an actual waterproof jacket question

I have two of these hooded rain jackets that are almost identical. One is from Anti Gravity Gear, and it is pure black sil-nylon. It is a bit oversized, so that contributes to ventilation from below. It is fairly waterproof, and it works for me.

One other one is near-black, and it is home-sewn from Pertex. It is not quite so waterproof and it breathes better. I wear it more around town where death from hypothermia isn't so likely.

–B.G.–

Silnylon is not going to be "waterproof no matter what". It's sufficiently waterproof for a tent, but with a jacket you'll need something better. In serious rain silnylon will wet out and your touch will pull the water through the fabric. There are ways of making silnylon more waterproof, but it's a lot of work and it may not look that good.

As for ventilation, silnylon (like most waterresistant fabrics) does not ventilate that well. For any jacket (regardless of what fabric you use) I would recommend large pit zips.

I would also use a heavier material for the shoulder and waist area if you're planning on using it with a backpack.

I thought silnylon was waterproof

@ Brett:
Waterproof is not a yes/no qualification. It's a question of how much. What we are typically talking about is how much water pressure can be exerted before it leaks through. A tent needs far less than a garment. The other factor that Mark referred to was the difference between havingthe fabric pressed against you in a garment or suspended in a shelter.

While it varies widely, Silnylon has a 'hydrostatic head' of around 1200mm, while most common WP/B membranes are in the 5000-20,000mm range. It you really want super waterproof, use a strong/thick plastic like a heavy duty painters cloth. That won't leak without a physical hole in it.

The Walmart Frogg Toggs jacket will not wet out and stays warm and dry against bare skin in the rain. So I can just wear a synthetic shirt underneath which really helps with ventilation. $30 :) Had in the rain at 13,000+ ft this month.

The problem with most silnylon shells is not so much the hydrostatic head as the condensation factor. Cold rain chilling the surface of the silnylon while near 100% humidity warm air is on the inside creates condensation inside the shell.

Silnylon capes/ponchos/skirts are open on the bottom to allow water vapor to escape, which is why they are so popular.

You still get wet, but if you have hydrophobic layers underneath, the water drains away and you can stay warm, although damp.