Condensation in produce packs: a WVTR cheat sheet

Quick answer: Condensation inside a produce pack is a mismatch between how much water vapour the product gives off and how much the film lets out, made worse by temperature swings. Macro-perforation, micro-perforation (EMAP) and dedicated moisture-management laminates each fix a different part of that mismatch, and none of them is free. Pick the fix by checking WVTR and OTR at stated test conditions (ASTM F1249 and ASTM D3985), not by film thickness or looks.

Why the inside of a produce bag fogs up

Fresh produce keeps respiring and transpiring after it is packed. In a sealed or lightly perforated film, that moisture has nowhere to go, so headspace relative humidity sits close to saturation for most of the pack’s life. Under these near-saturation conditions, even minor temperature fluctuation may cause condensation inside the packaging resulting in sliminess, decay, increase of microbial growth and browning of produce surface.

Modified atmosphere packaging (MAP) is built to slow that respiration down, but the fix creates a second problem. MAP is a technique widely used to extend shelf life by slowing respiration, but its use can become a problem due to water accumulation as condensation inside the package and at the surface of the product, promoting microbial growth and sliminess. You can’t separate the gas question from the moisture question. They’re driven by the same film.

Micro-perforated films are the usual answer for reaching a stable modified atmosphere quickly, and microperforated packaging films are commonly used in fresh produce packaging to increase O2 and CO2 gas permeability and achieve equilibrium MAP. The catch is that the same holes don’t do much for water vapour. Commonly used plastic films, even with enough micro-perforations to provide a modified gas composition, are more permeable to O2 and CO2 than to water vapour, so relative humidity in the packaging headspace could not be controlled lower than 100% while packaging of high transpiring produce such as mushrooms and strawberries. Getting the gas balance right doesn’t automatically get the moisture balance right.

Three ways to manage that moisture, and what each one costs

Packers generally reach for one of three film strategies, and each trades one problem for another.

Macro-perforation (visible holes, usually several millimetres) lets air move freely. To prevent moisture build-up, the packaging is often macro-perforated. However, this sacrifices modified atmosphere, accelerates dehydration and shortens shelf life, and even with macro-perforations, there is likely to be condensation in the package. It solves fogging partially, at the cost of the shelf-life benefit MAP was supposed to deliver.

A standard laminated pouch with a low WVTR keeps the modified atmosphere stable but traps moisture. Cucumbers are often packed in laminated stand-up pouches with a low water vapor transmission rate, which restricts excess moisture release and can accelerate microbial decay. As StePacPPC’s CTO Gary Ward put it when describing the trade-off packers face: “With traditional laminated pouches, achieving modified atmosphere for shelf-life extension comes at the expense of optimal moisture control.”

The third route is a film built to do both jobs with separate mechanisms rather than one perforation pattern doing double duty. StePacPPC’s Xgo PAZ pouch, now used by grower Wholesum for organic snacking cucumbers, is built this way: the Xgo Modified Atmosphere film works by incorporating three functions in one pouch: modified atmosphere, modified humidity, and condensation control. Before rolling it out commercially, the company ran 16-day shelf-life tests under simulated supply chain conditions. The published results give a sense of scale: organic baby cucumbers in the PAZ pouch achieved a 29 percentage-point improvement in overall quality compared with a leading top-seal format, and organic mini cucumbers recorded a 10 percentage-point advantage over produce packed in conventional macro-perforated packaging. Those are the grower’s own commissioned test results, reported through trade press, not an independent lab study, so treat them as directional rather than a spec.

The two standards that actually decide this: ASTM F1249 and ASTM D3985

Before changing a film, get the WVTR and OTR data sheet, and check the test conditions printed on it. Figures without conditions aren’t comparable to anything.

WVTR is usually generated by a global ASTM test method to measure the water vapor transmission rate through plastic films and sheeting using a modulated infrared sensor, and ASTM F1249 works with sheets and films up to 3 mm thick, and can be used with single or multilayer synthetic or natural polymers and foils, including coated materials. Published WVTR data for food packaging commonly uses 38°C and 90% or 100% relative humidity, or 23°C and 85% RH, depending on the lab and the end use; one widely cited patent example reports WVTR values at 38°C and 90% RH and at 23°C and 85% RH obtained according to ASTM F1249 for the same film, and the two numbers were not the same. Ask which condition a supplier used before you compare two data sheets.

OTR is measured separately, under ASTM D3985, typically using a MOCON or equivalent instrument at 23°C and 0% or 85% relative humidity. This matters because some barrier resins perform very differently across that humidity range. EVOH loses barrier at high humidity, so a value measured at 0% RH can look much better than the bag performs in a humid warehouse. A cold, dry-climate OTR number on a data sheet tells you almost nothing about how that film behaves in a humid produce cooler.

The laminate estimator is useful here for a quick sanity check on an indicative stack’s barrier and weight before you request lab data. The material properties table is a starting reference point for typical ranges by polymer family.

Diagnosing an existing condensation complaint

Pull the temperature log for the shipment. Near-saturation headspace tips into visible condensation with a small temperature drop, so a reefer excursion or a warm dock stop is often the actual trigger, not the film.
Check the perforations are not blocked. Micro-perforation holes are tiny: 20 to 100 micron holes cannot be readily seen with the naked eye, so it is impossible to prevent occlusion of the microperforations either by the produce or by adhesive labels placed over them. A blocked hole does more than stop fogging control; when holes are blocked, the principal route for gas transmission through the film is blocked, which leads to anaerobic conditions and fermentative reactions.
Get the actual WVTR and OTR figures, with test conditions, for the film in use. Compare against the produce’s published respiration class rather than against a generic “high barrier” claim.
Check whether the film has an anti-fog treatment, and what it’s rated to. Anti-fog layers are usually surfactant coatings that spread condensed water into a transparent sheet rather than droplets; published fog-time test data under EN166/168 for one commercial anti-fog film ran from around 10 seconds up to roughly 150 seconds before fog formed, when tested over a 50°C water bath. If the pack has no stated fog time, it probably has no anti-fog function at all, just a plain film.
If moisture and atmosphere are both off target, separate the two jobs rather than widening the micro-perforation and hoping. A two-mechanism approach (gas control plus a separate moisture-absorbing or moisture-release layer) is the basis of the newer 3-in-1 pouches on the market.

Causes and fixes at a glance

Symptom	Likely cause	First fix to try
Fog forms within hours of packing, clears later	Normal transpiration meeting a cold headspace at pack-out	Pre-cool produce fully before sealing; check pulp temperature, not air temperature
Fog appears after a specific transport leg	Temperature excursion during that leg pushed headspace past dew point	Pull the cold chain log for that leg; fix the handling step, not the film
Pouch is clear at the DC but fogs on the shelf	Retail ambient swing versus backroom cold storage	Consider an anti-fog-treated film or a film with higher WVTR at the shelf-exposed panel
Fog plus soft or slimy spots	Condensation sitting on product surface for extended periods	Move from plain micro-perforation to a moisture-management laminate or increase macro-perforation area
Inconsistent fogging across a batch of the same pack	Micro-perforations occluded by product or labels	Check label placement against the perforation map; inspect perforation count on affected packs
Off smell plus fog, no visible decay yet	Perforations blocked, pack has gone anaerobic	Treat as a line or label-placement fault, not a film spec fault

Where barrier paper fits, and where it doesn’t

Barrier paper is getting commercial attention as a plastic-coating alternative for dry or low-transpiration packs such as bakery, snacks and some produce overwrap, but it is not a drop-in substitute for high-humidity fresh produce film. Manufacturer-reported WVTR ranges illustrate the gap: EcoShield Barrier Paper falls in the range 0.61 to 0.69 g per hour per square metre, a comparable polycoated paper in 0.47 to 0.71, and commercial waxed paper in 6.5 to 6.9. Converted to the more familiar daily rate, that is roughly 14.6 to 16.6 g/(m²·day) for the repulpable barrier paper against 156 to 166 g/(m²·day) for waxed paper. The source does not state the test temperature and humidity used, so treat the comparison as a relative one between the three papers, not an absolute spec, and ask the supplier for the full ASTM or TAPPI method before you write it into a specification.

Typical WVTR range of paper-based packaging materials

For a pouch holding a transpiring vegetable at near-100% headspace RH, a repulpable paper in that WVTR range would let moisture out faster than a plastic laminate, which could help fogging, but paper’s much higher OTR and its poor performance under wet-crush conditions are reasons it is not yet common for mini cucumbers or bagged salad. The unit converter is handy for moving between g/(m²·day), g/(m²·hour) and gauge or micron thickness when you’re reading data sheets from different suppliers.

Material comparison for barrier laminate selection

Structure	OTR (cc/m²·day)	WVTR (g/m²·day)	Recyclability
PET/AL/PE	<0.1	<0.1	Not recyclable in most systems
PET/VMPET/PE	0.5–2	0.5–1.5	Not recyclable in most systems
Mono-PE with EVOH	1–5	1–3	Designed for PE recycling stream
BOPP/CPP	800+	3–6	PP stream where accepted
Kraft paper/PE	Very high unless lined	5–15	Limited, mixed material

These are typical structure ranges, and suppliers should be asked for OTR results under ASTM D3985 or ASTM F1927, and WVTR results under ASTM F1249, with the test conditions checked each time. None of these rows is a produce pouch structure as-is; they’re a reference point for how far apart common barrier families sit before you add perforation.

Common mistakes

Chasing a lower WVTR to stop fogging. A lower WVTR traps more moisture, not less. The fix for condensation is usually a higher, better-targeted WVTR or a separate moisture-absorbing mechanism, not a tighter barrier.
Reading an OTR number without its humidity. A dry-condition OTR figure on EVOH or similar barrier resins tells you little about a humid cold room. Always ask for the RH the figure was measured at.
Assuming macro and micro-perforation solve the same problem. Macro-perforation is a moisture release valve that gives up atmosphere control. Micro-perforation is an atmosphere control tool that barely touches moisture. Confusing the two leads to specifying the wrong one.
Ignoring label placement over perforations. A correctly specified film with an occluded perforation performs like a different, worse film.
Adding an anti-fog additive without checking food contact status with your own regulatory team. Anti-fog coatings are typically surfactant chemistries and migration behaviour varies by formulation; this is a decision for your quality and regulatory function with the applicable food contact rules, not a packaging engineering call alone. Where packaging waste or recycled-content rules affect your choice of film, check the current text at ppwreu.org rather than assuming last year’s requirement still applies.

What this means for you

Start with the temperature log, not the film, when a condensation complaint comes in. If the pack is genuinely under-specified, decide which job is failing: atmosphere control, moisture release, or both. Ask every film supplier for WVTR under ASTM F1249 and OTR under ASTM D3985, with the exact temperature and relative humidity stated, before comparing options. Don’t swap in a lower-WVTR film to fix fogging; that makes it worse. For high-transpiration items like mini cucumbers, a film built with separate gas and moisture mechanisms has published trial data showing a measurable quality improvement over single-mechanism macro-perforated or top-seal formats, but validate it against your own supply chain conditions rather than taking a grower’s commissioned test as a guarantee.

Sources

Values are typical figures from the cited sources. Real performance depends on the grade, the test method and the conditions. Check the supplier's data sheet and test your own structure.