Thanks for reaching out. I haven’t had the chance to use the Squamish Hoody, but Arc’teryx states that the Squamish features their “athletic” fit which is supposed to be very similar to the Alpha FL’s “Trim” fit. So, if you are comfortable with the Squamish in just a T-shirt, I’d recommend sizing up to a size Small for the Alpha FL if you’re considering layering more than a T-shirt underneath. Although it’s patterned so that you can freely move your arms, the Alpha FL doesn’t have a whole lot of volume to it, and the thickest insulator I find myself layering underneath is a 60 g synthetic midlayer like the Arc’teryx Atom LT or Patagonia Nano Puff Pullover. Hope this helps, and let me know if you have any other questions.