The specially engineered WindStopper membrane not only provides full protection from the wind with a 0 CFM rating, it also stabilizes the microclimate close to the skin by allowing excess moisture to escape while preventing unwanted moisture from coming back in. Emphasis is placed on breathability.
Sitka outer layers are originally treated with a Durable Water Repellant finish (DWR), which keeps the outer fabric from becoming saturated so that the breathable barrier can do its job. This coating needs to be replenished once per season, or more often if the piece gets a lot of use or washing. If water is no longer beading up on your shell, it's time to repeat it.
In general, washing your gear in cold or warm water with mild non-deoderant powder laundry soap (non-toxic, biodegradable types preferred) and hand drying it are the best ways to clean Sitka products.
Freeze it and then pick it off with a dull object. Rub the remainder out with a water and white vinegar solution and then wash with detergent.
Like most synthetics, Sika Gear will melt or burn when exposed to flames or direct heat. Use caution when around fires, stoves and hot metal surfaces!