Waterproof, “Waterproof” and Water Resistant

waterproof vs water resistant

With all the fabric advancements in the past years terms like waterproof and water resistant get thrown around a lot. But reality is such that not many products that use these terms are really literally waterproof. Waterproof as in I have dropped this backpack overboard into the sea with all my clean undies and my camera inside and I didn’t even flinch because I know water wont get inside. So to clear things up and explain what we think when we say that DryTide backpack is waterproof, I am going to divide all “waterproof” products into a few groups and explain what can you expect from each of these groups. And just to make things easier I am going to stick to bags and backpacks but this could be valid for just about anything actually – jackets, covers, sleeping bags, bivouacs, tents…

Why Regular Fabrics Won’t Stop Water?

First a short explanation about the fabrics in general. Fabrics generally aren’t waterproof. The reason for this lies in the construction – fabrics are woven together from individual fibers that even if they are woven really tightly leave some room in between for water to get in.

The width of these fibers is measured in denier. A single strand of silk is approximately one denier. So a 500 denier fiber is 500 times thicker that a single strand of silk and for instance  thinner than a 1000 denier fiber. The thickness of the fibers determines how waterproof the fabric will be. The reason for this is that even when the fibers are woven together very tightly, there are still pores in between them. The thicker the fibers the bigger these holes are and easier it is for water to get through the fabric.

Even if these pores are really small and water normally can not get through, with some added pressure water will eventually get in. Pressure can be a result of various conditions like really strong rain, underwater pressure, the pressure and friction created between different parts of fabric while wearing it etc… This is why even the best and most expensive materials used in top of the line products, like for instance mountain jackets, are not 100% waterproof. And they shouldn’t be. More on that later.

Waterproof Rating Explained

The amount of pressure needed to “push” the water through the fabric is usually measured in millimeters. The number is generated during testing when a 1’ by 1’ tube is placed over the fabric and then slowly filled with water. Once the water column get higher the water pressure on the fabric increases and the material will eventually starts leaking water. The height of the water column before the fabric starts leaking is waterproof measure for that fabric. For instance a 7K/7.000 mm fabric would start leaking once the water in the tube would reach 7m. Of course, these tests are made in ideal conditions.

So, to make the fabric 100% waterproof the only option is to have no pores in the fabric at all. The best way to achieve this is by laminating the fabric. The obvious downside of this is the lack of breathability. These kind of fabrics do not “breathe”, so when we get to active outwear producers must balances protection with breathability. But when it comes to backpacks this is not an issue. You need a really good backpack for wet conditions to be 100% waterproof.

Now armed with all this knowledge and aiming for that 100% waterproof goal that we can have with backpacks, I can set four different levels of backpack “waterproofness”:

Regular materials – level 0

For a really short time just about any material will protect the inside of a bag from moisture and water. So running really fast from your house to your car in light rain with a regular backpack on your shoulders won’t do any damage to the things inside it. But since nothing on the backpack is waterproof or at least water resistant anything more than that is not recommended. Also, with bags like these term waterproof is usually not even used.

Water resistant and waterproof – level 1

Here we have bags made out of waterproof or water resistant materials. I would put these materials into two groups: materials that try to stop water and have at the same time some other functions like for instance be at the same time breathable, look or feel a certain way, have requirements about weight, shape, processing, softness etc… and then there are materials that just want to stop the water. The first level of waterproofness is achieved with materials from the first group. Depending on the price, technology and choice of these materials they will stop water for a different periods of time. But in most cases after a while they will start leaking. Either because of the friction between the material that pushes the water inside, or they get less waterproof when they get a bit older and worn out, or they just simply lose their waterproof capabilities after they are exposed to water for a longer period of time. Since these materials are not 100% waterproof the bag production itself is also not 100% concentrated on making bags waterproof, there would be no real use in that (but more on that in the next waterproof level). These bags are ok for use in lighter rain and when a dry place is somewhere close, so you know you won’t have to expose these bags to the rain for a long time.

Waterproof – level 2

Here are materials from the second group. Materials with almost sole purpose to be waterproof. They will never ever leak water, even when they are exposed to it for a really long time because they have no pores in the fabric. You can literally leave the material in the water for a month and it will not leak. But…there is always a but. The type of material used when producing a bag is not everything when it comes to making a backpack waterproof. Backpacks are not made out of single piece of material and even if they would be there is the question of openings: main compartment, pockets… you need them to put your belongings into the bag. So what is the problem? All the common production techniques involve stitching of materials and stitching makes holes. There are various ways to later seal these holes but usually, after a while, you will get leaks.  So in level 2 I have put backpacks that use waterproof materials but also use stitching and possibly sealing for construction and they use regular zippers. These backpacks work good also in heavier rains and can be exposed to rain for a longer time, but eventually water can get in. Also, dropping them into the water is not a good idea.

100% Waterproof – level 3


Here we have the ultimate. Materials that are 100% waterproof and production technique that does not use stitches to put the panels together when producing a backpack. So what is used instead of stitches? The process is called Ultrasonic Welding. This is a more complicated and relatively rare construction technique. The machines used for ultrasonic welding are expensive, and they are not operated by your average sewing machine employee. The machine uses high level of ultrasonic sound waves that are directed into the fabric, these sound waves are then transformed into heat which causes two layers to bond together and form a 100% waterproof seal. No needle holes. Since this production technique is expensive it is only used for bags that really must be 100% waterproof. So in this last group we have backpacks that you can literally throw into the sea and let them float for hours and the inside will still be bone dry. You can hike all day through torrential tropic rainstorm and everything inside the backpack will be dry. You can go rafting, you can go kayaking, you can stand in the middle of the river fishing with your backpack sitting in the water and everything inside will be safe.

Waterproof backpack floating on water

DryTide backpack, you can throw it into a lake.

The Reality of 100% Waterproof Bags

Unfortunately stitching and construction is not everything. Backpacks need openings, how else would you put your belonging inside the backpack. This is where we come to the real problem. Even with ultrasonic welding and 100% waterproof construction water will try to get in through the openings and closing these openings and preventing the water to get in is the hardest part.

Regular zippers are the worst, they are easy entry points for water to get in as they have small holes in between the zipper teeth. An improvement over regular zippers are water resistant zippers that have fabric protection over the zipper teeth so raindrops can (ideally) not get close to the zipper teeth. But in real life, if you face stronger rain, after a while water starts getting through. If you put these into the water,  water will get in pretty fast.

roll down and zippers

30L backpack uses both roll down top and waterproof zippers

The Fold Down Closure

Right now the best technical solution to close waterproof bags is the fold down closure. You fold the top of the bag tightly 3-5 times and then buckle the ends together or to the side of the bag (see the full waterproof backpack instructions here) which prevents the fold from unfolding and the backpack is sealed. The truth is that even though all waterproof bags use this system it is still not 100% waterproof. Most of the manufacturers, shops, even users will claim it is, but honestly it is not 100%. This system is simple, malfunction free, you can not mess it up, and it works well in 99% of conditions. The 1% occurs when you leave your bag floating in water for a really long time with fold down closure floating underwater or if you force the bag underwater yourself. Waterproof bags trap air inside once you close them, this air helps them float on water so usually they will stay on the surface. But if they are submerged the water will slowly get in through the fold down. It is not a drastic flush of water that would instantly soak your things, it’s a slow process that also depends on how tightly you rolled the top of the bag. So if your backpack falls into the water don’t worry – system like this gives you plenty of time to save your belongings or even allows you to use your backpack in the water. This is just a warning that none of the dry bags are 100% waterproof in all conditions. Still, this is as close as we can get to 100% waterproof without breaking the bank and making these bags insanely expensive.


There is another closing system that is however not very popular. Partly because it is expensive, partly because it has other downsides. It uses special zippers that are designed not to let the water in. There are two types of these zippers.

The more affordable ones, but still expensive, are rainproof zippers, which we are also using in our DryTide backpack outside pockets. You will notice that closing this zipper feels harder and tighter than with regular zippers. This is because the extra pressure makes the zipper much tighter, closing both parts together and stopping the water. These zippers will withstand heavy rain without much problems and are much more reliable than the above mentioned water resistant zippers. These zippers are so good that they will also survive a shorter drop into the water. If you leave them in water for a longer period of time or if you push them underwater  they will slowly let water get in. The last option are 100% waterproof zippers that also work underwater. Using these would make the backpack 3 times more expensive, so we opted against them.

The danger with using zippers is that, if you are not careful and you don’t close the zipper all the way, it won’t do you much good. Even a small hole at the end of the zipper will let in more water that the fold down system in 10 hours of floating on sea.  This is why we have designed the DryTide backpack using both closing systems. The big main compartment uses the fold down system that is more practical and fail proof. The two external pockets on the other hand use rainproof zippers. This makes DryTide the most advanced and innovative backpack out there.

When Do You Need 100% Waterproof Backpack?

Do you really need a 100% waterproof backpack? It is as much a matter of needing it as it is a matter of the peace of mind. There are countless situations and sports where you really do need a waterproof backpack. But there are even more situations when owning a dry backpack will give you a peace of mind. When you are transporting expensive cameras, your laptop, your phone, your money, papers, or even just dry clothes, you want to be sure that your belongings are safe. As long as there is a chance of water getting in, you will be nervous. Even if the chance is small. I know I was. My backpack was at the back of the pickup truck and it was raining hard on the way from Padang airport to the ferry port. Even though the backpack should hold the water out it was not a dry backpack and all I could think about the whole drive was “I hope all my clothes aren’t soaking wet when we arrive.”


Running underwater with waterproof backpack on the back

When do you need a waterproof backpack?

Having a 100% waterproof backpack is knowing that is better to be safe than sorry. It is knowing that even if your backpack falls into the sea, you can just simply pick it up and go on as if nothing happened. It is an insurance that you don’t need until you need it :).

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