where the moisture comes from
- Ground moisture rising into a vehicle parked on grass or bare soil
- Bilge water and damp lockers on a boat
- Cushions, mattresses and carpet put away with moisture already in them
- Water tanks and plumbing left full over the storage season
- Rain leaks at roof seams, vents and window seals — small on a house, decisive here
The plan for this room
Ranked by what it costs you. Work down the list — most rooms are fixed before the paid options.
Empty it before storage: cushions, bedding, fabric, food and anything paper comes out and goes somewhere heated.
Open every internal door, locker and drawer so air can move through the whole interior.
Stand cushions on edge rather than laying them flat, so both faces can breathe.
Drain the water system and the bilge, and leave taps open.
Park on gravel or a hard surface rather than grass, or put a barrier under the vehicle.
Put calcium chloride tubs in — this is exactly the small sealed volume they are designed for. Two or three, low down, checked monthly.
Leave roof vents cracked open under a rain cover so there is some background air exchange.
If shore power is available, a small desiccant dehumidifier on a humidistat is the reliable answer — compressor units will not work at storage temperatures.
Fix roof seams, vent seals and window gaskets before the storage season. A slow leak over five months does more damage than a fast one over five days.
What will not work here
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A fitted cover with no ventilation
A sealed cover traps moisture against the body and holds it there all winter. Covers need to breathe or vent.
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A compressor dehumidifier in an unheated vehicle
It ices up below about 65 °F and collects almost nothing. Desiccant is the only type that works here.
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Sealing it up tight
With no air exchange and no heating, every gram of moisture inside stays inside for the season.
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Leaving soft furnishings in "just this once"
They are both the most vulnerable items and the largest moisture reservoir. Removing them is the highest-value free step.
What is your reading telling you?
- Dew point
- 53 °F
- Mold risk, cold walls
- Moderate
Still inside the accepted range, but this is where cold surfaces begin to sweat.
Through the year
| Season | What the room does | What to do about it |
|---|---|---|
| Winter | Every surface is effectively a cold wall; a single occupant can saturate the air overnight | Ventilate deliberately every day; absorbers genuinely work at this volume |
| Spring | Condensation from winter storage becomes visible as mildew on upholstery | Open everything up, dry it out, and clean before the season starts |
| Summer | Cooking and showering in a tiny volume spike the humidity instantly | Roof vents open, extract at the source, and cook outdoors where possible |
| Fall | Layup begins, and a sealed damp cabin over winter is how interiors are ruined | Dry it thoroughly, leave absorbers, and ventilate through the winter rather than sealing it |
Where to measure in this room
The measurements are the same as a house, but the volume is so small that everything moves faster and a single hygrometer covers most of it.
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Anywhere central, at head height
A cabin is small enough that one reading is genuinely representative of the air — unlike a house, where it never is.
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The underside of the mattress and any locker
The classic failure point: a mattress on a cold, unventilated base traps moisture underneath. Lift it and feel; slatted or vented bases exist for exactly this reason.
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Behind and under upholstery against the hull or wall
Thin walls mean the surface behind a cushion is close to outdoor temperature. Check for damp and for mildew on the backs of cushions rather than the fronts.
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Over a winter layup, with a logging unit if possible
A cheap hygrometer left inside over the layup tells you whether your winter arrangement is working, and it costs nothing to check in spring.
Common questions
How do I stop my RV getting damp in storage?
Remove all soft furnishings, drain the water system, leave lockers and internal doors open, park on a hard surface, and put calcium chloride absorbers inside. With shore power, a small desiccant dehumidifier on a humidistat is better still.
Do moisture absorbers work in a caravan or boat?
Yes — this is the situation they are designed for. A small sealed volume is exactly where an ounce a day is meaningful, unlike in a room.
Why does my boat smell musty after winter?
Cushions and fabric hold moisture, every surface is effectively a cold surface, and there is no air exchange for months. The smell is mildew growing slowly on soft furnishings.
Should I cover my RV for winter?
Only with a breathable or vented cover. A sealed cover traps moisture against the bodywork and holds it there through the whole storage season.
What is the best way to stop condensation in an RV or boat over winter?
Ventilation plus absorbers, not sealing. Leave roof vents cracked or fit purpose-made vent covers so air exchanges continuously, lift mattresses and cushions off their bases so air can circulate underneath, and put calcium chloride tubs inside — a small enclosed volume is where those genuinely work. Sealing a cabin completely for the winter is how interiors get ruined.
Does a small dehumidifier work in a camper?
Yes, if there is power available, and a small unit goes a long way in that volume. Where there is no shore power, absorbers and ventilation are the practical answer. Thermoelectric mini dehumidifiers, which are close to useless in a room, are actually reasonable in a space this size — the volume is small enough that their tiny capacity is proportionate.
Sources
- US EPA — Mold and moisture guidance — the 30–50% range and the 10 sq ft remediation threshold
- ASHRAE Standard 55 — Thermal environmental conditions for human occupancy — indoor comfort and humidity limits
- US DOE 10 CFR 430 — Dehumidifier test procedure — the 2019 move to 65 °F rating conditions