TL;DR: sous vide bags suck and shouldn’t be used for filament storage, and past two weeks, genuinely offer no advantage over just leaving your filament out on a shelf.

So anyway, here I am trying to figure out JUST HOW moisture permeable my sous vide filament storage bags are, because it sure looks like they’re ~useless for filament storage.

Let’s back up a little – the concept that a plastic bag that holds water might be permeable to humidity is a little weird, but it’s totally a thing. Different plastics have different water vapor permeability. If you’ve ever seen reusable silicone kitchen bags like Stasher brand – I found out the hard way that silicone in particular is EXTREMELY moisture permeable. I like to store cut avocados in my fridge, sometimes for days at a time if I don’t have breakfast a few days in a row. When I store half an avocado in a regular ziploc, it comes out days later just about exactly how it went in – nice, moist, and not even very brown if I left the pit in. However, an avocado half stored in a Stasher bag gets dry and rubbery after just two or three days. It’s basically drying out in the fridge as if it were just left open on the shelf, because silicone is just SO permeable to water vapor.

So what if my sous vide bags are JUST like that? My theory now is this: sous vide bags are too moisture permeable to be any good at filament storage, and pulling a vacuum on the bag isn’t particularly necessary in the first place. In fact, what you ought to be doing is using metallized, moisture barrier bags with decent desiccant, and not worrying about evacuating the air at all.

I ran some quick numbers just to get a sense of how much moisture could possibly be in a bag that you didn’t vacuum evacuate. I’ll assume that there’s a liter of air left in there, which I feel is generous. If we also assume that the ambient air is at 75 Fahrenheit and 70% relative humidity (also pretty generous), that works out to about 15 grams per cubic meter of water in the air, or about .015g trapped in the bag with our nice dry filament. From the CNC kitchen experiment linked in the previous part, Stefan’s PETG lost 1.4 g out of 692.4 when dehydrated. 0.015/1.4=0.0107, so the moisture trapped in the bag as you seal up your filament on a warm humid day is worth about “1% waterlogged” on a not-full spool. And that’s ignoring that the desiccant you put in there will pull much of that out before the filament gets a chance to.

I devised an experiment to see: I’d put one equally dry desiccant packet (moisture indicating of course) alongside a logging hygrometer in each of two bags: one sous vide bag fresh out of the box, and one metallized bag from my polymaker spool. I squished both down so they contain approximately the same amount of air left, but I did NOT pull vacuum on them. The idea is that what little air remains in the bag should be quickly dried by the desiccant, then over time any moisture that does enter through the walls will eventually saturate the desiccant. Whichever bag leaks more should saturate the desiccant sooner, resulting in an earlier rise in humidity than the other bag.

The experiment began in the evening on 8/8/25:

Note that even moments after filling the bags, the desiccant is doing its work and the sous vide bag shows 45% while the ambient humidity is 66%. Luckily, the metallized bag doesn’t block bluetooth so well that Home Assistant can’t hear both sensors, so we’ll have live logged data throughout the experiment.

Right away the results were interesting. Ambient humidity is plotted in red, blue is in the silver bag, yellow is in the sous vide bag. The silver bag starts out a little lower in RH but the two bags quickly converge as the desiccant does its work. But even just an hour in, the silver bag starts pulling away, dropping faster than the sous vide bag in RH. We don’t necessarily expect RH to be monotonic here because the temperature isn’t constant, but it turns out that it is for quite some time.

After two weeks, the picture is quite clear:

The silvered moisture barrier bag is definitely letting in SOME amount of water vapor, but it’s easily maintaining RH below 15% after hitting a low of 10%. The sous vide bag on the other hand is asymptotically approaching ambient humidity, and in recent days as temperature has gotten high and ambient RH low, the two lines have nearly touched.

So it seems safe to say the sous vide bag is quite moisture permeable in comparison to the moisture barrier bag which is doing its job pretty well. If you want the raw data, here it is in slightly inconvenient format:

This raised a side-quest question which was: how do other bag types compare? I know sandwich bags store avocados better than stasher bags. What about partly-silvered ESD bags (the normal kind you usually get electronics in)? I added some bluetooth sensors to the fleet and prepared some more samples.

This time, I used smaller desiccant packets hoping for a faster response. You can see this one is quite saturated (it’s blue when dry). The results were quite swift and conclusive.

Right away there are some obvious conclusions: The esd bag starts off more humid than the sandwich bag but quickly overtakes it and stays there, so I’m comfortable saying that it’s less moisture-permeable than the sandwich bag.

The stasher bag, in contrast, starts off INCREASING in humidity, despite the desiccant, until ambient temperature (and humidity) start dropping. But no matter – after a few days, all three bags have converged on ambient humidity and only barely stay below from that point forward.

Conclusions

1. Sous vide bags are a CRAP way to store your filament. They don’t actually block moisture outside of a couple days, and vacuuming out your bag prior to sealing it accomplishes almost nothing besides taking up less space in a box.
2. Moisture barrier bags are much better, but you really do need desiccant for them to work. For long-term storage, I’ll probably put two or even three packets in to make sure humidity stays acceptably low for months or years.
3. Stasher bags are very bad for storing stuff that should stay moist.

Next Experiments

1. How do real-world storage scenarios compare?
2. How does dew point work in practice?
3. Maybe make a chilled mirror hygrometer