Keeping outdoor gear dry in the wilderness: update on Ultra-Sil dry sacks

Making sure that your spare clothes, pile, sleeping bag and food remain dry even if it rains the whole day or you fall into a creek is an essential part of wilderness safety. One of the best compromises between effectiveness, flexibility and weight reduction seems to pack the essential items inside one or more lightweight, waterproof synthetic sacks.

Last year I made a bit of online research about this topic. Back then, the most promising solution seemed to be the Ultra-Sil Drysacks by Sea To Summit (STS). There are 7 models, from 1 litre (weight 20 grams) to 35 (weight 65 grams).

Here is a detail of the internal seams and buckle of an Ultra-Sil sack (click on the image to see the larger version). Since a few comments (linked below) in the reviews of those sacks weren’t positive, I contacted Sea To Summit to know more. Here is what Aaron Newman, STS International Sales Manager, answered.

Strider:: the Ultra-Sil Drysacks looks like a great solution for serious wilderness backpacking. Besides quite positive reviews, however, I also read that, at least in ²⁰⁰⁵⁄₂₀₀₆:

"The bad news is that the new polyurethane coating seems to have added little incremental water resistance to the fabric over standard weight silnylon / Sil Cordura".

what do you think?

Aaron: A number of years ago, the designers at Sea to Summit added a polyurethane coating to the existing range of silicon fabrics being used then in our stuff sacks only. This was not only to add water resistance to the fabric, which any sort of addition to layers will tend to do, but as importantly. It also enabled us to add a seam tape which then gave us the ability to make a waterproof range of sacks, as opposed to only the fabric being water resistant.

Water resistant and water proof are two issues that need to be outlined firstly to highlight the difference between them - which is as you would guess significant.

The original products which were being referred to in the article were our silicon stuff sacks, and were never intended to be anything more than water resistant. When seams are sewn, of course what happens when you punch a needle and thread through fabric to sew it together, is that you create holes within which water can reasonably easily enter.

So our stuff sacks were only ever claimed to be resistant to water. With the addition however of a polyurethane “PU” treatment, it meant we could then add an additional PU tape over these sewn seams, in effect closing up the holes, and with the addition of our unique roll top closure, making a water proof sack.

It is also important to note that waterproof does mean a number of different things. Our belief is that, while definitely being waterproof, our Ultra-Sil Dry Sacks are not so waterproof that we are happy for them to be submerged. Having such a light fabric, and subsequent light layer of polyurethane means that the product should be used in lighter applications than our heavier duty products.

For this reason, we do offer many other dry sacks in our range, including a sack for us in true open-water scenarios, called the Big River Dry Sacks. These are heavier than the Ultra-Sil sacks, but of course on the upside offer exceptional waterproofing in much more severe scenarios.

So the PU coating as mentioned in the quote did add only a little incremental water resistance o the fabric, but also produced the lightest weight waterproof Dry Sack.

Strider: Another thing I’d like to know is what is the best way to repair the bags, should they be punctured in the field: duct tape or what?

Aaron: We have worked closely with INVISTA, owners of the brand CORDURA, to develop the Ultra-Sil fabric and offer consumers the best chance of a product resisting damage, be it from punctures, tears and abrasion.

But we all know that from time to time, even the best products come to grief at the hands of hard users and abusers! So my advice to repair a silicon product in the field is to have in your repair kit, a tube of silicon sealant, such as Sea to Summit’s Ultra-Seam-Sil or similar. This is a flexible silicon sealant which adheres to either the inside or outside of a silicon product. If a roll of duct is all you have in the repair kit, then that will also do the trick, but either application, it is best to ensure it is applied to the inside of the sack for better adherence and protection.

Strider: What about cleaning? I remember days when I inadvertently put my stuff bag on the side of very resinous trees or spilled gravy over them…

Aaron: Avoiding contact with unknown substances is always the best option, but I will add that most spills can be cleaned up simply with water. The silicon finish on the exterior of Ultra-Sil makes it very hard for anything to adhere. If plain water doesn’t work, I would carefully use hot soapy water as a secondary measure. If these methods didn’t work, it may be time to farewell the product and be thankful it didn’t happen to your expensive brand name jacket!

Strider: How long do these sacks last? See for example this “got worse with use” comment at Backpacking Light.

Aaron: Like anything, Ultra-Sil products will offer you plenty of useful life with the appropriate care. Just like the tyres on your car or bike however, with continuous use they will offer you faithful service, but one day will wear out. If you can also think of Ultra-Sil products being in a different category than something like our Big River products - with the Ultra-Sil benefit of being fast and ultra light weight products. Whoever might make them, Ultra-Sil will tend to have a shorter life in exchange for being much more lightweight and transportable while they are in service. Big River on the other hand is constructed with much harder scenarios in mind!

Strider: So the final recommendation is…

Aaron: Considering their weight and design, with Ultra-Sil Dry Sacks you get a very dependable and worthy product when used correctly. Filling it up with water for example is not recommended, and if you are looking for a product to transport water, then check out some of our options designed for that. Some of the tests seen on websites where filling a Ultra-Sil Dry Sack full of water confuses me; the sacks are not designed to do this, while our Kitchen Sink or Folding Buckets are perfectly suited to such tasks.

Strider:One last question: what are exactly the differences with your eVent dry sacks? What are the technical differences, and which sacks are better for which scenarios?

Aaron: The two products series are slightly different. The eVent Dry Compression Sacks and the eVac Dry Sacks both blend the benefits of a light weight dry sack with compression, that is reduction of the amount of excess air via straps or other means. Previously there were two other options, one being using non-waterproof fabrics, where the air could escape through openings in the product or the fabric itself, such as our Ultra-Sil Compression Sack. The alternative was to use dry sack with a valve to expel air from. The downside of this was that the valve was often heavy, and would break.

So we came up with the principal of using a waterproof sidewall fabric and a breathable base fabric, where water could not enter either fabrics. In other words, they were waterproof, however, the base fabric would expel air, just like a technical water proof jacket or pants expel vapour when you wear them. This eliminated the need for a manual valve!

The first version of STS eVent compression product used a round base and compression straps to help force air outwards. This is perfect for sleeping bags, clothing, or other bulky items. We have since added a lighter version without the straps and changed the base to oval instead of round, which adds a versatility to packing in backpacks.

The sidewalls use a slightly heavier fabric than the Ultra-Sil - which adds to the overall weight of product, but the range of STS eVent sacks are still significantly lighter than standard dry sacks.

Strider’s conclusion

Personally, I can only agree with the general principle that the lighter your gear is (unless you’ve spent a fortune commissioning it to NASA) the more you must be careful with it. In the field, I keep sacks like these inside the backpack or the tent 95% of time, so I’m fine with sacrificing some robustness for lightness. “Waterproofness” as defined above by Aaron is also enough for practically all my own hikes, including winter ones. Durability may be a more serious issue: besides wasting money, buying a synthetic product that doesn’t last long means to pollute more than absolutely necessary. I’ll report more about this if/when I’ll have personally tested these sacks in the field: in the meantime, of course, you’re all encouraged to add as a comment below your answer to this question: “are your STS Ultra-Sil Dry Sacks still waterproof (as explained above, and if used as explained above) as when they were new?”