Before closing an RV or boat for winter storage, remove everything that holds moisture — cushions, bedding, rugs, damp gear — or prop it up on edge so air reaches both sides. Drain the fresh-water system and the bilge rather than leaving them full. Place two or three moisture absorbers sized to each separate compartment, not one for the whole vehicle, and crack a roof vent under a rain cover instead of sealing the unit airtight. Reopen it on the first dry day in spring, air it out fully, and check every absorber before storing anything back inside.
Why a closed-up RV or boat fails differently than a house
A house has heating, occupants moving air around, and someone checking in daily. A stored RV or boat has none of that for months at a stretch, so whatever moisture is sealed inside on the day the door closes is still there — often worse — on the day it reopens. RVs, boats and stored vehicles covers why the thin, poorly insulated envelope makes this worse than a house; this page is the specific pre-storage routine that keeps it from happening.
The goal going into storage is simple even where the execution takes an afternoon: leave as little moisture inside as possible, give what remains somewhere to go, and give the interior at least some air exchange rather than none.
What comes out before the door closes
Cushions, bedding, curtains, towels and any clothing or gear that has been damp at any point all hold water long after they feel dry to the touch, and a sealed, unheated interior gives that residual moisture nowhere to go but into the air around it. Pull all of it out and store it somewhere heated and ventilated for the season rather than leaving it aboard — it is both the easiest step and the one with the biggest effect, because it removes a moisture source rather than just managing one.
Food, paper goods and anything else that can mildew or attract pests belongs in the same pile. A pantry left stocked over winter is a second, smaller moisture-and-mildew problem on top of the one this page is about.
Cushions and mattresses: propped, not stacked flat
Anything that cannot be removed — a fixed dinette cushion, a built-in mattress — should stand on edge rather than lie flat against a wall or the floor. A cushion laid flat only ever dries, or absorbs moisture, from its top face; the underside sits against cold fiberglass or a damp locker floor in still air for months, which is exactly where mildew starts. Standing it up, even just leaning it against the opposite wall, lets air reach both sides.
The same logic applies to anything upholstered built into the space. If it cannot be angled or stood up, at least slide a thin spacer — a folded towel, a length of wood — underneath to break the direct contact with a cold surface.
Draining tanks and the bilge
A full fresh-water tank or a wet bilge is a standing reservoir that raises interior humidity all winter on its own, on top of any leak risk from a freeze. Drain the fresh-water system, the water heater and any holding tanks completely, and leave taps open rather than closed so residual water has somewhere to evaporate to rather than sitting trapped in a line. On a boat, pump the bilge dry and check it stays that way — a slow drip from a shaft seal or a through-hull fitting can refill it faster than expected over a season nobody is watching.
Open every internal locker, cabinet and drawer once the tanks are down, for the same reason the cushions get stood on edge: a closed locker is its own small sealed space, and air needs a path into it as much as it needs one into the main cabin.
Sizing moisture absorbers to the compartment, not the vehicle
The mistake worth avoiding is buying one large absorber for the whole interior. Moisture absorbers only ever work in a small, sealed volume — a calcium chloride tub removes roughly an ounce of water a day, which is why the whole category is close to useless in a room but genuinely effective in a closet. An RV or boat interior is not one space by that measure; it is several. The main cabin is one sealed volume, and each enclosed storage locker, cargo bay or under-berth cavity is its own separate one, cut off from the cabin's air the moment its door or hatch is shut.
Place two or three absorbers in the main cabin — low down, where damp air settles, with clear space around each one — and add a smaller unit to any locker or bay that closes off completely rather than assuming the cabin's absorbers reach it. See DIY vs. store-bought moisture absorbers for how a calcium chloride tub, silica gel and a rock salt bucket actually compare — the contained tub is the reasonable default here, since a spilled bucket of brine inside a stored vehicle can sit undiscovered for months.
The pre-storage checklist
Remove everything soft
Cushions, bedding, towels, rugs, curtains and clothing all come out and go somewhere heated for the season, not just the obviously damp items.
Prop what cannot be removed
Stand built-in cushions and mattresses on edge, or at minimum slide a spacer under them to break contact with a cold surface, so air reaches both faces.
Drain the water system and the bilge
Empty the fresh-water tank, water heater and holding tanks, leave taps open, and pump a boat's bilge dry.
Open every locker, cabinet and drawer
Each closed compartment is its own sealed space. Leave them open so air moves through the whole interior, not just the main cabin.
Place absorbers by compartment
Two or three in the main cabin, plus a smaller unit in any locker or bay that closes off completely on its own.
Crack a roof vent under its rain cover
Leave one vent open a small amount rather than sealing the unit airtight, so there is some background air exchange through the season.
Park on a hard surface
Gravel or pavement rather than grass or bare soil, which lets ground moisture rise into the underside of the vehicle all winter.
Why a cracked roof vent beats sealing the unit completely
Sealing an RV or boat as tightly as possible feels like the safer choice, but it traps whatever moisture is already inside with no way out for the whole storage season, which is precisely the condition mildew needs. A roof vent cracked open under its rain cover — a small, protected gap rather than a wide-open hatch — lets a slow, continuous trickle of air exchange happen the same way a house's background ventilation does, without letting rain or snow inside.
It will not dry a genuinely wet interior on its own, and it works alongside the absorbers rather than instead of them — a sealed unit with absorbers still traps humidity that a cracked vent lets bleed away gradually, and a cracked vent with no absorbers still leaves standing moisture sources unmanaged. The two together cover what neither does alone.
Storage methods compared
| Method | What it needs | Best for |
|---|---|---|
| Sealed unit, no vent, no absorbers | Nothing to set up | Nothing — traps every ounce of moisture already inside for the whole season |
| Cracked roof vent + moisture absorbers by compartment | A rain-covered vent and a few tubs, checked monthly | Most stored RVs and boats with no shore power |
| Small desiccant dehumidifier on a humidistat | Shore power | Anywhere power is available — removes far more water than absorbers can, and does not ice up the way a compressor unit does below about 65 °F |
Checking in through the storage season
A monthly check, where the location allows it, catches the two things that go wrong quietly: an absorber that filled faster than expected and has stopped working, and a leak at a roof seam, vent seal or window gasket that a fall inspection missed. Absorbers in a genuinely damp compartment can saturate well before the season is over, and a tub that is full is doing nothing for the weeks until someone notices and replaces it.
If a monthly visit is not realistic, err toward more absorbers placed at the start rather than fewer, and toward the desiccant-dehumidifier route in the comparison above wherever shore power makes it possible — it needs far less monitoring than tubs that can quietly run out.
The spring reopening checklist
Open it on a dry day
Choose a low-humidity day to break the seal, not a damp or rainy one, so the outside air helps rather than adds to the problem.
Open every door, hatch and window
Air the whole interior for at least a few hours before doing anything else, including the lockers and cabinets left open all winter.
Check every absorber before storing anything
A saturated tub or a bag of silica gel that never got recharged means the interior may have run damper than planned for part of the season — inspect cushions and soft goods closely before bringing them back in.
Look and smell for what the winter left behind
A musty smell, dark spotting on fabric, or a soft or swollen spot in wood trim all point to a compartment the season's routine did not fully reach.
Reconnect water systems and refill gradually
Check hoses and fittings for winter damage before restoring full water pressure, rather than assuming a system that was fine in fall is still fine now.
When it is worth the shore-power option
A desiccant dehumidifier on a humidistat is the more reliable answer wherever shore power reaches the storage spot, because it removes water continuously rather than in the small fixed amount a tub can hold before it needs replacing. It is worth the setup cost for a boat or RV stored for the full winter in a genuinely humid climate, or for anyone who cannot commit to the monthly check the absorber-only route depends on.
For a shorter storage window, a dry climate, or a spot with no power at all, the cracked-vent-and-absorbers routine from this page is enough on its own, provided the pre-storage steps — emptying, propping, draining — are done properly first. See winter indoor humidity levels for the same 30–50% target applied to a heated space, which is the number to aim for once the vehicle is back in use rather than in storage.
Common questions
How many moisture absorbers does an RV need for winter storage?
Two or three in the main cabin, placed low, plus a smaller absorber in any locker, cargo bay or under-berth cavity that closes off completely on its own. Each sealed compartment is its own small space and needs its own absorber rather than relying on ones placed elsewhere in the vehicle.
Should I leave a roof vent open when storing an RV or boat for winter?
Yes, cracked slightly under its rain cover. A small continuous gap lets some background air exchange happen through the season, which a fully sealed unit does not get, while the cover keeps rain and snow out.
What should I remove from an RV or boat before storing it for the winter?
Cushions, bedding, towels, rugs, curtains, clothing, food and paper goods — anything that holds moisture or can mildew. Whatever cannot be removed, such as a built-in mattress, should stand on edge rather than lie flat against a cold surface.
Is it worth draining the water tank if the RV will not freeze?
Yes. Even where freezing is not a risk, a full tank is a standing source of moisture that raises interior humidity through the whole storage season. Draining it removes that source regardless of outdoor temperature.
What is the first thing to check when reopening an RV or boat in spring?
Open every door, hatch, locker and window and air the interior out fully before doing anything else, then check each moisture absorber to see whether it saturated during the season — a full tub is a sign the compartment it sat in ran damper than the routine accounted for.
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
- US EPA — A Brief Guide to Mold, Moisture and Your Home — still, sealed air as a condition that raises mold and mildew risk, applied here to a closed-up RV or boat cabin