Hibernate smart: 5 steps to stop RV mold growth before it starts

Mold may flower in the summer, but it’s planted in the winter. Or to riff off an old country proverb, “December cold brings May mold.” I use the terms “flower” and “planted” metaphorically, of course. Mold neither blossoms nor roots. It’s a fungus, and it eats things. If you accidentally nurture it, it’ll try to eat your RV, too. So if you improperly store your RV over the winter, you could be turning it into a veritable fungus greenhouse. (Looking at you, plastic tarps!)

Mold growth begins with a spore—or, more likely, a few bazillion of them. They’re everywhere. In fact, you’re probably inhaling a few right now (thank your nose hairs for keeping your lungs clean).

A spore needs five things to survive: an organic food source, warmth, darkness, oxygen, and moisture. Your RV provides all of these ingredients thanks to its wood-based construction. You can’t control most of these things, but you can control the moisture. That’s the key to preventing mold growth in your RV: Keep it dry!

Here’s why you don’t want mold growth in your RV

Just in case this is your first winter as an RV owner, here’s why you really, REALLY don’t want mold or mildew in your rig. To borrow a phrase from Sun Tzu, “Know thy enemy.”

Health conditions

Mold can aggravate or create certain health conditions, such as lung infections. People with mold allergies can have Chronic Inflammatory Response Syndrome (CIRS), which is the Mayo Clinic’s way of saying a stuffy nose, itchy throat, phlegm-y cough, and all the other bad memories of childhood.

Resale value

Selling an RV with a mold infestation is like selling a cracked TV: No matter the price, no one wants to watch The Walking Dead through a dark spider web. Many RV owners simply will not buy a rig with a mold infestation, because once mold has a foot in the door, it’s very difficult to shut it out entirely.

How to prevent mold growth in your RV over the winter

Here are five simple steps you can do to keep your RV mold-free. If you plan on using your RV during the winter and not just storing it, check out these additional tips from Gail Marsh.

1. Clean and dry your RV thoroughly

Before putting your RV into hibernation, clean and dry all surfaces.

• Wipe down any suspect surfaces: floors, counters, windows, etc.
• Mop up any standing water in sinks or shower stalls.
• Apply an anti-mold spray to bathroom stalls, kitchen counters, sinks, and other surfaces exposed to organic matter.
• Dry out your RV by running a dehumidifier inside for at least 24 hours before locking it up. This will suck out moisture from the building materials as well as the work surfaces.

2. Insulate cold-condensing surfaces

Any surface in your RV that gets cold quickly is a prime candidate for condensation. The usual suspects are windows, skylights, insulated wheel wells, roof fans, and all corners. (Most RVs have lots of aluminum structure in the corners, and aluminum gets cold quickly.)

Insulating your windows, skylights, and roof fans is the simplest, best thing you can do to reduce condensation in your RV over the winter. You can buy commercial insulators (read Dustin Simpson’s post on those here) or MacGyver your own.

3. Inspect your pipes

Imagine a slow leak: one drop every five minutes. Hardly noticeable, right? Well, over the course of a 4-month winter, that’s more than two gallons of water, enough to saturate your subfloor and turn your OSB (oriented strand board) into garden mulch.

Any leak, no matter how small, can severely damage your RV. If you suspect a slow water leak, here are three methods to scout it out:

• For the freshwater system, set up a timed pressure test. Pressurize the fresh water system to 80 psi (65 psi for older RVs) and then shut off the pressure source. Wait at least 10 minutes. If the pressure drops, you have a leak.
• Also for the freshwater system, you can spray soapy water or a leak detection fluid on joints. If water or air escapes, bubbles will appear in the fluid.
• To find a leak in the DWV (drain-waste-vent) system, try using a septic tracing dye. However, besides water in your P-traps, you shouldn’t have any sitting water in your RV’s drain pipes over the winter.

If you want to get really fancy, borrow an infrared heat scanner and look for blue spots around the pipes, indicating cool water. And if you don’t want to do any of these tests yourself, ask any qualified RV technician to perform the pressure drop test, which is standard for all RVs.

4. Ventilate your RV

The ideal relative humidity for interior spaces is 30-60%. “If there are no cold-condensing surfaces and the relative humidity (RH) is maintained below 60 percent indoors,” says the North Carolina Department of Health and Human Services, “there will not be enough water in those materials for mold to grow. However, if the RH stays above 70 percent indoors for extended periods of time, mold will almost certainly grow.”

Did you catch that? Above 70% RH, mold kicks off its shoes and turns on the TV. Almost all states outside of the Southwest average greater than 70% RH in the morning, which sets up a perfect diurnal cycle for mold growth.

Here are three ways you can reduce the humidity inside your RV during storage:

• Run a dehumidifier. If you have access to 120V electricity, run a dehumidifier every week or two.
• Unpack the desiccant. You can use desiccant buckets and packets like DampRid to absorb moisture from the air. Desiccants are best used in enclosed areas like drawers, cabinets, and small bathrooms. And don’t cheap out. You need to replace containers every 60 days. And if you plan on only using desiccants to dehumidify, then you probably need 3–4 for an entire RV.

• Heat the air. If you heat up air, you decrease the relative humidity. You’ll need electric hookups, obviously, but you might consider a specialty space heater like the Air-Dryr. I haven’t used one myself, and I would only consider it as a backup solution.
• Ventilate the interior. If you store your RV outdoors, you might need to ventilate it every so often. Pick a warm, windy day with as low humidity as possible. Open all doors and windows to encourage airflow. If you can, turn on your bathroom exhaust and roof fans, too.
• Track the humidity. If you’re a data-is-beautiful kind of person, you can buy an indoor hygrometer, which will track your relative humidity and temperature.

5. Invest in an RV cover to prevent mold growth

‘Tis the season for the perennial question, “Is it OK to cover my RV with a tarp?” My personal opinion is that a tarp is a case where the cure may be worse than the disease. Tarps aren’t breathable; they trap water. If you’ve ever worn a cheap plastic poncho in a rainstorm, you know what I mean.

If you’re afraid of a roof leak, better to re-seal your roof than Band-Aid it with a cheap plastic tarp. It almost goes without saying, but you need to be 100% sure that your RV is leak-free before putting it away for the season. You can sometimes find water leaks with the simple hose test, or you can rely on something more accurate like a Seal-Tech test.

To quote an old friend of mine, “Don’t step over dollars to pick up pennies.” Over the life of your RV, a good cover will pay for itself.

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