I just finished an experiment on my Patagoina Houdini which significantly extended its waterproofness. The weight increase and breathability decrease weren’t measurable. I used McNett Silnet on the inner top seams and zipper fabric.
Water repellency- is a fabric's ability to make water "bead" and roll-off. Effective DWR is desirable in a fabric as water soaking into the outer fibers will inhibit breathability. DWR coatings can and do wear or wash-off (they can be renewed or re-activated) whereas waterproof inner coatings or membranes will to a greater or lesser degree last the lifetime of the garment. In the case of the Patagonia Houdini, the fibers are encased in silicone so the DWR doesn’t need reapplying for the life of the garment and there is no waterproof inner coating or membrane.
Waterproofness – is a fabric's ability to prevent water from passing through the fabric under force. The Nextec Epic 4 Summit Nylon has a waterproofness rating of 1.8 PSI. The Patagonia Houdini material seems to have a similar PSI rating.
Misting rain averages up to .4 PSI of force on the fabric. Drizzling rain averages up to 2.6 PSI of force on the fabric. Because of the requirement to pass 5 CFM of air, the Houdini’s pore size will only block misting rain from passing through the fabric and block heavy drizzle for an extended period before it starts to pass through. My objective was to extend the time period I could use the windshirt in heavy drizzle.
I put on a long sleeve absorbent shirt and the Houdini. I then rotated slowly in the shower until I could feel moisture pass through different areas of the windshirt. The only areas that moisture passed through were the top seams and the zipper (very heavy leakage). The arm seams and side seams never leaked because the shower head pressure didn’t reach those areas.
I used McNett seam seal on the inside top seams and all of the zipper fabric. This allowed the same extended shower time without experiencing any water pass through except for very minor leakage through the zipper teeth. The downsides are that you can easily see the inside Silnet coated seams through the shear fabric (looks like the fabric is soiled) and the Silnet seams do not feel as pleasant against bear skin as untreated cloth. I cut out the labels and so there was no increase in weight as a result of applying seam sealer (besides that the labels were a leak point).
I take multi-month expeditions to Alaska. I carry a WPB suit in addition to my windshirt. The DWR wears off my WPB suit after 30-50 miles of abrasion from the pack straps and under brush. By extending the range of use of my windshirt, I am trying to reduce the use of my WPB so as to extend its DWR.
Has anyone else done something similar to their windshirt? If so, were the results worth the effort?

