Frost Layer Formed Between Gore-Tex Shell and Puffy Jacket?

Went winter hiking today in temperatures between 10-15°F and layered as usual: merino base layer, Patagonia puffy, then a Gore-Tex shell on top (with the pit zips open). When I took off the shell back at my car, I noticed a very thin but clearly visible layer of frost over almost all of my puffy jacket. I wasn’t particularly cold, maybe sweating a tiny bit. Can someone help me understand what might have gone wrong? Thanks!

You didn’t do anything wrong; your sweat got trapped under your shell and froze. If it wasn’t snowing, you might not need to wear the shell if your other layers provide sufficient wind protection.

Teo said:
You didn’t do anything wrong; your sweat got trapped under your shell and froze. If it wasn’t snowing, you might not need to wear the shell if your other layers provide sufficient wind protection.

Do you think the sweat passed through the merino base and all the way through the puffy insulation to the exterior? It’s more likely just condensation from the air forming on the warmer exterior surface of the puffy.

I don’t wear a hard shell jacket unless I’m expecting precipitation because I know I’ll sweat. A few weeks ago, it was 23 degrees when I left the car. Wind chills were at or below zero at the top. When I got back to the truck, the back of my fleece and my pack were wet. Since you can’t vent more, you could try stowing the shell while you are moving.

If you’re just going for a hike, don’t wear a puffy if you expect to sweat. Wear it when you stop hiking and start to feel cold. Also, don’t wear a hard shell unless it’s windy or raining, as it will keep any sweat you do have locked in.

Only you can judge if you felt too cold or too warm and sweated too much, but the fact that you had ice on your shell and weren’t cold suggests that you were insulated so well that your body heat didn’t escape to melt the ice. This is similar to how horses can have snow on their backs without it melting—no body heat is escaping their coat. Whether the ice came from sweat going through the layers and freezing or from cold air condensing with the warm air inside of your jacket, it shouldn’t matter too much.

Definitely a build-up of condensation from the trapped warm air in your puffy layer. I recently spent a few nights outside in the Arctic Circle, with temperatures ranging from 0° to -25°F, and experienced this every morning inside my Gore-Tex bivy sack, even though I was sleeping relatively ‘cold’ compared to a normal night in a sleeping bag and bivy.