TPU Coating – In or Out?

Early versions of the DIY Packraft plans can be used to make a packraft with the heat sealable TPU coating on either the inside or the outside of the tubes (see the Fabrics page for definitions of the abbreviations used here), but later versions of the plans and kits (“V2” and later) are designed to be made with the TPU side of the fabric facing out.

There are pros and cons to each approach, depending on what kind of fabric you are using and how you plan to use your packraft. Consider:

Heat sealable TPU coating on inner surface of tubes:

  • Woven nylon layer protects the airtight TPU membrane from puncture
  • Bottom layer of floor can be made of same pieces of fabric as side tubes, leaving no seam between the tubes and floor on the outside of the packraft
  • Woven nylon will soak up water if fabric has no waterproof coating on other side, making it heavier after paddling
  • Fabric threads are exposed to abrasion and may weaken over time

Heat sealable TPU coating on outer surface of tubes:

  • Slippery TPU coating provides abrasion resistance and waterproofness
  • Tie-downs, spray decks, and other accessories can be attached easily by heat-sealing to the TPU surface
  • Air pressure could separate TPU coating from woven fabric layer over time if it is not a high quality TPU coating
  • If TPU coating is breached, air pressure will be lost even if fabric is not punctured

I have tried both TPU-in and TPU-out configurations and, like the best packraft manufacturers, I have settled on the TPU-out configuration.

 

Note that any extremely lightweight inflatable boat is bound to develop leaks regardless of how it is put together; even the best name brand packrafts fail regularly, especially when used in moving water, but the nice thing is packrafts are easy to repair. It’s a trade-off between weight and durability. If you want a tougher boat, you’ll have to make it heavier. If, on the other hand, you are prepared to deal with the fact that by paddling over submerged rocks and wood you will occasionally put holes in your boat, then you can get by with a very light packraft.

Finally, note that the thickness and quality of the fabric and coating is just as important as whether the coating faces in or out. Fabrics in the DIY Packraft shop are custom made to our specifications with packrafting in mind; some fabrics available elsewhere have such a thin coating that they’re not even airtight, so it’s important to test samples before you decide where to purchase supplies.

4 thoughts on “TPU Coating – In or Out?

  1. “I have samples of all of Seattle Fabrics coated fabrics, so if you have questions about them, feel free to ask (most of them are unsuitable for packraft construction).”
    I live near Seattle and am planning to visit the shop what am I looking for in unsuitable materials

    Heat Sealable 30 Denier Nylon Ripstop
    30 Denier 3 oz/sq yd. Heat sealable like the pack cloth, oxford and taffeta, but this is super light weight. This is the lightest heat sealable fabric that we have come across. With a “kiss coating” on the face and heat a heat sealable coating on the back, this super light weight ripstop is useful for applications where extremely light weight fabric is required, but a high degree of water resistance is necessary.

    or
    70 Denier 4.3 oz/sq yd. Heat sealable like the pack cloth and oxford, but this is super light weight. Useful for applications where weight is a factor, but a high degree of water resistance is necessary.

    Any suggestions or comments on the materials that I should be looking at

    1. Hi Ken,

      The white 30 denier is what I used to make my first packraft and it worked well. It shrinks a small amount when heat sealed, so a few wrinkles are unavoidable, but it’s still a good option. I chose it over the black 70 denier because the TPU on the 70D didn’t seem to be bonded as strongly to the nylon, and the 70D didn’t have that PU coating on the other side (that coating gives a second layer of airtightness and also stops the fabric from absorbing water). If you plan to do much river paddling where you will scrape up against sharp rocks and sticks, I would choose a heavier fabric.

  2. Will the TPU side of the fabric heat bond to the non TPU side of your 210D fabric? Or do both fabric sides have to be TPU?

    Carl

    1. Hi Carl – the TPU only forms a strong bond with the TPU-coated side of the fabric. It will bond weakly to the non-TPU side, but the bond is not strong enough to rely on.

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