In reply to Andrew Lodge:
> I was always told to never wash anything you want to stay waterproof with detergent.
Detergents wont damage your clothes, they'll just mask the DWR coating. There is an industry that has sprung up around misinformation when it comes to cleaning waterproof clothing. In reality if you put your gtx jacket through the wash with detergent and give it a couple of good rinse cycles it will still be waterproof and if it's relatively new it will still be water repellant. The waterproof layer is a membrane in between the outer and inner, unless you're detergent is sulphuric acid or has razor blades in it you wont destroy the waterproof membrane. Any remnants of detergent left on the jacket will hamper it's ability to bead up and theoretically reduce the breathability. Washing in tech wash and tx direct or similar will give you the most optimal results, it will clean it without leaving impurities and then enhance a worn down water repellant layer making it a little more breathable and prolonging the time before the outer layer wets out.
I have to admit I have never done side by side comparrisons with reproofed jackets where one has had the detergent drawer cleaned out before washing and the other hasn't. I vaguely remember asking the nikwax rep that when I was younger and he didn't think it was important. I'm inclined to agree.
My own opinion is that if say you're wearing a 10 year old GTX jacket with a pack pressing on your back, shoulders and hips and the odd duct tape repair, when you're struggling through a wild hoolie and the wind driven rain is dancing up your sleeves and down your neck the precise level of breathability doesn't really matter as much as the salesmen make you think. It'll keep most of you more or less dry and what more can you ask?
Soap flakes (disolved) are just as good as tech wash, cheaper and a bit more environmentally friendly. I don't know why they still sell it so diluted. I stopped reproofing my gtx jacket years ago and it still keeps me pretty dry.