How to Keep Moisture Out of an RV (Winter, Storage and Mold)

Two adults put 4 to 6 pints of water into an RV’s air every 24 hours just by breathing, before anyone cooks or showers. Keeping moisture out of an RV means removing that water faster than you produce it: vent it outside every day, hold indoor humidity between 35 and 45 percent, and warm the cold surfaces where it turns back into liquid.

Where the water inside an RV actually comes from

Almost none of it comes in through the roof. In a rig with no leak, the four sources are your lungs and skin, unvented propane combustion, cooking, and showering, and they add up faster than the volume of a travel trailer can absorb. A 30-foot trailer holds roughly 1,000 cubic feet of air, and at 68 °F that air can carry only about 1.1 pints of water vapor total before it saturates.

Here is what a normal winter day puts into that 1,000 cubic feet:

Source	Water added	Notes
Two adults, 8 hours asleep	2–3 pints	Respiration plus perspiration; rises with activity
Two adults, 16 hours awake	2–3 pints	Higher if anyone showers or exercises
Unvented propane heater, 9,000 BTU, 8 hours	About 5 pints	Burning 1 lb of propane makes 1.6 lb of water
Propane cooktop, two burners, 45 minutes	0.5–0.8 pint	Plus whatever boils out of the pot
One 5-minute RV shower	0.3–0.5 pint	The rest goes down the drain
Wet coats and towels drying inside	2–4 pints	A wet dog counts too

The bottom row is the one people create for themselves. Two loads of laundry hung over the shower rail put 2 to 4 pints into the cabin over a day, which is why a vented or condensing RV washer dryer combo that finishes the drying inside its own drum changes the moisture arithmetic more than any habit change does. Add the middle rows and a couple running a Buddy-style heater overnight is making 10 to 12 pints of water a day inside a box that can hold one. The excess has to go somewhere, and it goes onto the coldest thing in the RV.

This is also the single strongest argument for a vented heat source. An RV furnace draws combustion air from outside and pushes its exhaust, including every ounce of combustion water, out the sidewall vent. A portable catalytic or blue-flame propane heater dumps all of it into the cabin with you. If your winter heat is currently a portable propane unit, the RV furnace guide covers getting the built-in furnace running again, and that swap alone removes about half of a cold-weather moisture problem.

Why moisture lands on windows, corners and under the mattress

Condensation forms wherever a surface is colder than the dew point of the air touching it. At 68 °F and 50 percent relative humidity the dew point is 48 °F, so every surface in the RV below 48 °F is collecting water whether you can see it or not.

Indoor air at 68 °F	Dew point	Condenses on
30% RH	35 °F	Almost nothing but single-pane glass on a hard freeze
40% RH	43 °F	Window glass and aluminum frames
50% RH	48 °F	Glass, frames, outside corners
60% RH	54 °F	All of the above plus wall cavities and under cushions
70% RH	58 °F	Everything on an outside wall, including the mattress base

RV construction guarantees cold surfaces. A laminated sidewall is roughly R-5 to R-7, the aluminum framing inside it conducts straight through as a thermal bridge, and single-pane glass with 25 °F outside sits around 30 to 35 °F. The predictable wet spots, in the order they appear:

• Window glass and aluminum frames. The frames run colder than the glass, which is why water pools in the track and rots the sill panel below it.
• Outside corners and the roof-to-wall joint. A corner has more exterior surface than interior surface, so it loses heat faster than the flat wall and can sit 6 to 8 °F colder.
• The plywood under the mattress. Body heat drives vapor down into a platform with zero airflow, sitting against a wall at 35 °F.
• Inside wardrobes and cabinets on an outside wall. Closed doors cut them off from the heated air, so the back panel stays at wall temperature.
• Behind the dinette cushions in a slide-out. Slide walls are thinner than main walls and their floors have less insulation.
• The metal ceiling trusses. Cold stripes across the ceiling, spaced every 16 inches, are the framing telegraphing through the panel.

What is a good humidity level for an RV?

Hold 35 to 45 percent relative humidity in winter and under 55 percent in summer. Mold needs a surface humidity near 80 percent to germinate, and keeping room air below 50 percent keeps even the cold spots away from that threshold. Below 30 percent, cabinet doors shrink and trim joints open up, so drying harder is not automatically better.

Buy two cheap digital hygrometers rather than one, at about $10 each, and put one at head height in the living area and the second in the bedroom or the closet on the coldest outside wall. The gap between them tells you more than either number alone: 42 percent up front and 61 percent in the bedroom means the bedroom air is not moving, and that is a fixable ventilation problem rather than a whole-rig humidity problem. Stirring that one room is worth more than another dehumidifier, and a 12 volt fan mounted over the bed keeps vapor off the cold outside wall for a few watts a night.

The daily routine that keeps an RV dry in winter

1. 1

   Wipe the windows before you open anything

   First thing in the morning, run a microfiber cloth or a small squeegee down every pane and along the frame track. Expected: cloth wrung out damp, dry tracks, no water reaching the sill trim. Two minutes.

2. 2

   Crack two roof vents at opposite ends of the rig

   One open an inch at the front, one at the back, so warm wet air leaves at the ceiling and dry air is drawn in low. Expected: window glass clears within 20-30 minutes on a cold morning.

3. 3

   Run the vent fan on speed 1 before you light a burner, not after

   Start the fan first, cook with lids on pots, leave it running 10 minutes past the last burner. Expected: no fog on the glass while dinner cooks.

4. 4

   Shower with the bathroom fan on high and squeegee the walls

   Squeegee the shower surround and the door into the pan, then leave the bathroom door open afterwards. Expected: the surround is dry to the touch in 15 minutes instead of 2 hours.

5. 5

   Heat with the furnace overnight, not with an unvented propane heater

   Set the furnace to 60-62 °F for the night and use the portable heater only in the daytime with a vent open. Expected: you stop waking to running windows.

6. 6

   Read both hygrometers before bed and act on the higher one

   Above 55 percent, open the bedroom vent an inch or run the fan for 10 minutes. Expected: both readings back into the 35-45 percent band within half an hour.

7. 7

   Once a week, open everything that stays closed

   Lift the mattress on edge, pull the dinette cushions off, open the wardrobes and the outside-wall cabinets, and let them air for two hours. Expected: no dark patches, no damp smell, no beading on the back panels.

The step people skip is the first one. Wiping condensation off glass is not cosmetic: that water runs into the track, sits against the sill, and soaks the lauan behind it, which is why so many older trailers have soft, dark wood under the windows on the north side.

Ventilation, dehumidifier or desiccant: which one for which season?

Ventilation is the answer whenever outside air is colder than inside air, a dehumidifier is the answer when outside air is warm and damp, and desiccant is only for a sealed, unoccupied rig. The reason is that cold air is dry in absolute terms: outside air at 20 °F and 70 percent humidity, warmed to 68 °F indoors, arrives at about 10 percent relative humidity and can absorb 5 or 6 pints on its way back out.

Situation	Outside conditions	Use this	Why the others fail
Winter camping, occupied	Below 40 °F, any humidity	Vents plus a vented furnace	A compressor dehumidifier frosts its coil below about 65 °F; desiccant absorbs 1 pint over weeks against your 6 pints a day
Coastal or Pacific Northwest rain	45–60 °F, 90% RH	Compressor dehumidifier, 20–30 pint class	Ventilating brings in air already near its dew point and adds water
Humid summer heat	Above 80 °F, 60%+ RH	The air conditioner, on low fan speed	Low fan keeps the coil colder and wrings out more water per hour; see keeping an RV cool in summer
Sealed storage, no power	Anything	Calcium chloride tubs plus cracked vent covers	No power for a dehumidifier, nobody there to open a window
Storage with shore power	Anything	Small dehumidifier on a humidistat, drain hose to the shower	Tubs fill and overflow unattended

Two numbers make the choice obvious. A thermoelectric mini dehumidifier, the kind sold as an RV unit for $50, pulls 8 to 16 ounces a day, which is less water than one person exhales in a night. A compressor unit in the 20-pint class pulls 20 to 30 pints a day but draws 200 to 350 watts continuously, so it is a shore-power tool, not a boondocking tool. Sizing one against your rig’s volume and your power budget is the whole job, and the dehumidifiers sized for RVs comparison covers which capacities and noise levels are realistic in a space this small.

Desiccant deserves an honest limit. A 10.5-ounce calcium chloride tub holds roughly one to two pints of water before the crystals dissolve into brine, and it takes 30 to 60 days to get there. That is genuinely useful in a closed storage rig producing nothing. It is meaningless against two people breathing.

Airflow beats capacity in a space this small. A 14-by-14 roof vent fan on its lowest exhaust speed moves enough air to change the volume of a 30-foot trailer in 3 to 4 minutes, which no dehumidifier at any price can match for removing a shower’s worth of steam. If the rig only has a passive vent lid and no powered fan, that upgrade does more for winter condensation than anything else you can buy. If the fan is already fitted and the glass still streams, a roof vent that has stopped pulling air is usually the 3 to 5 amp fuse on the fan’s own control board rather than a dead motor.

What to put under an RV mattress to prevent mold

Put a ventilating mat between the mattress and the platform: 3/4-inch open-weave matting such as Hypervent Aire-Flow or Den-Dry, cut to the mattress footprint. It creates a continuous air gap so vapor driven down by body heat can move sideways and evaporate instead of condensing on cold plywood.

The problem is specific to RVs. A house bed sits on a slatted frame in a room with warm floors. An RV mattress sits on a sealed plywood or lauan deck, often directly over an outside wall or an unheated storage bay, and two people put a pint of water a night into it. The plywood stays at 35 to 45 °F, the dew point of the air trapped in the foam is around 55 °F, and the underside of the mattress runs wet all winter.

Three fixes, in order of how well they work:

1. Ventilating matting, roughly $18 to $25 per linear foot. A 39-inch-wide roll cut to a queen platform runs about $130 to $170. It is the only option that keeps working when you never touch it.
2. Rigid foam board, about $25 a sheet. A half-inch sheet of XPS at R-2.5 cut to fit raises the plywood-side surface temperature above the dew point. It stops condensation but traps no airflow, so it is a cold-climate fix rather than a humid-climate one.
3. Wooden slats or furring strips with 2-inch gaps, under $20. The cheapest air gap there is, though a thin RV mattress will telegraph the slats and you will feel every one.

Whatever goes under it, stand the mattress on edge once a week in winter and leave it for two hours. Pull it right out of the rig for storage season.

How to keep moisture out of an RV in storage

Dry the rig completely before you close it, then leave it slightly open. A camper parked wet in October and sealed until April is a sealed incubator: the ground gives off moisture, the sun heats the roof and drives vapor around inside, and nobody is there to open a window.

1. 1

   Dry it for 48 hours before you close it up

   Park it home from the last trip, open every vent, window and cabinet, and run a fan through it for two days. Expected: shower surround, sink cabinets and the fridge interior all dry to the touch.

2. 2

   Strip out everything that holds water or feeds mold

   Bedding, towels, cushions if they will fit in the house, all food including sealed dry goods, and every liquid. Expected: nothing organic and nothing absorbent left aboard.

3. 3

   Empty and dry the plumbing

   Dump and rinse both waste tanks, drain the fresh tank and water heater, and finish winterizing if you are in a freeze zone. Expected: no standing water anywhere in the system, P-traps holding antifreeze rather than water.

4. 4

   Prop the fridge doors and pull the drip tray

   Use the factory door bar or a wedge, remove the drip tray, and wipe the gaskets with a baking soda solution. Expected: both doors held open an inch, fin area dry.

5. 5

   Leave every interior door and drawer open

   Wardrobes, bathroom door, under-sink cabinets, overhead lockers, drawers pulled out an inch. Expected: air can reach every interior surface, no closed cavity anywhere.

6. 6

   Crack the roof vents under their covers

   Vent covers let a lid stay open an inch in the rain, so warm air keeps escaping at the ceiling all winter. Expected: air moving out the roof, no rain entering.

7. 7

   Set out calcium chloride tubs, one per compartment

   One in the bathroom, one in the bedroom, one in the main living area, sitting in something that will catch an overflow. Expected: crystals dissolving steadily, collected brine below the fill line at each check.

8. 8

   Check it monthly, whatever the weather

   Twenty minutes with a flashlight: window sills, corners, under the mattress platform, the fridge, and the smell of the air when you first open the door. Expected: dry surfaces and neutral air. A musty smell on entry is the earliest warning you will get.

An awning is the other storage-season mistake. Vinyl awning fabric rolled up damp will show mold within a week in warm weather, because the roll traps water against itself in the dark. Extend it on the next dry day, let it dry fully in the sun, then roll it. If it is already spotted, wash vinyl with mild soap and water or a 1:4 bleach-to-water solution, and use the fabric maker’s own cleaner on acrylic like Sunbrella, since bleach strips its water-repellent finish.

Do RV covers cause mold?

A breathable cover does not cause mold. A plastic tarp does. Polypropylene and similar breathable cover fabrics let vapor pass out while shedding rain; a poly tarp traps moisture rising from the ground against the sidewall, where it condenses on the fiberglass and grows mildew on the seals and the cover contact points.

Three rules make a cover safe. Never cover a wet rig, because the cover then holds that water in for months. Choose a cover with zippered vent panels and use the ones on the roof, not just the door zips. And park on gravel or a pad rather than on grass or dirt if you have the option, since bare soil under a trailer releases moisture continuously all winter. If the rig has to sit on grass, a sheet of plastic on the ground under it, weighted at the edges, cuts that source off.

How to test for mold and moisture in an RV

Use your nose first, then a moisture meter, then a lab only if you need proof. A musty smell in a closed RV means active growth somewhere, and the smell reaches you long before a stain does. Ignore the smell for a season and the growth has usually moved from the surface into the wall.

A pin-type moisture meter costs $25 to $40 and is the most useful tool in this whole article. Press the pins into bare wood or through a soft panel:

• Under 15 percent moisture content: normal, dry wood.
• 16 to 19 percent: elevated. Something is wetting it, though decay has not started.
• 20 percent and above: wet enough to support mold and rot. Find the source now.

Take readings in the same six places each month so you have a baseline: the sill under each window, the front and rear outside corners at floor level, the plywood under the mattress, the floor at the entry door, and the wall behind the shower. A pinless meter scans through panels without leaving holes but reads a shallower depth, so pins remain the honest tool for a suspect area.

Home mold test kits that grow a culture on a settle plate tell you almost nothing, because mold spores are present in every building’s air. If you need a defensible answer, for an insurance claim or a purchase inspection, hire an inspector who does moisture mapping with an infrared camera plus tape-lift or air sampling sent to a lab.

How to get rid of mold in an RV

Scrub hard surfaces with detergent and water, dry them within 48 hours, and replace porous materials rather than trying to save them. The EPA’s mold guidance sets a practical line at about 10 square feet: below that most owners can handle the cleanup themselves, and above 100 square feet or anywhere the growth is inside a wall cavity, bring in a remediation contractor.

Wear an N95 respirator, nitrile gloves and sealed eye protection, open the vents, and run the fan on exhaust so spores go out rather than around the cabin.

Walls, ceilings and hard surfaces. Detergent and warm water on a stiff brush removes surface growth from fiberglass, vinyl wall board, laminate and painted panel. Rinse, then dry with a fan pointed at the area for a day. Skip bleach here: the EPA does not recommend it for routine cleanup, it cannot penetrate porous material, and its water content feeds whatever remains below the surface.

Inside the wall. If a panel is soft, stained through, or reads above 20 percent on a meter with the surface visibly clean, the growth is behind it. That panel comes off. Anything soaked, batt insulation, lauan, particle board shelving, gets cut out and replaced, because those materials cannot be cleaned. This is also the point to find the water source, since interior condensation and a leaking roof seam look identical from the inside.

The refrigerator. Wipe the gaskets, the shelf channels and the fin area with a solution of one tablespoon of baking soda per quart of warm water, then leave both doors propped for 48 hours with a fan nearby. Black growth on a fridge gasket almost always means the unit was closed damp. If the fridge is also running warm or smelling of ammonia, that is a different fault entirely and RV refrigerator troubleshooting covers the diagnosis.

Soft goods. Mattresses, upholstered cushions, carpet and curtains with visible growth get thrown out. There is no cleaning method that reliably removes mold from foam, and a mattress you save is a mattress that re-seeds the rig in three months.

FAQ

Frequently asked questions

How do I stop condensation in my RV during winter?

Cut production and increase ventilation at the same time. Heat with the vented furnace instead of an unvented propane heater, which alone removes about 5 pints of water per 8 hours. Then crack two roof vents at opposite ends of the rig, run the vent fan on speed 1 while cooking and showering, and wipe the windows every morning. Aim for 35 to 45 percent relative humidity on a hygrometer at head height.

What is the best moisture absorber for an RV?

It depends on whether anyone is aboard. In a sealed storage rig, calcium chloride tubs such as DampRid work well: each 10.5-ounce tub takes up 1 to 2 pints over 30 to 60 days. In an occupied rig they are pointless against 6 or more pints a day, and the answer is a compressor dehumidifier on shore power or open vents on battery. Renewable silica gel units are best for a single closet or cabinet, not a whole RV.

How do I keep mold from growing under my RV mattress?

Create an air gap. Cut 3/4-inch ventilating matting such as Hypervent or Den-Dry to the mattress footprint, or lay half-inch rigid foam board or slats with 2-inch gaps on the platform. Then stand the mattress on edge for two hours once a week through the winter, and remove it entirely for storage season. The plywood under a queen bed sits at 35 to 45 °F against an outside wall while the air trapped in the foam has a dew point near 55 °F.

Will a dehumidifier work in a cold RV?

A compressor dehumidifier stops working below roughly 65 °F because its coil frosts over rather than condensing water, and many units cycle into a defrost mode that removes nothing. Desiccant-rotor dehumidifiers keep pulling water down to about 35 to 40 °F but cost more and run warm. In a cold, occupied RV, ventilation combined with vented furnace heat removes more water than either type.

How long does it take mold to grow in an RV?

24 to 48 hours on damp cellulose materials such as lauan panel, cardboard-backed insulation or a paper-faced cabinet, provided the surface humidity stays near 80 percent and the temperature is between 40 and 100 °F. That is why the 48-hour drying rule matters after any spill or leak, and why an RV closed up wet in the autumn can smell musty by the time you get to it in November.

Does an RV cover cause mold?

A breathable cover with roof vent panels does not. A plastic or poly tarp does, because it traps ground moisture against the sidewall where it condenses. Never fit any cover over a damp rig, open the cover's roof vents rather than only the door zips, and park on gravel rather than grass or bare soil where you can, since the ground under a trailer releases moisture all winter.

Why is there condensation only on some walls of my RV?

Those walls are colder. Outside corners lose heat through two exterior faces and can run 6 to 8 °F below the flat wall next to them, aluminum framing conducts straight through the laminated sidewall as a thermal bridge, and slide-out walls are thinner than main walls. Closed cabinets and wardrobes on outside walls are cut off from heated air, so their back panels sit at wall temperature. Leaving those doors ajar often fixes a stubborn wet patch on its own.

Moisture control in an RV is really a heat problem wearing a water costume: every wet surface is a cold surface, and every extra pint of vapor came from something you did inside the rig. Fix the heat source first, add the airflow second, and buy equipment third. For the parts that touch the rest of the rig, the climate and appliance guides cover furnace, air conditioner and fridge issues, and the RV propane guide explains why burning a pound of propane indoors gives you a pound and a half of water back.

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