Condensation inside an RV: Why sealing it up makes it worse

The water is you

An RV carries its own weather system, and the makers quantify it. Winnebago: "One source suggests that breathing and perspiration alone can generate as much as three pounds of vapor per person per day!" Keystone put a family figure on it: "With normal activities like breathing, cooking, and bathing, a family of four can release up to three gallons of water vapor into the air each day."

That is not an RV-specific quirk. The US Department of Energy state the same thing about houses: "Just by breathing and perspiring, a typical family adds about three gallons of water per day to their indoor air." The load is a property of people, and it does not care what they are sitting in.

Cooking and showering add to it, and so does any unvented flame. Airstream state the ratio plainly: "Un-vented combustion equipment, such as propane stovetops is a source of moisture within the RV. For every gallon of fuel consumed, approximately one gallon of water vapor is evaporated into the air."

Which is worth knowing before blaming the furnace. A vented RV furnace is sealed combustion: its burner chamber vents outdoors, and it neither adds nor removes interior moisture. A catalytic or otherwise unvented heater is a different device entirely, and it adds water at roughly one for one.

What the makers say the target is

The figures converge, which makes them usable.

The two Airstream figures together are the useful pair: the same coach interior that is fine at 55 percent in summer will grow water on its windows at 45 degrees outside. It is not the humidity that changed, it is the surface temperature.

Why it lands on the cold spots

Dometic define the mechanism the way a manual would, and their wording is worth reading for the last sentence: "Air contains water vapor which condenses when temperature of a surface is below Dew point." They then add that the manufacturer "will not be responsible for damage caused by condensation forming on ceilings, windows, or other surfaces." That is a maker telling you, in a manual, that this is normal physics with real consequences.

Jayco teach the same thing to owners directly: "When the temperature reaches the 'dew point' the water vapor in the air condenses and changes to a liquid form. Proper ventilation or the use of a dehumidifier (customer supplied) will assist in controlling the condensation."

So the wet patches appear where the surfaces are coldest, and in an RV that is the frame itself. Winnebago: "metal structure within the sidewalls and around openings conducts the cold into the vehicle where it condenses the moisture in the air. Removing this moist air is the best way to prevent this condensation from forming." Airstream add the operating consequence: "avoid nighttime setbacks of 10 degrees or more", because a cold coach overnight means cold surfaces everywhere.

This is the signature that fools people into hunting a leak: moisture on interior metal, on window frames, at wall corners, and behind furniture, with no water anywhere near the roof.

The part that catches everyone: sealing it up makes it worse

Every instinct says to keep the cold out. The makers say the opposite, and they say it in the same words.

Winnebago: "Proper ventilation of your coach is the most effective way of condensation prevention. Modern construction methods, with improved insulation and weather stripping, create a living space that can be virtually airtight when closed up, allowing little or no moisture laden air to escape."

Grand Design put the diagnosis even more bluntly: "Condensation inside your RV indicates that proper ventilation is NOT occurring", and that moisture "must be carried outside by ventilation, or removed with a dehumidifier."

Read those together and the logic is unavoidable. A better-sealed coach traps more moisture, because the water has to go somewhere and there is nowhere for it to go. The EPA reach the same conclusion from the health side, listing condensation on walls behind furniture as a cause of hidden mould, not just leaks.

What the makers say to actually do

Jayco give the most concrete instructions of any manual found for this page: "Crack open windows and roof vents to allow warm moist air to escape." Then, for the two activities that add the most water: "Open the bath roof vent (if so equipped) approximately 1/2 inch when showering" and "Use the range hood fan (if so equipped) when cooking or washing dishes."

There are rates behind that if you want to check whether your fans are up to it. The DOE cite ASHRAE 62.2 as requiring local ventilation in bathrooms of at least 50 CFM intermittent or 20 CFM continuous, and in kitchens at least 100 CFM intermittent. A weak fan that never vents outdoors is a fan that does not count. The EPA also endorse air movement as a remedy in its own right: "Increase ventilation or air movement by opening doors and/or windows, when practical. Use fans as needed."

Dehumidifiers, and when they stop working

A dehumidifier helps, and the makers of them publish which one. ENERGY STAR give capacity by condition: 20 to 30 pints per day for a slightly damp space under 2,000 square feet, rising to 25 to 40 for very damp and 30 to 50 for wet. An RV is small, so the bottom of that range is the right end of it.

The catch is temperature, and it is specific. Compressor dehumidifiers stop working in the cold. ENERGY STAR caution that performance degrades around 65 F and that frost forms below it; manufacturer documentation puts the hard low limit at 41 F for units tested. Desiccant units are the cold-weather option: one maker documents operation from 1 C to 37 C and a 7.5 litre per day rating at 10 C.

Which means the common plan of running a dehumidifier through a cold winter in a sealed coach runs into the exact condition the unit cannot handle. In cold weather the answer is ventilation, or a desiccant unit, or both, and the makers of the coaches say so in their cold-weather sections as well: Jayco and Grand Design both pair cool-weather use with ventilation or dehumidification as a requirement rather than an option.

What happens if you leave it

This is the reason the page exists, and a manufacturer states it better than any blog does. Winnebago: "In more severe cases, you may develop dry rot or structural damage to the wooden components in the coach if the moisture is not contained. This type of damage often goes unattended, as it may not be readily visible and is usually quite expensive to repair." The delamination guide covers what that damage looks like when it reaches the sidewall.

The DOE describe the same failure in a building: "moisture condensing inside a wall or in the attic can lead to wood rot and permit the growth of mold." Inside the assembly, not on the surfaces you can see.

Two makers treat condensation as serious enough to write it out of the warranty in named terms. Airstream: "Condensation and the results of condensation including water damage and the growth of mold or mildew" are not covered. Forest River state they disclaim responsibility for damage "due to condensation" specifically. When a warranty singles something out by name, it has happened often enough to be worth naming.

Mould, and the bleach question

The EPA lead with the cause rather than the cure: "The key to mold control is moisture control", and "If you clean up the mold, but don't fix the water problem, then, most likely, the mold problem will come back." RV makers put the growth window at 40 to 100 F, which is most of the year in most of the country. The EPA also give a clock: drying wetted materials within 24 to 48 hours.

On bleach, the federal sources disagree and this guide will not pretend otherwise. The EPA: "The use of a chemical or biocide that kills organisms such as mold (chlorine bleach, for example) is not recommended as a routine practice during mold cleanup", adding that dead mould still causes reactions so it must be removed rather than merely killed. The CDC give a permitted dilution: "Use no more than 1 cup of bleach in 1 gallon of water", with a warning never to mix bleach with ammonia. OSHA give their own figures, a quarter cup per gallon for light contamination.

Three federal agencies, three positions. What they agree on is the part that matters: the moisture source is the problem, and cleaning without fixing it guarantees the return.

Storage over winter, and what is not documented

Airstream address the storage question in their own terms: manage humidity if you are not going to keep the RV warm, and leave a vent open during storage. Jayco and Grand Design both tie cold-weather use to ventilation or dehumidification.

What no maker found for this page states is the claim that circulates widely, that heating an unattended stored RV causes mould. No RV or appliance maker document says that. Nor does any maker say that wood-composite construction never dries once wet. Both are plausible and both are common in forums, and neither has a manufacturer behind it, so neither is asserted here. What the manufacturers do say is narrower and sufficient: manage the moisture, by ventilation or by removing it, and do not assume warmth alone solves it.

What this costs

Ventilation costs nothing, which is why it is easy to skip. A hygrometer to measure the humidity rather than guess at it is a few dollars, and it is the only way to know whether you are above or below the 60 percent line. A desiccant dehumidifier for winter is a modest purchase, and a compressor unit for the rest of the year is the same order.

The order to spend in: a hygrometer first, so you know which problem you have. Then ventilation, which is free. Then a dehumidifier matched to the season, remembering that a compressor unit stops working at the moment winter starts. What you are avoiding is not a damp weekend. It is the slow rot that Winnebago describe as unattended and expensive.

Sources

• Winnebago, Condensation Prevention in your RV, for the three pounds of vapour per person per day, the 60 percent target, ventilation as the most effective prevention, the airtight-coach mechanism, the metal structure conducting cold, and the dry rot and structural damage warning.
• Keystone RV, Managing Condensation and Mold in Your RV, for the three gallons per day figure for a family of four, and the 40 to 100 F mould growth window.
• Airstream, Managing moisture and preventing moisture-related damage, for the 60 percent target, the 35 percent cold-weather figure, the one gallon of water per gallon of unvented fuel, the nighttime setback advice, and the dehumidifier drainage point.
• US Department of Energy, Guide to Home Ventilation, for the three gallons per day from a typical family, the condensation inside a wall leading to rot and mould, and the requirement that combustion gases vent outdoors.
• US EPA, A Brief Guide to Mold, Moisture and Your Home, for moisture control as the key, the 30 to 50 percent ideal range, the recommendation against bleach as routine practice, the 24 to 48 hour drying window, condensation behind furniture as a hidden mould cause, and air movement as a remedy.
• CDC, Mold Clean Up Guidelines and Recommendations, for the permitted bleach dilution and the warning never to mix bleach with ammonia.
• ASHRAE Technical FAQ on recommended humidity levels, for the 65 percent ceiling in mechanically dehumidified spaces and the no-limitation caveat where dehumidification is absent.
• Jayco 2025 Class A owner's manual, for the dew point explanation given to owners, cracking windows and roof vents, the half inch bath vent while showering, the range hood while cooking, and the cool-weather ventilation requirement.
• Dometic Comfort Control Center 2 operating instructions, for the dew point definition and the manufacturer's disclaimer of condensation damage.
• Grand Design Lineage Series M owner's manual, for the statement that condensation indicates ventilation is not occurring and that moisture must be carried outside, and the 40 to 100 F mould window.
• US Department of Energy, selecting ventilation systems, for the ASHRAE 62.2 local ventilation rates of 50 CFM for bathrooms and 100 CFM for kitchens.
• Airstream 2024 Trade Wind owner's manual, for the warranty exclusion naming condensation, water damage, mould and mildew.
• Forest River travel trailer and fifth wheel owner's manual, for the disclaimer of damage due to condensation.