good advice!
a cheaper option (but the same thing!)
use soap flakes (any brand) it usually says suitable for breathable materials.
(
DO NOT use washing up liquid or normal washing powder/liquid , it will simply wash off any proofing!
then when rinsed well yes
Nikwax or
Grangers re-proofer for Synthetic coated materials
(check on-line you might be able to get a free sample!)
after applying tumble dry on a warm heat, (not hot!) this helps "set" the re-proofing.
after doing this (and when dry) water should bead and fall off.
when you get it mucky again, wash it down with a damp cloth. if it's bad wash in
soap flakes you might not need to re apply re proofer for a while!
(as a guide I was told by a Berghaus rep(when working in Blacks), jackets in serious mountain use should be washed after use, and re-proofed after around 10 weeks) so for the average user Bivibag re-proofing perhaps should be done once/twice a year if that!
If you really don't like washing kit make sure you sleep on a mat or sheet of some sort to keep the bag clean.
oh and don't eat/drink whilst in your bag!
as a final note, Goretex works best in cold wet/windy conditions, as your heat pushes the water vapour through the fabric.
in hot humid conditions you will find sweat will not pass through the fabric as well. so it can appear to be leaking or not breathable.