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. Aluminium framing and single glazing 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
-
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.
-
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
Autumn
The decisive month. Empty, dry, drain, ventilate and place absorbers before the cold arrives.
Winter
Check monthly if you can. Absorbers fill faster than expected and stop working when they do.
Spring
Open it up on the first dry day and air it thoroughly before putting anything back in.
Summer
In use and ventilated. Watch condensation on windows overnight when sleeping aboard — four people in a small volume is a lot of moisture.
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.
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