A few years ago, I bought a gun safe. When I did I noticed that there is very little accurate information on fire ratings. For that reason, I thought I would write this post to share my opinion on the subject. Before I start, I have no experience testing gun safes (though I have several years experience testing electronics) however, I also do not work in the gun safe or other safe industry. With that in mind here it goes. The safe review guy (by the way if you are buying a gun safe read his site, it is excellent) correctly says that gun safe fire proofing can be split into three categories. The categories are drywall, ceramic fiber, and cement. He also says that cement is most effective followed by ceramic fiber with drywall being the worst. However, he uncharacteristically does not back up that opinion well likely due to the fact that little information is available. In my opinion he is correct but I would like to take the time to show this to the best of my ability. To do that I used excel to perform heat transfer modeling on all three types of safes. There are two basic effects which keep the inside of a safe cool in a fire. These are insulation effects (high thermal resistance or low heat transfer coefficient) and evaporative cooling. Both cement and drywall rely on both while ceramic fiber relies on only thermal resistance. What happens in a fire – ceramic fiber In a fire, assuming the safe is well sealed, the safe will quickly reach thermal equilibrium. This means that the temperature of the outside wall of the safe is the same as the fire and the interior is what it was before the fire. The temperature inside the wall of the safe (inside the ceramic fiber) is in between (at the midpoint it is half way between the fire temp and the inside of the safe temp). At this point the heat will slowly transfer through the wall and this effect can be calculated readily if the outside temp, inside temp, and thermal resistance of the insulation are known. I did this for a safe with 2.5 inches of ceramic fiber insulation and discovered that in a 1200 degree fire at ideal conditions the safe would stay under 350F for 38 minutes and at 1500 degrees for 27 minutes. If you want the details of the analysis I can provide them. What happens in a fire – concrete The concrete example is a bit more complicated. In this case the heat from the fire will attempt to heat the concrete. However, once it gets above the boiling point of water the water contained in the concrete will evaporate which will cool the concrete. This effect will continue until no more water is left. Once that happens the safe will start to heat beyond the boiling point of water far more quickly than the ceramic lines safe (since concrete does not insulate as well). To model this phenomenon I assumed that the safe had 2 inches of concrete all around would heat evenly all around. I then assumed that each infinitely thin layer of water contained in the concrete would evaporate evenly until all the layers evaporated. This results in the first layer evaporating very rapidly since there is very little insulating it from the heat of the fire. However, as more water evaporates the heat has to pass through more of the concrete which insulates the water resulting in less energy reaching it. This means each layer takes longer to evaporate as the heat penetrates further into the safe wall. This sounds complicated but it results in the average water layer being 1 inch (half way) in and I can assume all of them are. Using this logic I discovered that in a 1200 degree fire a 2 inch lined concrete safe would use up its supply of water in about 3 hours 45 minutes. After this the safe would last a few more minutes due to the insulating properties of concrete. At 1800 degrees this time drops to around 2 hours. Drywall is similar to concrete but has much less water. 4, half inch layers surrounding the safe lasted 7 minutes in a 1200 degree fire. How do I make my safe last longer in a fire? Concrete safes do not have enough insulation to benefit an there is not much that can be done inside the safe to help. This is not true with ceramic fiber lined safes. In these the contents matter greatly. This is because a fire basically causes heat energy to transfer through the safe walls and this will cause whatever is inside to heat up. This means the more stuff in there the more energy needed to heat it up and the longer the contents will last in a fire. One last note, it occurred to me when I was doing all this that the best fire resistant safe would have concrete on the inside and a ceramic fiber layer on the outside. This would prevent heat from getting to the concrete layer and any that did would be dissipated by the evaporative cooling of the water in the concrete. There might be issues of the water evaporating into the ceramic fiber and causing it to lose its insulating properties. If ceramic fiber was used on the inside and concrete on the outside the safe would not last much longer than the concrete alone but would have even more moderate temperatures on the inside for a while. I later found out media safes are made this way. Hopefully this helps someone but if anyone wants more details or has questions I’ll answer them as best I can.