How We Think About Storage.. The Storage Spectrum. Is Storing Pu’erh at a Constant 70RH in a Box OK?

Pumidors are bad.
Sealed storage is bad.
No airflow is bad, you’re suffocating the tea.
Too much airflow is bad, you’ll suck out the aroma.
Your tea is drying out.
Heated storage is bad.

Other than a few select controversial posts, storage articles tend to be subject of vigorous debate and polarized opinions. In my humble opinion there is often too much spoken absolute certainty without leaving room for nuance. It’d be dumb to weight every possible storage setup equally, but I do think it’s good to be open to a wide band of possibilities working. Certainly when it comes to something as uncertain as storing tea in the various climates of the west. Once you get past the do’s and don’ts storage should be thought of more of a range.

A few cakes in storage
A few cakes in storage.

Storage is Really a Range

There are some fundamental truths to storing tea of course. Strong smells will ruin it. Storing cakes in a bathtub will not be good for your tea. But once we agree on the basic fundamentals it’s also likely that a range of setups will not ruin your tea and turn out varying degrees of OK. In other words, storage is not really black & white. People’s preferences of certain storage (some people prefer Malaysia, others may like Kunming) is also subjective and complicates matters. This all goes to say that there is no one way or path when it comes to storing pu’erh in the west.

I think it’s pretty unfair to also loop all pumidors into the same categorical dismissals. If someone pushes their pumidor in an attempt to recreate HK traditional like conditions, should we treat the person storing it in the low 60s RH as unsafe? There’s also the assertion that it involves constant fussing that I find to be inaccurate for at the bare minimum of myself. I suspect the goal of most is to create something passive, and not to talk our tea to sleep every night.. My own setup requires very little effort beyond spending around $20/yearly on Boveda packs and putting them in.

Take Marco’s hotbox storage, which in his initial experiments stored tea at 32C (90F). I’ve seen some people dismiss this setup categorically. Maybe you’re not sold on the long-term effectiveness of raising the heat to the levels he’s tried..But would you feel better if the tea was being set to 24C (75F)? My guess is that many (myself included) would. To me this indicates that people aren’t as skeptical about the hotbox storage setup itself.

Judging Solely Off of Smell Can Go Wrong

One aspect I think gets overrated is judging how awesome your tea is aging off of smell. I think this approach is appealing because it’s intuitive and if you’ve stored tea in an enclosed space, the smells can get quite wonderful when temperature and/or humidity are at certain levels. Tea will usually smell more fragrant with higher heat and higher humidity. My stash certainly smells better in summer than in winter. Increased fragrance is seen as a net positive, but I’ve come to think of it as more of a mixed signal. Smelling fragrant means your tea is aging (this is good!), but it also could mean your humidity is edging up. An intensely aromatic stash can also serve as an important warning sign. Tea storage is about balance. You want your tea to age, but you don’t want to ruin it either. It’s important to make sure your parameters are safe and that you are not just chasing after a more fragrant smelling box. Do yourself a favor and make sure you’re measuring the conditions your tea is living in and take the necessary precautions.

Storage Containers
Storage Containers.

Is a Constant Measured 70RH Safe in an Enclosed Box?

One thing that is underrated and should be discussed a lot more is the actual parameters the tea is being stored at. I think we should be spending more time criticizing, arguing and tweaking the numbers people are working with to a fairly granular degree…

Is 70RH safe? I think this is an interesting question to think about when considering your options for storing tea.

One mistake I see people just setting up their pumidor doing is copy/pasting the outdoor humidity conditions from HK, Taiwan or Malaysia and assuming it is ideal for their own stash.The logic is easy to understand. But trying to exactly replicate humidity can significantly increase the risk. If you were to average the relative humidity of these places year round, you’d end up with 78RH for HK, 81RH for Kuala Lumpur, and 77RH for Taipei. Even with a slight reduction in humidity, if you based your parameters off of those you’d likely probably end up concluding you want your tea to be at least at 75RH.. But if you took that number and stored tea in an enclosed pumidor box at that humidity. You’d get mold and it would probably not take very long…

Why is that? Anecdotally it is true that pu’erh is successfully stored at humidities 70 or above regularly in Asia. But the setups are inherently different. Maybe the most important trade off in a pumidor is lowering natural ventilation and airflow in order to increase the humidity. This change in setup makes a difference even when the parameters might be similar. In my opinion, it’s better to look at what are generally considered acceptable parameters for people storing tea in a similar way as you.

Two Seattle Storage Setups

One of my most regular tea buddies is Garrett. He lives around a mile away and I walk over to his place regularly for tea. He even has the same wine cooler as me (we both got it at Goodwill). We have been dealt the same situational hand. But I’ve bought a lot more pu’erh than him which leads us to significant differences in how we store tea..

Garrett has under ten cakes and he bumps up his humidity using soaked terracotta shards whenever his tea gets beneath 70RH. When I ask about his humidity, it is often between 70-75RH. This is far more aggressive than me. I have a large stash and keep my tea around the mid 60s RH and will only add more humidity if it moves beneath 62RH. If it gets much above 70RH for a few days I’ll usually try to open my pumidor for an hour or two. He can push parameters more because he’s naturally opening up the wine cooler and checking his cakes because he drinks from them, whereas I can easily go a few months before opening up a pumidor. It can be argued that despite pretty different humidity levels both our approaches make sense and may even represent similar risk levels. Situation beyond location matters too.

Humidity (+/-3)

The useful dew point calculator puts the mold risk for 70F/70RH at 163 days to mold. Bumped up to 70F/73RH and mold risk comes out to 64 days or about 2.5x as fast. 70F/67RH shows no risk. While the dew point calculator should not be taken as the holy grail, it does emphasize how a few points of humidity can make a significant difference.

I recently revisited some of the parameters being used by long-term pumidor users in my pumidor survey a few years back. One thing that surprised me is that many of the users stored their tea at 70RH up to 72RH mold-free. While the sample size is small, it certainly seems possible to store your tea at the low 70s RH. But how risky is it?

  • I’ve seen shade casted on hygrometers before. It is true that these are not always accurate. But they are inexpensive and measuring is far preferable to guessing, even with a margin of error. Calibration is also not that difficult, and is far easier than cleaning up any sort of mold disaster. My advice: calibrate and use multiple hygrometers in different spots of your storage.
Hygrometers. Important tools, but need to be calibrated.

Speed. Humidity vs. Temperature

One final aspect to consider is speed. People willingly storing pu’erh at higher humidity are doing so to increase the speed of aging. But what it this is altogether unnecessary? The new thing in the west is heated storage i.e. Marco’s hotbox (I’m all for illicit substance references). HK/TW/Malaysia are not only humid but they’re also much hotter than most of the west. These heated experiments in the west are still very early and the jury is still out, but at bare minimum they do seem to advance the tea fairly quickly. Raising temperature is also less likely to raise mold risk, compared with raising humidity. This adds appeal as an alternate, safer way to speed up the aging.

The Margins & My Answer

I do think in theory with a bit of regular opening up, airing out, and checking the edges the box 70RH is safe for a hobbyist that exercises caution. If I were to give it a score, I’d just 70RH is maybe 70-75% safe, whereas 65RH is like 90% safe. I also would not store my tea much higher than 70RH. For me, this added risk is not worth the reward and effort of checking my tea and I’d much rather keep my tea centered around the mid 60s RH for my situation. I think we too easily lump 60-75RH as the same storage when there is a huge difference between the two ends of it. I think the risk tends to be overstated at the lower end of that range, and the risk is underrated at the upper end.

So why am I not comfortable storing my tea at a constant 70RH? Storing tea at around 70RH might be safe in most scenarios but it also moves the tea a lot closer to a danger zone. We know that a few points of humidity can make a large difference. How confident are you that your hygrometer is reading out correctly? How often do you check on your tea? It’s quite common for a hygrometer to be off by over a few points of RH. What if a corner of the pumidor is a bit more humid and you get hit with condensation. Or there’s a spike and it unknowingly goes up to 75RH while you’re out of town? Keeping the tea a little lower adds a buffer zone and makes me feel a lot more comfortable about avoiding the moldy downside of storing pu’erh.


12 responses to “How We Think About Storage.. The Storage Spectrum. Is Storing Pu’erh at a Constant 70RH in a Box OK?”

  1. James,

    I think we are in sync regarding storage parameters. I have stored tea like that for several years without problems. I have also seen definite progression in my pu’erh teas over this period. My main concern is keeping the teas alive and in good shape, not aging them quickly. To that end I have purchased teas in various ages to drink now, later, and hopefully we’ll into the future. I think having a strategy in place for consumption as well as storage is the key. You need to be buying the right mix of teas then storing them in a way that they progress into what you want over your life.


  2. James,

    This is one of the most balanced and practical articles on puerh storage I’ve read. It contains much solid advice that is well backed up with an open mind to the possibilities of puerh storage. As such, I doubt this will spur much debate but rather contains many truths based on both experience and good science. Great job!

    I have a handful of upcoming articles still in rough draft on puerh storage that touch on various aspects of the above article. I should get to publishing these soon…

    Much Peace

  3. I tried a lot, watched a lot, noticed a lot and read a lot about this subject and I also came to my personal best solution when it comes to storage. First and foremost it really depends on where you live. If you are lucky and live somewhere in Southeast Asia you don’t need to think so much about it but if you live in Europe or generally within a Country where there are more longer dry periods and all four types of Season then you should think about whats best for you or let me say what works best.

    I did a lot of tests and also watched friends with their humidors and how those (Pu-erh) teas developed over a longer time period. To be honest I am not a fan of using a shared open storage within a broken fridge or wine cooler plus humidity bags etc. The reason is simple: Everything is fusing together and they all become one thing with less differences. I mean of course after some rinses you will discover a difference between Sheng A and B but not as much, as if you would store them separately without interfering with each others profile and character.

    For me I am too picky and I want each and every tea to be experienced as they are and should be. I don’t want aromas mixed up. I want diversity.

    Next thing: Whole cakes or parts/pieces? First and foremost for me the best solution storing tea in non Asian / none rich in humidity type of Countries is a doybag/doypack/airtight zip bag. I store each and every tea within such zip bags (like those ones Yunnan Sourcing and all the major tea shops use) I even use them for whole Liu An Baskets (but big zip bags).

    I did many tests and it is really strange but those ones I broke into pieces and stored in such bags aged the best and are still until this day rich in aroma and taste. But the same tea I kept as a whole cake (also in a big zip bag) smell dry and are weak in taste and aroma.

    Another good way are clay caddies but they need too much space and are too expansive. But on a long term basis for me the best way is to store parts and pieces or completely broken off cakes within a doypack and those I store inside a closed big cardboard box with nearly no odor to it. Because everything should stay as neutral as possible to not interfere with those teas character.

    For me this works the best and the good part is that those bags not only keep the character and profile full intense and original they also age well in such a storage way.

  4. Great article, James. I’m definitely on the small scale and low effort side of storage: a couple boveda packs in the winter when it is at its driest, pushing 65rh at the highest with temps between 65-68F.
    I do both enjoy the hot and humid summers in the US-ian South because my pu tastes so good, but do worry about this 75rh at 70-80C indoors. I do leave the container with my pu open just a crack that time of year. Made it through the first summer with no mold and only good tea. Here’s to hoping for many more. I’d hate to have to worry about bringing the humidity down in the summer as well as up in the winter.

  5. Something I’m not seeing here is a discussion of the “relative” in %RH. To snag the first few sentences from Wikipedia:

    Relative humidity (RH) is the ratio of the partial pressure of water vapor to the equilibrium vapor pressure of water at a given temperature. Relative humidity depends on temperature and the pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air.

    Because the equilibrium vapor pressure of water increases with temperature, a given %RH is not a measure of how many grams of water are in a kg of air (unless you also specify the temperature). Even worse, the relationship between temperature and vapor pressure is nonlinear (the vapor pressure of water in air increases faster at higher temperatures).

    To put it slightly differently, at 20° C and 100% RH a kg of air will contain about 15g of water. At 30°C a 100% RH kg of air will contain about 28g of water. But a kg of air at 75%RH and 20°C will contain about 11g water and a kg of air at 75%RH and 30°C will contain 21g of water. The “distance,” in g H2O/kg air, between 75% and 100% RH, gets bigger with every increment in temperature… at 20°C your kg of 75%RH air only needs an additional 4g of water to be at the dewpoint, while at 30°C you have to add 7g to get there.

    To put it Yet Another way: at lower temperatures, a system at a given %RH requires a smaller perturbation (in absolute humidity) to get it to 100%RH than the same system does at a higher temperature.

    Also, you mention quietly what I think should be shouted loudly: OUTDOOR WEATHER STATION DATA IS NOT THE SAME AS INDOOR CONDITIONS, EVEN IN NON-AIRCONDITIONED WAREHOUSES. My observations in Florida suggest that non-A/C rooms with at least 1 story above them will be ~5°C cooler than the weather station data, and have RH about 10% less.

    • I guess I would ask what are the practical implications of looking at relative humidity this way that you see? That you’re margin of error is lower when your tea is being stored at lower temperatures?

      As far as outdoor conditions, I agree and have talked plenty about it in the past including this post.. People who just look at outdoor climate data to determine their own indoor storage are foolishly not seeing the full picture. (point 5)

      • Sorry about the delay in replying to this.

        I guess I would ask what are the practical implications of looking at relative humidity this way that you see?

        Here is what my storage conditions in coastal S. Florida look like over the last 36 hours. Notice that the RH ranges from 55-73%, while the temp varies between 77-82°F. The current reading of 68% @ 77°F messes up my argument a bit: the RH is high (for the temp) because it’s raining right now. But the ranges are relatively constant, and if you were to watch this device over the course of a day without rain, you would find that the low temps are at night, and the low RHs in the daytime. They’re inversely correlated, that is. The actual grams of water/cubic meter of air are much closer to constant.

        People who devise regulation schemes for pumidors tend to look at that outdoor weather station data, see the extremes of RH and temperature, and try to make both of them at or near their max values (already too high, because outdoors) simultaneously. In this way they wind up with a lot more water in the air than they want, if what they want to is reproduce tropical or semi-tropical (depending on which region’s weather station data they’re looking at) conditions.

  6. The debate goes on and I really do enjoy the ongoing dialogue and sharing of techniques and views around storage.
    I have to say that I have my storage pretty much stable around 68% humidity with seasonal ventilation. Thus far I have avoided any enclosed box storage but have experimented with some zip bagging or wrapping which I still remain open minded about.

    I think storage techniques and parameters will always remain somewhat subjective and for some my 68% humidity may seem dry, however I will state that a 10 year old traditionally stored 7572 that I’ve recently dipped into has retained its qualities well under such conditions.

    The Marco Hotbox

  7. I finally set up a pumidor — a small fridge. Placed a bunch of cakes in it.
    I’ve noticed a few days later that the smell is dominated by the wet stored cakes, which have a strong stoage smell on them. I wonder if this is going to ruin the young green cakesI have there? Will they absorb the funky wet storage smell?
    Iwonder if people tend to store various types of sheng together, or separate by storage type.

    • I’m in general not too anal about it, but it’s not a bad idea to store similar cakes on top of one another.

      If your fridge is like 80% traditionally stored cakes, I’d expect it to kind of all take on that smell though.


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