The Definitive Guide that You Never Wanted: Foam Padding
It’s eight o’clock in the morning as you set out on your daily trek. A half mile hike through the concrete jungle followed by the wait. As you stand idly and enjoy the view of the city, your bus comes down the street. Being full, the driver looks indifferently past the small crowd waiting at the stop, apathetic to the fact that all of you are going to be late to work. On a bad day, a few more buses pass by before one finally has enough open space to stop for you. Already defeated, you step into the bus single file.
Fun fact of the day: “Sandpeople always ride single file to hide their numbers” — Obi Wan Kenobi.
Where were we… If you’re lucky, you have a seat on the bus. Otherwise, you’re standing for the next 30 to 40 minutes as the bus crawls through a packed tunnel into a city of 8.5 million people. Once you’re finally off the bus, you have to fight through crowd after crowd, weaving through the thousands of tourists standing (literally, they just stand there) between you and your office. Four blocks later, you’re finally at the office. Exhausted. You whip off your backpack. Plop into your desk chair with shoulders aching, finally ready to start the day. Rinse and repeat five days a week, every week.
One thing that can make this daily march a tiny bit more bearable is having a comfortable backpack. Several factors can affect how comfortable these backpacks are, including how well it fits on your body and the padding of the straps and back.
Before you go on… trigger warning for those with trypophobia. There is an image in the below section that might be a bit freaky. Anyways, when it comes to a backpack’s padding, there are only a few types of foams that are typically used.
- Open cell foam is a very soft and compressible foam that is often seen in high end self-inflating camping mattresses. Open cell foam is made by using an inflating compound, such as sodium bicarbonate, into a rubber mixture. The sodium carbonate releases gas that inflates the rubber when it is vulcanized. Open cell foam is essentially interconnected air chambers and will feel great at first contact. However, because it is so compressible, open cell foam is not great for backpacks that if they are carrying weight, which is all backpacks. Under the weight of a loaded backpack, the foam will compress to the point that it is ineffective at providing much comfort and padding. So while it is comfortable for lighter loads, it’s not great for backpacks with larger volumes. When shopping for backpacks squeeze the padded straps between your thumb and index finger. If it compresses to less than half the original width, it’s likely open cell foam.
- In contrast to the interconnected air chambers, closed cell foam is air chambers that are surrounded (or closed) by foam and not interconnected. Essentially, encapsulated pockets of air cells. Close cell foam, which is also called Ensolite, is more suitable for padding on the straps as it doesn’t compress as easily as open cell foam. Since the air cells are encapsulated, Ensolite doesn’t absorb water like open cell foam, which can soak up water like a sponge. Closed cell foam used to be an attribute of better backpacks, but is now very common. It comes in a variety of densities, typically ranging from 6 to 19 lbs per cubic foot. Some examples of close cell foams include EPDM (ethylene propylene diene monomer), which is used in cable jointing and many electrical applications. Another example people may be more familiar with is neoprene, which has applications in virtually every industry. Even knee braces I wear when I play basketball, because I’m old and broken, are also made of neoprene. Backpacks will frequently use a closed cell foam in the shoulder straps while lining the back with open cell foam to provide more comfort.
- Though closed cell foam provides better support for carrying loads, it is an expensive component to manufacture. To create, the liquid compound is poured into a flat frame to create sheets. These sheets are then cut down to the desired size and shape. Overall, the process is labor intensive and wastes a lot of material, driving up the costs. In response, compression molded foam was developed as a method to create closed cell foam. With compression molding, nylon is bound to polyethylene, and then thermally softened and compressed into the desired shape with a mold. The product is then cooled to solidify it.
- Now that we’ve seen the advantages and disadvantages of both open and closed cell foams, how can you get the benefits for both? Dual density foam utilizes both of them by having a layer of of open cell foam against the body (for comfort) with a layer of close cell foam to provide more support, providing comfort under both lighter and heavier loads. Many of Deuter’s backpacking bags (review to come in the near future!) use dual density shoulder straps and hip belts.
- Another type of foam that is often used in better backpacks today is EVA (Ethylene-vinyl acetate) foam, also known as PEVA (poly ethylene-vinyl acetate) foam. EVA, (C2H4)n(C4H6O2)m, sometimes also called expanded rubber or foam rubber, can vary in the amount of vinyl acetate used, which allows for various coplymers of differing densities. Other common applications of EVA range from use as a wax-like coating to drug delivery devices — the polymer has very low immunogenicity while still being non-biodegradable. It’s also used in other sporting equipment like boots, pads, gloves, and helmets. EVA is a very good materials to use for backpack padding as it is higher in density, durable, and springs back to shape against pressure. Some backpacks that use EVA foam include Boreas Gear bags, as well as the EVERGOODS CPL and MPL. We’ll have our hands on the Evergoods CPL at the end of the year, so be on the look out for a comprehensive review in a few months. They use a type of EVA called Zotefoam that has a more even distribution of density throughout the material.
TL;DR — Open cell foam is very comfortable but only under no weight as it is highly compressible, making it ineffective for backpacks. Closed cell foam has sealed air cells so it doesn’t compress like open cell, but doesn’t feel as great as open cell. Dual density foam takes both and laminates them together to get the benefits of both. EVA foam is a high density polymer that is durable and springy, the golden standard for backpack padding.
The next time you’re buying a backpack in-person, you’ll know what to look for in the padding. As always, it’s best to try on a backpack to decide whether or not you like the way it feels. REI has sandbags you can use in the backpacks to try out what it feels like fully loaded. Having a backpack that is comfortable can go a long way in making your trek more comfortable, whether it’s a daily commute or you’re tackling the Appalachian Trail.
Like always, please comment if you have any feedback or any additional questions!
Works referenced:
- White Mountain. https://www.whitemountain.com.au/backpack-construction/material-faqs.html. October 8, 2017.
- CGR Products. https://www.cgrproducts.com/open-cell-vs-closed-cell-foam/. October 8, 2017.
- Wikipedia. All of it. Again. October 9, 2017.
