Making a bivy sack

Daniel–just throw in the Tyvek on a regular spin cycle? Detergent?

It was a while back, I think it was just a normal cycle, cold water, no detergent.

You probably also want an older style top loading washing machine with an agitator. I think it's the agitation that you want to soften the tyvek. I have one of the newer front loading washing machines now, and I suspect they would not work very well for this (though they are great for washing a sleeping bag).