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What should I look for in a dry bag that will maintain its hydrostatic head (stay waterproof) forever?

I use a ~20L dry bag for a few reasons:

  1. To keep my electronics dry in my pack
  2. To wash my clothes
  3. To carry water from streams

So when I say "waterproof" I mean that it should both [a] keep water out and [b] keep water in.

I've owned a couple dry bags that were made of some thick nylon. Since nylon isn't waterproof, it's laminated with Polyurethane (PU). The problem with this is that after some years of abuse, eventually the PU delaminates and the dry bag doesn't hold water anymore.

photo of the outside of a green dry bag photo of the white inside of the dry bag with the TPU has worn away in some places
This 4-year-old, 20L Sea To Summit "Lightweight Dry Sack" is made out of 70D nylon. The nylon material is intact and has held-up well to years of use The inside of the same dry bag has a white TPU coating with a 10,000mm hydrostatic head. This waterproof lining has delaminated from the nylon, making the dry bag no longer waterproof.

This is curiously limited to dry bags: I've owned an MSR dromedary bag (which is a 1,000 denier nylon that's laminated with PU) for over a decade, and the PU has not delaminated. Why?

Are there any dry bags that are designed with 3-layers: a waterproof PU layer wedged between two nylon layers? Or is there some other dry bag material that's naturally waterproof and therefore doesn't require any waterproof lining that may fail in the future? What's different between the construction of a dromedary bag and a dry bag that makes dromedaries last forever while dry bags fail after a few years?

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  • See also pangolinswithpacks.com/… May 17, 2023 at 0:33
  • The MSR bag is probably seeing a lot less abrasion from just having water compared to a drybag with stuff in it. MSR is also designed for durability while S2S for lightness.
    – noah
    May 17, 2023 at 7:39
  • Check out dyneema dry bags from Hyperlite gear or other similar makers. But the fancier you go, the more it costs. If this lasted 4 years of constant use, I'd call that pretty good. Another brand is Watershed drybags, but they aren't lightweight.
    – noah
    May 17, 2023 at 7:40
  • @noah thanks for the recommendation. On quick check of Hyperlite's website, it does appear that their Dyneema "composite" fabric is, again, waterproofed with a polyethylene lining. My understanding is that it's not a matter of if a lined bag delaminates -- it's a matter of when. Do you have any recommendations on waterproof dry bags that are made from a material that's not a composite made with a waterproof lamination? Something that will never delaminate because it can't? May 17, 2023 at 13:54

1 Answer 1

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The most durable and waterproof drybags are heavy and stiff. Carrying excessive weights of water in yours is likely to have accelerated damage.

My large kayaking one (too large for regular use) is made of a sort of heavy duty tarpaulin fabric. The stiffness is good because when you roll it over it's incapable of creasing, so it handles immersion better than the light ones. It's a 20-30 litre bag, and easily strong enough to fill like a bucket, giving some idea of its strength. Unless using it as a full-size rucksack liner, you wouldn't really want to use it in another bag - it's just too stiff to pack down nicely. It's probably at least 4× the weight per unit volume of one like yours.

The ones I regularly use are coated, like yours. You can get better life by not using them for things with harsh corners, or wrapping those things to protect the bag. For the most sensitive things I double up: my kayaking first aid kit is packed in a drybag that then goes in an outer drybag with other emergency kit. That protects the outside.

Using yours to fetch water, if you come anywhere near filling it, will stretch the fabric. This may not be enough to start delamination, but will certainly cause it to spread. This is likely to be your biggest problem.

Water bladders are generally flexible plastic rather than fabric. They're quite easily pierced, and would abrade from the outside on rocks etc. They survive by being in a protective pocket in your backpack.

In your case I'd use a small drybag for gadgets, and wrap those in clothing inside it. I'd do that anyway for padding against knocks, but it will stop the corners of things like charging plugs from scratching the coating. Alternatively I often just use resealable (ziplock) bags for this. I'd fetch water and wash clothes in a dedicated bag, again smaller - a 20 litre sack of water is unwieldy around the camp. Personally I tend to have several litres of drinking water bottles or bladder and would use that for fetching, but have no need to wash clothes in it. This bag could be a more robust drybag, but some leakage might be acceptable there anyway, so a smaller lightweight one would help.

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  • Note that woven fabrics (i.e. nearly all fabrics) always need coating as the weave is inherently porous. The coating can be applied in different ways, as a lamination of a sheet, as a liquid, or even as a melt as used in some vinyl fabrics. The latter is probably the strongest, but the thickest and heaviest
    – Chris H
    May 18, 2023 at 13:43
  • indeed in my searches it appears that the less-lightweight bags are PVC-lined instead of TPU-lined. I have been unable to find any decent articles that tested the durability of vinyl vs PU waterproof fabrics over years of use. I think such an article would be immensely valuable. May 18, 2023 at 15:11
  • @MichaelAltfield my commuting bike pannier looks like it's vinyl-coated fabric, similar to my big drybag, though perhaps a coarser weave under the coating, and perhaps a thinner coating. The fabric has done pretty well (better than the main zip, which I've replaced) but is now tearing where the handle is sewn on; it's also got a few cuts, but I'd expect that a pannier is more exposed than a drybag - it's hit the ground a few times.
    – Chris H
    May 18, 2023 at 15:17

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