From The Best Fireproof Document Safe | Reviews by Wirecutter:

Humidity can be an issue with these safes, as they tend to capture and hold in moisture, which can damage their contents.

Someone commented:

Is this really as good as it gets? Is the humidity concern a consequence of its fire rating or are these poorly designed? I would have appreciated if this was addressed in more detail because the amazon reviews are scathing and has me deeply concerned about this purchase. I don’t want to have to maintain my safe, I want to put stuff in and not touch it for months or years. Surely that’s possible?

I replied:

I think the humidity concern is a direct consequence of water resistance and the safes being made out of relatively thermally conductive materials (e.g. metal). The safes are roughly air/water tight so it's more likely that the moisture in the air inside the safe (slowly) condenses into liquid water over time. Or maybe moisture seeps inside the safes (via air) but, for some reason, can't similarly 'escape'?

Am I right? Or are these safes somehow acting as (slow) 'water pumps' and causing the humidity inside to increase over time? Or is there another explanation?

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    $\begingroup$ Welcome to Physics! I'm not sure this is a great fit for us, though; maybe Home Improvement would be a better fit? $\endgroup$ – Michael Seifert Feb 24 at 19:57
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    $\begingroup$ Anything that is air-tight will cause trapped moisture to condense as the ambient temperature changes the temperature inside. I don't really see what else could be particularly special about fireproof safes. $\endgroup$ – DKNguyen Feb 24 at 21:05
  • $\begingroup$ I would have explained it more or less the way you did. Otherwise one would have to recourse to the mental picture, that evil safe manufacturers design their products out of a motivation like "scheduled obsolescence", which is pretty ridiculous considering the fact that a safe is no safe if it's not safe. $\endgroup$ – oliver Feb 24 at 21:29
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    $\begingroup$ Some safes have a moisture retaining material in the walls. The idea is, that in a fire the water boils off (latent heat), giving the documents extra time before they catch fire. This moisture reduces the chance of fire damage but increases the chance of mold. I have such a safe, and it always smells a bit musty, but nothing has ever molded. It's not, though, comparable to condensation in an airtight box -- it's significantly worse and always smells musty. $\endgroup$ – tom10 Feb 24 at 21:53
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    $\begingroup$ This question is the subject of a discussion on Meta. $\endgroup$ – rob Mar 3 at 23:42

Anything that is air-tight will cause trapped moisture to condense as the ambient temperature changes the temperature inside. I don't really see what else could be particularly special about fireproof safes.

  • $\begingroup$ So – if a fireproof safe, or any (roughly) air-tight container, was stored in strictly controlled conditions, e.g. no changes in ambient temperature, we wouldn't expect any condensation? Or just much less? Would the container's material significantly affect our expectations? $\endgroup$ – Kenny Evitt Feb 25 at 21:32
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    $\begingroup$ If the outside conditions never changed I wouldn't expect it to be a problem. $\endgroup$ – DKNguyen Feb 25 at 21:33
  • $\begingroup$ Would the, e.g. thermal conductivity, of the material matter if the ambient temperature does change? In other words, would some materials (ignoring their own ability to absorb moisture) result in less condensation than others? $\endgroup$ – Kenny Evitt Feb 25 at 21:37
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    $\begingroup$ @KennyEvitt Thermal conductivity of the material would only slow down the change that would ultimately occur. Like a thermos. So if it was short, or the insulation was really good, you might be able to ride out a change in ambient with reduced internal changes until the ambient returned to what it was previously. $\endgroup$ – DKNguyen Feb 25 at 22:17

Fireproof safes hold a lot of moisture so that when the safe is exposed to fire, the boiling off of the retained water keeps the items inside protected from extreme heat (until the water boils away). A downside is that permanent extra water is that it keeps that contents of the safe permanently damp (unless the safe is frequently aired out).

That is, they use the latent heat of water to buy some time in a fire, hopefully so the safe can outlast the fire. As is probably obvious, all "fire proof" safes are only fire resistant, and some say that this boiling of the water is the primary factor in their time limit, and some more accurately label them as "fire resistant" (because you can't fight thermal equilibrium forever).

So, hopefully, this makes the answer to the question clear: "Why do fireproof safes “capture and hold in moisture”?" It is because the contents of these safes are surrounded by a moisture holding material, which over time is in thermodynamic equilibrium with the interior of the safe, raising the humidity to much higher than normal relative humidity and more extreme and consistent than normal condensation. In general, the water-holding material forms a shell around the safe, and if the interior were completely water tight the contents could remain dry, but in practice, this isn't the case (see item 6 in final paragraph for a possible reason).

I know this from the documents and experience with safes I have used, just as a normal user, but I was interested to find out a bit more (eg, I've always wondered with the musty safes, "can they really not do better than this?").

That this approach is used in modern safes, is clear, for example, from this statement: "There is a good reason for this. In order for a fire-safe to successfully resist fire, the special composite material sandwiched between the outer and inner steel walls gives off moisture. This creates steam inside of the safe."

But I wanted to know more about this, especially, how is this water held around the contents, and what is the nature of these "special composite materials", so to find that (without a "handbook of fireproof safe design"), I searched for old and relevant patents, and here are some interesting facts:

This concept was in use before this patent (pdf) written in 1868.

People also used pipes around the safe to hold the water, but that had it's own problems, as described in this patent (pdf)

Here is an example (pdf) from 1869 using plaster of paris to hold the water. I am happy to stop with this one example, but I also presume that there are more modern "special composite materials" for holding moisture.

Finally, a note on condensation: The other answer here and multiple comments bring this up so I'll address it directly and explain why it's not relevant, in my opinion. 1) By "condensation" I assume people mean that when the safe is opened and subsequently shut, humid air from outside the safe is trapped within, and then can cool down and release its moisture. 2) It's important to note that this is a small amount of moisture, at most a few grams, and would be easily managed in the normal way, which is by using a desiccant. 3) Yet, a desiccant pack won't work in a fireproof safe. 4) This condensation problem would not be any worse in a fireproof safe than in a regular safe or a sealed box. Yet the moisture problem is much discussed for fireproof safes and not for regular safes or boxes -- it's not that it doesn't exist for boxes but that everybody knows the solution for boxes (ie, desiccants). 5) It's also worth noting that paper is quite absorbent itself, and a box full of paper should act as its own desiccant, and can absorb a reasonable fraction of its own weight in water without issue, so more relevant is the ambient humidity (an infinite source of moisture), not condensation (a small source of moisture). (But this isn't relevant for gun safes). 6) People suggest that fireproof safes could be air tight, but this would risk getting into a vapor lock situation where if the safe was opened when the safe was colder than the air, then on closing the air would cool down inside the safe lowering the pressure, making the safe truly impossible to open other than by heating. (That is, you can't pry up the corner of a safe like you would a box, you'd need to fight against the pressure difference over the entire door.)

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    $\begingroup$ The Laws of Physics are perfectly happy to let steam reach temperatures above 350 degrees fahrenheit. $\endgroup$ – alessandro Feb 26 at 18:44
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    $\begingroup$ @alessandro: What you are referring to is from a quote within my answer. I assumed that the 350F was due to a pressure valve, or something like that, so the safe didn't turn into a bomb. I am only using the quote to show that people still use the latent heat of water as a mechanism to make safes fireproof, and I think it makes that point clearly, but you're right in that the 350F bit is confusing, so I took out that part of the quote. $\endgroup$ – tom10 Feb 26 at 19:14
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    $\begingroup$ @KennyEvitt: The water around the safe has a huge impact on the dampness within. I've experienced it, as well as the top quote in your question, and other people who review and talk about these. I think theoretically it would be possible to keep the moisture separate (like in the pipes patent I list, or other ways), but in practice, these safes are damp (and I assume that varies with cost and convenience too). Furthermore, I think most people have used well sealed bags and boxes, and the dampness for these is generally far less than with a fireproof safe. $\endgroup$ – tom10 Feb 26 at 21:08
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    $\begingroup$ I think that the review is a bit misleading. The point of concern is condensation. It does not really matter if the air inside get more humid, the point is condensation. A reiteration of condensation / evaporation cycles can wash out an indefinite amount of ink, just to say. By this point of view a very tight safe is worse than a leaking one, but only if closed in a day with high absolute humidity. $\endgroup$ – Alchimista Mar 4 at 17:26
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    $\begingroup$ @Alchimista: Your comment is helpful to me and has brought some clarity. Thanks. I have been assuming a framework where the fundamental "point of concern" is relative humidity (which then creates mold and the mold damages the documents). You are saying that the fundamental point of concern is condensation because the condensation results in fading or washing of the documents. Is that a fair summary of your point? (Condensation and relative humidity are related, but I want to think about these more -- mostly because condensation is complicated with absorbent materials in abundance). $\endgroup$ – tom10 Mar 4 at 18:20

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