Waterproof: The Most Overused Word in Outdoor Gear
Waterproof is one of the most overused and misunderstood terms in the outdoor industry. In this post I’ll break down what waterproof actually means for bikepacking bags and tramping packs – from fabrics and zips through to seam sealing – so you can make smarter buying decisions and have realistic expectations from your gear.
Do You Actually Need a Waterproof Bag?
If we take bikepacking for example – your sleeping gear and spare clothes are best kept completely dry. Generally, the best place to store these is on your handlebar. A dry bag strapped into a Heaphy Harness is a great option here and will ensure you can get a warm night’s sleep when you need it.
For both tramping and bikepacking, it’s worth thinking about what you’re actually carrying. Tools, cooking equipment, and packaged food can handle getting wet without any real issue. Electronics these days are generally fairly water resistant and would be fine in most bags. The things that genuinely need to stay dry – sleeping gear and clothing – are better protected with a lightweight dry bag inside your main bag than by relying on the bag itself being waterproof.
When buying waterproof bags you also need to consider the abrasion resistance – as soon as you get a small hole in the fabric, the bag will no longer be waterproof. Therefore, if you do need to keep other items dry then smaller lightweight dry bags not only make packing your bags easier, but act as a second line of defence.
For bikepacking I use a dry bag strapped into a Heaphy Harness to keep my sleeping bag and spare clothes completely dry. I then use lightweight dry bags inside my other bags as an extra layer of resistance if I am particularly worried about the gear inside getting damp. For tramping I use a pack liner or lightweight dry bags which double as pack organisers and help keep my gear easily accessible.
A yellow NZ Mountain Safety Council pack liner inside a Stealth tramping pack on the trail. A simple and effective way to keep gear dry without needing a fully waterproof pack.
Are Your Zips Actually Waterproof?
The term ‘Waterproof’ for zips is thrown around far too loosely, especially by some smaller outdoor gear makers. A lot of people describe a coil zip that has a PU coating as waterproof, and this is certainly not the case. The only thing keeping the water out it is a less than 0.5mm layer of PU coating which comes together to touch from either side at the zip closure. Add a bit of stress on the zip and it will soon open a gap for the water to go straight through. The slider will also let water through as the zip is not closed at that point.
Several big-name manufacturers that use these PU coated coil zips in fully welded bags and call them waterproof. Sure they will keep a lot of water out, but they are not waterproof. You can get truly waterproof and airtight zips but they are very expensive and require bonding the zip to the fabric, by means of high frequency welding or seam taping, so the water cannot get through the seams. You cannot assume that this additional process has also been completed. I am yet to come across anyone using airtight zips on bikepacking frame bags.
The zips I use in my frame bags have a PU coated tape and Nylon teeth which are offset to prevent water ingress. While not waterproof, they are very water resistant.
Coated Fabrics vs Laminates
There are two main types of fabrics used for bags made for the outdoors – coated fabrics and laminates.
Coated fabrics like Cordura and Ultragrid are woven fabrics with a coating that has been applied to the fabric as a very thin layer. The coating is generally on the inside of the fabric. For Cordura or Ultragrid the coating is PU (polyurethane), and generally on the inside of the fabric. Depending on the thickness of the coating, the coating can make the fabric very water resistant – but generally if enough pressure is applied water will seep through. The measurement of waterproofness is called Hydrostatic Head. To test Hydrostatic Head (HH) a 1 inch tube is capped with the fabric at the bottom and the tube is filled with water. Water is added until it starts dripping through the fabric – the measurement of the water level up the pipe in mm is the rating. For example, Ultragrid has a 1500mm HH rating and 500D Cordura is around 350mm HH.
Laminate fabrics such as Ecopak, Ultra and X-Pac are generally made up of multiple layers and laminated together. The Ecopak EPX200 fabric consists of a layer of 200d woven polyester, 45 degree cross ply yarns, and a 70d ripstop backing. There are multiple thin layers of waterproofing between the laminations and according to Challenge EPX200 is waterproof and airtight to 200psi (PSI is another common measurement of waterproofness).
Both coated fabrics and laminates that I use also have a DWR (Durable Water Repellent) coating on the outside face of the fabric. This is a coating which is applied to the fabric so water that lands on the fabric will bead and run off rather than being absorbed into the yarns. This coating can wear off over time but there are a range of products sold at outdoor stores which can help rejuvenate the coating.
Seam Sealing
In addition to considering the types of fabrics used, it is important to examine how the bags are seam sealed. There are three common methods for seam sealing – Pressure Sensitive Adhesive (PSA) taped seams, hot air seam sealing, and High Frequency (HF) welding.
PSA tape such as Challenge’s TNT seam tape is essentially a specialised reinforced sticky tape that is applied to a seam after sewing. The tape is pressed into the fabric, and the bond is formed using the adhesive on the fabric. This method is best for flat seams where the tape can sit flat against the fabric. If you try and go around a corner or curve the tape will form creases (i.e., small tunnels), creating a possible point for water entry.
Hot air seam sealing uses a machine to melt adhesive on a tape that is then fed through rollers and pressed into the seam to be sealed. Hot air seam sealing is most commonly used on lightweight drybags. This method can go around corners and is a lot more malleable than the PSA tape. It generally creates a good seal.
Both the PSA tapes and hot air sealed seams can be affected by moisture, dirt, chemicals and UV and therefore have limited lifespans.
HF welding is a construction method rather than seam sealing. It usually requires the use of a TPU coated fabric. HF welding uses heat created from the vibrations of high frequency radio waves to melt the TPU coating on a fabric, whilst pressing the area to be joined together. Generally, HF welders are very large machines and require special dies for each piece to be welded. Unfortunately, this method is not feasible for the small, typically bespoke and local manufacturers.
Most of the bags made by Stealth are unable to be made waterproof, due to the type of seams and fabric, but methods are taken where possibly to try to prevent water ingress. You will find this is the case for the majority of custom-made bike bags. However, I can manufacturer custom dry bags made from laminate fabrics and PSA taped seams which are highly water-resistant.
The Bottom Line
So, to bring it all together – whether your gear needs to be truly waterproof depends on what you’re carrying and how you’re doing it. For most bikepacking and tramping setups, a combination of water-resistant bags and lightweight dry bags – or a simple pack liner – for the critical stuff is a practical and durable approach. If you do need full waterproofing, understanding what goes into achieving it – the fabrics, the zips, and the seam sealing – will help you make a better buying decision and have realistic expectations about longevity. As always, feel free to get in touch if you have questions about any of the materials or methods I use in my bags.
