Insulation Strategy: Shell under or over down sweater?

Under rain shell.

I wear my rain shell under my insulation layer. It helps the insulation layer maintain loft by avoiding something from compressing it while also adding a little warmth because the shell acts a bit like a vapor barrier. Reverse it if the weather is bad.

It depends on why I'm putting on the insulation layer.

If it's a dry day, if I'm using my shell as a windshirt, and I'm wanting to add insulation on a break from hiking, I'd throw on the sweater/jacket/vest on over my shell. Why? It's faster and easier, and I don't want to get any of my sweat on my insulation, especially if it's down.

If I'm sitting around at camp and don't expect to take off the insulation, I'll wear my shell over my insulation, if anything to protect the insulation's delicate low denier shell fabric from twigs or ash.

^ same- for breaks it goes over (non-rain event), at camp it goes under

Assuming the shell is properly sized so that it doesn't compress the high loft jacket, over the top is warmer in my experience. That said, if it's to get a quick warm up, and then I will be moving again, putting insulation over the shell is more efficient.

–Mark