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Keeping Your Critical Gear Dry (Part 3)

Keeping Your Critical Gear Dry (Part 3)

by , May 19, 2015

Part 2 of the “Keeping Your Critical Gear Dry” series courtesy of Jim Wood discussed gear sacks as a potential method for protecting your important gear from the elements. In part 3, we’re scoping out silnylon sacks and oven roasting bags…

Silnylon Sacks: The Unfortunate Realities

Inspired primarily by the lightweight backpacking movement over the past few years, a number of manufacturers now offer waterproof sacks made from “standard weight” (30 or 40 denier) silicon impregnated nylon (“silnylon”). Ray Jardine also includes instructions for building waterproof clothing bags in his book Beyond Backpacking using this fabric, along with a design that’s essentially the same as the Golite Paddler sack mentioned above.

I believe that these sacks are worth highlighting in part because of their growing popularity, but also because I suspect that many backpackers have accepted the claims of water resistance that are being advanced by some manufacturers and are using them under the belief that they will adequately protect critical gear. Unfortunately, for the most part, they will not.

I’ve worked with silnylon for several years and have constructed dozens of gear items from this fabric. The most significant is the tent I now use – a design that includes two doors with zippers, mesh windows, awnings, storm doors and other fairly complex features. I’m also in the process of writing a separate article entitled “Silnylon: The Inside Story” that will discuss this material’s history, physical properties, manufacturing processes and legal issues. Further, in connection with this upcoming piece, I’ve conducted a series of water resistance tests on a wide variety of silnylon fabric samples, both new and used. While there remains more work to be done, I nonetheless feel that I have a pretty good understanding of this fabric’s properties and problems.


As noted above, silnylon is waterproof, but only to a limited degree. Any “waterproof” fabric will eventually leak if enough pressure is applied either to the water that’s attempting to penetrate from the outside, or to the dry inside surface of a fabric that’s pressing against an external layer of moisture (the case, for example, with a stuffed sleeping bag that’s pushing against the walls of a silnylon sack that’s wet on the outside). A fabric’s water resistance is usually gauged with a device such as this that’s used to measure a property known as the “hydrostatic head”. This factor reflects the maximum amount of pressure that a fabric can withstand before it begins to leak and is commonly expressed either as the height of the water column (usually in millimeters), or as the number of pounds per square inch of water pressure necessary to initiate that leakage.

Hydrostatic Head tester M018

As a point of comparison, consider that even the lightest of the commercially available polyurethane-coated nylon gear sacks have hydrostatic head values that usually range from 5,000mm to 10,000mm (note also that heavier-duty dry sacks intended for paddling use typically have much higher values). In contrast, the hydrostatic head of standard weight silnylon, according to most manufacturers’ specifications, is only 1 to 2 pounds per square inch, which translates (for consistency with industry conventions) into a range of 700mm to 1,400mm. In other words, standard weight silnylon, on average, is less than 15% as water resistant as even the lightest of the conventional coated nylons.

If you own a silnylon gear sack, you can perform a simple experiment to get a sense of its poor water resistance. First, make sure that the seams are sealed with a product such as McNett’s SilNet (more on seam sealing techniques below). Next, turn the sack inside out to best simulate real-world conditions, fill it with water, and watch what happens. You’ll probably find that even the minimal water pressure inside the sack will cause some leakage through the fabric. To roughly quantify the pressure, consider that water weighs 0.036127 pounds per cubic inch and if the water is say, 12″ high in the sack, the pressure at the bottom of the sack will be about 0.43 pounds per square inch (or ~305mm of hydrostatic head). But you’ll probably find that leaks also occur even near the top of the sack, where the water pressure is far less.

I recently conducted this experiment on a relatively new sack that’s made from standard weight silnylon. As you’ll see from the photos below, a great deal of water leaked through the fabric on even the top side of the bag, where the pressure was quite low. I should also point out that these results were not unique to this particular sack. I’ve hydrostatically tested more than two dozen samples of  silnylon from multiple sources, and they have all displayed similar characteristics.

keeping your critical gear dry


The silnylon hydrostatic head values noted above are for fabrics that are new. My testing suggests, however, that water resistance can deteriorate with use. As an example, I’ve used two commercial silnylon stuff sacks (made by Equinox) as food bags on and off for the past three years. When new, both were water resistant to about 1500mm, confirmed by testing identical sacks that have never been used. After only perhaps 20 days on the trail, their hydrostatic head values have been reduced to less than 550mm according to my measurements. They have become, in other words, not much more than splash resistant. These findings are generally in line with those of the parachute industry (for whom silnylon was originally developed), which acknowledges that silnylon typically becomes much more air-permeable over time, mandating the eventual retirement, for safety reasons, of parachutes made from this fabric. It should be noted, however, that the effects of various kinds of stresses, such as physical tension on the fabric and exposure to ultraviolet light, on the long-term water resistance of silnylon is an area that is deserving of more study.


With a thickness of only about 1½ mils, 30 or 40 denier silnylon is pretty thin stuff, a fact that can limit its durability when used in high stress packing applications (such as serving as a stuff sack for a sleeping bag). The resistance to punctures is probably greater when items are packed only loosely, since the fabric is under less tension, but it is still much lower than most other nylons.


To be even modestly waterproof, the seams of a silnylon gear sack must be sealed. To my knowledge, no manufacturer has yet perfected a method for sealing silnylon seams at the factory (though Sea to Summit may have developed a way to tape hybrid silnylon/polyurethane seams – see more below), mostly because silnylon is so slippery that almost nothing will stick to it. As a result, it’s usually left to the end user to perform this task, most often with the SilNet product noted above. Because of the thinness of the fabric, however, there’s often a fair amount of flex at the seams, which means that the user will usually need to apply multiple coats in order to form a durable seal. I’ve found that single coats of SilNet are typically insufficient and will usually allow water to seep through the seams.

Another issue is that most commercially-produced silnylon gear sacks are built using a single felled seam that runs the length of the bag, and one or more rows of simple seams that are used to join the circular bottom of the sack to the main body. Simple seams are probably used for this purpose because it’s difficult to sew the stronger felled seam in this situation (see above for a discussion of the differences between these kinds of seams). The problem is that simple seams allow even more movement of the sides of the fabric against the seam than do the felled seams, which makes them difficult to seal successfully. Because these bottom seams must bear the full weight of the sack’s contents (resulting in a lot of fabric stretching), the cured seam sealer will often break open with use. I’ve sealed and re-sealed the bottom seams of my silnylon sacks several times, but the sealer continues to split, so I’ve mostly given up on the idea of these bags remaining waterproof.

keeping your critical gear dry

A different approach, which I now use to build my own silnylon bags, is to eliminate the sewn-in round bottom altogether. Instead, I simply double the length of the sack over to form the bottom, then secure the edges with two felled seams. This design creates a slightly lower volume sack, but makes the seams much easier to seal. By the way, before leaving this subject, it’s probably worth noting that seams on silnylon stuff sacks are best sealed on the outside surfaces only. Cured seam sealer on the inside creates a high-friction surface that can make gear stuffing difficult. The sealer is also more likely to deteriorate as it rubs repeatedly against items that are inserted and extracted.


keeping your critical gear dry


Except for a new product that’s highlighted below, all commercial silnylon sacks that I’m aware of use simple drawstring closures that are not very secure for the reasons previously discussed. Even using the candy cane method of tying these drawstrings, the knots tend to work loose on silnylon sacks rather easily, primarily because the fabric is so slippery. As noted above, thick rubber bands tend to work better, but can still slip on silnylon.


For protecting critical gear, the bottom line is that although they might be tempting to use because of their low relative weights, gear sacks constructed from standard weight silnylon are simply not, in my opinion, up to the task (at least when used by themselves).

If you already have such sacks, however, it’s possible to improve their water resistance somewhat by using the treatment process described in a previous article. You will want, however, to apply the coating only to the outside of the fabric since it creates a high-friction surface when cured. Even so improved, however (I’ve already tried it ), such sacks are still not durable enough to meet my requirements. I wish it weren’t so, because like all backpackers, I’d love to save the weight.

Oven Roasting Bags

Oven roasting bags, such as those made by Reynolds are an interesting alternative to silnylon for lightweight waterproof gear sacks and seem to be developing something of a following among ultralight hikers. These clear bags, unlike their common household plastic cousins, are not made from polyethylene, but rather, are constructed from a heat-resistant blend of two nylons, according to a Reynolds spokesman. This material, even though it’s only about 0.4 mils thick, makes these bags quite strong for their weight. They are also much less “stretchy” than polyethylene bags, so they work better for containing stuffed gear like sleeping bags. These bags can even be sewn using a loose stitch (perhaps 4 or 5 per inch) to create drawstring casings, and if they’re punctured, can be easily repaired in the field using duct tape, which sticks well to the nylon surface.

keeping your critical gear dry

The Reynolds bags are available at most supermarkets and come in two sizes: large (16″ x 17.5″) and turkey (19″ x 23.5″). The large size bags weigh about 0.2 oz and are useful as general purpose gear sacks. The turkey size bags weigh 0.5 oz and are large enough to hold most down sleeping bags, and maybe a couple of clothing items as well. Another company, PanSaver, also sells oven bags in two sizes: 18″ x 24″ and 24″ x 30″. Their smaller bag is about the same size as the Reynolds turkey bag, while their larger size is probably spacious enough to serve as a pack liner.

I haven’t tried the PanSaver bags, but the Reynolds bags have a notable weakness. It turns out that the bottom seam is not nearly as strong as the nylon film itself and will split with relative ease. If you have one of these bags, you can perform your own test very simply. Just fill a bag with 4″ or 5″ of water, close the top, then shake it a few times. The bottom seam will most likely fail under even this modest stress. Likewise, I also found that about half my sample bags leaked water through these seams right out of the box.

If you want to use an oven bag as a waterproof barrier, I’d suggest that you strengthen and seal the bottom seam. Identifying this seam, however, can be a bit confusing, since these bags appear to have two lower seams. At the very bottom edge, you’ll note that the bag is closed where the two sides are joined. This closure, however, is not actually the bottom seam and can be opened with a little massaging. It’s formed during the manufacturing process and is just the point where the two faces of the bag stick together during the cutting operation. The real seam is located about ¾ inch above the bottom of the bag.

keeping your critical gear dry

You can reinforce and waterproof this seam in at least a couple of ways. One option is to apply a wrap of duct or 2″ wide clear packaging tape around the seam as shown above. Before applying the tape, however, you’ll want to either turn the bag inside out or cut the excess material at the bottom of the bag to just below the real seam.

Alternatively, the seam can be sealed using McNett’s Seam Grip, but you’ll want to turn the bag inside out before applying this sealer (otherwise, you’d need to seal three seam faces, rather than just one). It also helps to fasten the bag to the tabletop during this process using masking or other easily removed tape (duct or packaging tapes can be very difficult to unstick). After application, allow the Seam Grip to cure for at least 12 hours for the best results. When the sealer is dry, its surfaces will stick tenaciously to one another, so before you remove the hold-down tape, you’ll want to dust the cured adhesive with talcum powder to eliminate the tackiness.

Despite the interesting properties of these bags, I would nevertheless be hesitant to rely upon them as my primary means for protecting critical gear. They are still too easy to puncture for my liking and require seam sealing, which is not always 100% effective. For an ultralighter traveling in a fairly dry environment, however, and especially if supplemented with a pack liner, these bags are probably worthy of consideration.


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