Some questions before I start my cuben fiber backpack project

There are two type of tape for cuben. One is 1/2″ double sided tape used to attach two pieces together if you do not want to sew. The 1″ wide single sided tape is used to tape over seams after they are sewn to make them waterproof ( and it will make them stronger). Because of all the pieces involved in making a pack it becomes very hard if not impossible to waterproof. I would just sew it together as the double sided tape is hard to work with- if you miss-align something you’re stuck with it. I wouldn’t bother trying to tape to waterproof, I would just use a trash bag as a liner or one of the plastic liners sold by MLD and others.

Thanks for your reply jimmyjam! That clarifies a lot. People seem to tape and reinforce here and there so it got a bit confusing reading about it.

I’ll stick to sewing it together (and then maybe reinforce with tape)

Another question: when I made my little silnylon tent I seam sealed all the seams with silicone, couldn’t that be done to cuben too, just for waterproofing? I guess the tape helps the cuben from ripping though.

And lastly, I found some 34g/qm and some 50g/qm cuben. What would you recommend to use for a backpack?

Well, sewing tends to elongate under stress with Cuben (or the new Dyneema stuff.) Tape tends to creep over time and actually will loosen over about 10-15 years. I have had some I needed to simply remove. So, I would recommend both where any type of strength is needed.

I am not sure Cuben/Dyneema is the best choice for packs. It does not resist abrasion well. Corners, edges, odd shaped items will quickly abraid through the plastic film leaving it less than waterproof. Anyway, fabric for a pack is not the heaviest part. Straps, hardware, foam, any netting, stiffening/frames, etc, can weigh more than the pack body. An old Blast only weighs 4oz, but really needs features.

Silicone will not stick to Cuben/Dyneema. So, it can be difficult to seal.

If you’re working with pure cuben, bonding reinforcements and taping after is probably a good idea.  If you’re using hybrid cuben, and especially the 150D face stuff, sewing will be just fine.  Consider felling any vertical load bearing seams, which will eliminate any issues with stitch hole stretch.

Cuben can be sealed, inside or out, with aquaseal.

While not as sexy, have you considered DP X-pac fabric instead? Cheaper, similar weight, and easier to design with since stitch hole elongation isn’t as much a consideration. More waterproof to boot. The nylon face fabric will hold a little water in comparison though.

I just put the finishing stitches into a new pack a day or two ago that is constructed of multiple fabrics including Cuben Fiber.  I did my first, baste stitch using a fine apparel thread with a wide spacing to get everything lined up, then went over it with standard, heavy thread on the next pass.  I did not tape, nor did I use a felled seam.  If I was going to recommend one of those two techniques it would be felling however.  That makes a nice, clean finish, and will probably weigh less than tape.

Re. tape:

The tricky thing about taping over sewn seams is managing what carries the load.

As a seam is loaded, it stretches.  This is due in part to the fibers in the fabric and thread stretching, but also to various bends and curves in the fabric near the seam straightening out.  If you just casually lay the seam flat and tape over it, those bends will be trapped by the tape and won’t be able to straighten under load.  This means the tape will take the full load.  This isn’t necessarily bad, as there are many claims that taped seams are stronger than sewn, but sewing doesn’t suffer delamination over time as tape can.  And if the tape is doing all the work, it begs the question, why sew?  I once asked Joe at ZPacks that question and his response was, “in case the tape fails”.

In order for the sewn seam to share the load with the tape, you have to tension the seam before applying the tape.  Clamping is probably the most effective, but tightly stuffing the pack is easier.

If your only interest is waterproofing, consider sticking w/ sealant.  Cuben is a polyurethane (PU) laminate, so any PU (not silicone) seam sealer will work.  Do some searching for tips and tricks on using sealant.

Re. fabric:

This is ultimately up to you, depending on what you want from the pack.  Look at what other packs of similar size and design use.  Given your stated influence by the HMG line, consider copying their fabric selection.  Given your volume and use of frame, you have the potential for some heavy loads – I’d go w/ the hybrid cubens, especially on the bottom.  The funny thing about that, since the lightest  hybrid (50D) weighs 3osy, is that you’ll be in the same weight range as woven fabrics.