It is to be expected that a heavily used jacket with a #3 main zip, like the 2.1 year old Rab Xenon pictured here, will have zipper failure within the useful life of the garment. While manufacturers continue to use these zips on weekly-use pieces, for reasons of weight, cost, and pliability, repairs will be necessary.
Some makers will replace the zipper. I’ve had good luck with Patagonia doing this under warranty. Anymore it’s too much of a headache to be without the jacket for weeks. With some coats it’s worth replacing the zipper. On something like the Xenon picking seams would be a nightmare, due to thin fabrics and the appropriately tight and sturdy stitching. The jacket has a number of ember holes in it already, and the Primaloft One probably has until the end of 2014 until the insulation is shot. Thus the easy, if messy, answer: turn it into an anorak.
First, zip the zipper shut. It’s ideal to do this conversion when the zip is not so far gone that this is no longer possible. Pick a zipper height; I like 16-18″ from chin down to end.
Sew a tight bartack across the zipper from seam to seam. This one is 2mm wide with a .8mm stitch length.
Thin thread is a good match for thin fabrics. I used 100% poly embroidery thread.
Next, sew over the zipper seam on both sides, nailing the upper to the draft flap. No draft flap? Make one from some spare fabric. Double stitch both sides. finish with a bartack at the bottom of the open side to take the main force when pulling the anorak on and off.
Finally, cut the zipper out, carefully. Fuse the sides, very carefully, with a lighter. As can be seen above, this often goes a little wrong with uber-thin fabrics, but this isn’t a big deal with synthetic insulation.
Continue wearing. It’s ugly, but using Primaloft for a town coat is foolish, due to the way it accelerates loft degradation. Get a wool, fleece, or soft shell for that purpose.