How to get mildew out of a tent

To get mildew out of a tent, brush the dry growth off outdoors, hand wash the affected panels in lukewarm water with a mild soap, treat the patches with an enzyme odour eliminator or a dilute vinegar spray, then dry the whole tent completely within a day. That stops the smell and the spread. The black stain itself usually stays, and the tent's coating may already be damaged underneath.

Is it mildew, or is the coating failing?#

Mildew shows as spots, "tiny black or dark blue/green spots" in Homecamp's description, usually in patches where the tent lay folded and wet, with a damp smell. A sour smell plus a tacky inner face that sticks to itself when folded is a different problem: the polyurethane coating breaking down by hydrolysis. Cleaning does nothing for that. The look-alikes are set out in a table in why is my tent sticky?, and an old tent can have both.

The two are linked. Both come from the same mistake, a tent packed away damp. Big Agnes says damp storage "may cause your tent to smell, leak, or delaminate", and MSR says mildew can "compromise its waterproof coating". Neither maker covers mildew damage under warranty.

How to get mildew out of a tent, step by step#

1. Work outdoors. Pitch the tent or hang it in the open air, with the affected panels where you can reach both faces.
2. Brush off what you can, outside. Use a soft brush, as both canvas makers advise, and no more force than that. The EPA warns that dead mould "may still cause allergic reactions in some people", so killing it is not enough: it has to come off, and not indoors.
3. Hand wash the affected panels. MSR: warm water, a sponge and "a mild, non-detergent soap", with the tent pitched. Big Agnes rules out hot water, bleach, liquid detergents and dishwashing liquid. MSR adds no detergent, dry cleaning, machine washing or tumble drying: "any of these methods can remove the tent's waterproof coatings".
4. Treat the patches. Spray an enzyme odour eliminator at the maker's ratio, or a dilute white vinegar solution. The ratios are in the table below. Leave enzyme cleaners on; do not rinse them off.
5. Dry completely, the same day if possible. Pitched or hung, out of strong heat. GEAR AID says to avoid heat and direct sunlight with its cleaner, because "heat may harm the microbes", so a breezy shaded spot suits that product best.
6. Check, then store. Use the sniff and bowl tests further down. Store loose, cool and dry, as in storing gear between trips.

What each maker tells you to use#

MSR and Big Agnes, both makers of synthetic tents, give no killing agent at all: clean it, dry it, keep it dry. The stronger chemistry comes from a retailer's blog and from canvas tent makers.

GEAR AID, whose cleaner was formerly sold as MiraZyme, lists tents among items "that should not be soaked", so its spray ratios apply: 2 ounces per 16 ounces of water for "mold/mildew smells", the fabric left "noticeably damp" but not saturated, and the mix used within 72 hours. It says the product will not break down a PU coating, but "soaking these items in water can have that effect".

Does bleach kill mould on a tent?#

Yes, at a strong enough concentration. A claim that circulates on forums, that bleach only whitens mould and never kills it, is not what the evidence says. The review by Rogawansamy and colleagues cites a study in which 2.4 percent sodium hypochlorite inactivated the spores of six fungi, including Aspergillus niger and Penicillium chrysogenum, to undetectable levels after 5 minutes on non-porous surfaces and 10 minutes on porous ones.

The real question is how strong the solution on your tent would be. Household bleach is 5 to 9 percent sodium hypochlorite, according to the CDC. Here is what each recipe works out to:

Even REI's spray, the strongest recipe any tent source offers, is a fifth to a third of the strength shown to work in 10 minutes, and it is wiped straight off. Vango's well-diluted Milton is weaker again, which fits Vango's own modest claim for it.

So there is no bleach dose that is both sure to kill mould in a fabric and approved by the companies that make the fabric. The EPA, for houses, calls bleach "not recommended as a routine practice during mold cleanup", and says removing the growth matters more than killing it. Our position: on a coated synthetic tent, follow the maker's instructions and use no bleach. If your tent's maker explicitly allows hypochlorite, as Vango does, use it at their dilution and never mix it with vinegar or any other cleaner, which the CDC warns "can release vapors that may be very dangerous to breathe in".

Does vinegar kill mildew?#

Partly, and less than most tent advice implies. Rogawansamy and colleagues tested undiluted vinegar, 4.0 to 4.2 percent acetic acid, against two moulds common in indoor air. It inhibited Penicillium chrysogenum, with a mean clear zone of 15 mm around the test disc. It had no inhibitory effect on the growth of Aspergillus fumigatus. It also suppressed spore production in the first and not the second. The authors suggest its "lack of persistence on surfaces" may limit vinegar to cleaning non-porous materials.

That was full strength. Homecamp's 1 part vinegar to 4 of water is one fifth of it, and Montana Canvas's half cup per gallon, about 1 part in 33, is one thirty-third.

Vinegar is still a reasonable choice for the treatment step. It is cheap, no tent maker here warns against it, and Montana Canvas prefers it to bleach, calling it "less abrasive than bleach to the actual canvas fibers". But treat it as a cleaning aid that may slow some moulds, not as a guaranteed kill. The drying afterwards is what stops regrowth.

Will the black spots come out?#

Usually not completely. None of the makers above promises that the stain will go, and GEAR AID's cleaner is "designed to eradicate odors arising from mold and mildew", not the mould itself. The realistic target is a tent that no longer smells and no longer spreads, with faded marks where the patches were.

What matters more is what is under the stain. Big Agnes lists leaking and delamination as results of damp storage. Laboratory work shows why a PU-coated tent is vulnerable: Magnin and colleagues isolated Alternaria, Penicillium and Aspergillus strains that could use polyester polyurethanes as their only carbon source, the polyester-based chemistry that "PU" means on a tent label. Their tests ran two months on solid plastic samples, not tent fabric, and weight losses were small, 1.3 to 8.9 percent. That justifies checking a mildewed panel for leaks, not guessing how much coating a stain has eaten.

After cleaning, run the bowl test below on every panel that was mildewed. If the coating has gone, the options are the same as for a sticky tent: recoat or replace the fly. Homecamp advises reproofing straight after any mould treatment; on a synthetic tent, a water repellent only helps the outer face, as reproofing rain gear explains.

Canvas tents are a different problem#

Cotton is a plant fibre, so on a canvas tent the mould can feed on the fabric itself, not just on dirt and coating. Montana Canvas calls its canvas "a natural fiber" that is susceptible "especially if left in a wet environment". Homecamp calls mould "a significant drawback of canvas tents compared to plastic ones", even with the anti-mould treatment many canvas tents carry.

That is why the canvas makers' advice is more aggressive: a dedicated mould cleaner, a vinegar solution, a soft brush, repeat until clear, and for severe cases Homecamp suggests an overnight soak. Montana Canvas reads pinkish colouring in the canvas as the mould inhibitors "trying to kill the active mold spores", and rules out pressure washers because they can damage the fibres and the coating.

How fast does mildew grow on a packed tent?#

MSR's figure is "as few as 24 hours" on a wet, crumpled tent. The EPA says that if damp materials are dried within 24 to 48 hours, "in most cases mold will not grow". A tent packed wet on Sunday morning and unpacked on Tuesday evening is past both.

The rule that follows, also in camping in the rain, is to have a wet tent out and drying within 24 hours and to treat 48 as the limit. Condensation counts too: a tent struck at dawn with a wet inner is a wet tent, for the reasons in tent condensation.

For storage, the EPA's indoor target is relative humidity below 60 percent, ideally 30 to 50. Our storage guide ranks the garage and the loft as the two worst common places to keep gear, for humidity and heat respectively.

Test it yourself: is it gone, and is the tent still waterproof?#

Two checks, after the tent has dried completely.

1. The bag sniff test. Bundle the cleaned section loosely, put it in a closed bag in a warm room for a day, then open the bag and smell it. A musty smell that comes back means growth is still active in the fabric: repeat the treatment and dry it again.
2. The bowl test. Hold a cleaned section of floor or fly as a bowl, pour in about a litre of water and leave it for ten minutes. Damp patches on the outside show where the coating has failed. It tells you whether the mildew cost you the waterproofing, which the stain alone cannot. A new coating carries a hydrostatic head rating of the kind in waterproof ratings explained; a ten minute bowl test is the home version.

If both pass, the tent is usable even with faint marks. If the bowl test fails, plan a recoat or a new fly before the next wet trip.

Common mistakes#

Putting it in a washing machine. MSR and Big Agnes both rule it out; MSR says it can remove the waterproof coating.

Using bleach because it is in the cupboard. The tent makers that address it forbid it, and the strongest tent recipe published is still well below the strength shown to kill spores.

Mixing vinegar and bleach. The CDC warns never to mix bleach with other cleaners. Use one or the other, never both together.

Rinsing off an enzyme cleaner. GEAR AID says rinsing "would remove the microbes before they complete their task". Leave it on and let it air dry.

Drying it in hot sun or a tumble dryer. MSR rules out machine drying. Strong heat speeds coating hydrolysis, and GEAR AID says heat may harm its microbes.

Treating and then packing it away. A tent put away damp after cleaning is back where it started.

Frequently asked questions#

Can you use bleach on a tent with mildew?#

Not if the maker says no, and MSR and Big Agnes both do. Bleach kills mould at 2.4 percent hypochlorite, the strength shown to kill spores on porous surfaces within 10 minutes, but no tent source recommends anything close: REI's 1 part household bleach to 10 of water is 0.45 to 0.82 percent. If your maker allows a hypochlorite product, use their dilution and never mix it with other cleaners.

Does vinegar kill mold on a tent?#

Not reliably. In a laboratory test, undiluted vinegar at about 4 percent acetic acid inhibited Penicillium chrysogenum but had no effect on Aspergillus fumigatus, two moulds common indoors. Tent recipes dilute vinegar 5 to 33 times. It is a reasonable cleaning step, but drying the tent completely is what stops mildew coming back.

How do I get the mildew smell out of a tent?#

Wash the affected panels by hand with a mild soap, then spray an enzyme odour eliminator and let it air dry without rinsing. GEAR AID's ratio for mould and mildew smells is 2 ounces per 16 ounces of water, applied until the fabric is noticeably damp but not saturated. Dry it out of strong sun, then check with a day in a closed bag.

Will mildew stains come out of a tent?#

Usually not completely. Neither Vango nor GEAR AID promises to remove the mould itself, only to stop the spread and the smell. The stain matters less than whether the coating beneath it still holds water, which a ten minute bowl test answers.

Is a mildewed tent safe to sleep in?#

The EPA notes that dead mould can still cause allergic reactions in some people, so the aim is to remove the growth, not just kill it: brush it off outdoors, wash the panels and dry the tent fully. If someone who will sleep in it is sensitive to mould, a heavily mildewed inner is a candidate for replacement rather than cleaning.

How long can a wet tent stay packed before it gets mildew?#

Plan on less than a day. MSR says mildew can start on a wet, crumpled tent in as few as 24 hours, and the EPA says drying damp materials within 24 to 48 hours prevents growth in most cases. Unpack and dry a wet tent the day you get home, and treat 48 hours as the outside limit.