Mark and Eric, I thought the Atom LT wouldn't be anywhere near as warm as my Montbell Thermawrap Parka, partly because of the Wind Pro with Hardface panels. Hardface is an excellent material for around town or high abrasion uses and holds up very well long term without pilling (it's also great for pet owners in that it doesn't hold animals hairs), but I have found that it is less warm than Wind Pro - I speculate that the Hardface coating decreases the thickness of the insulation.
For example, I have a Wind Pro with Hardface jacket that weighs about 14 oz, and a Wind Pro Micro Fleece Jacket that weighs the same, and the Micro Fleece jacket is much warmer alone and under a shell. The Hardface jacket though always looks more presentable because it never pills.
According to Malden Mills, Hardface doesn't really add any warmth or much wind resistance - that comes from the Wind Pro itself.
Part of this Backcountry review confirmed it for me:
"The atom makes a nice, light layer, but its not much of a cold-weather jacket. Wind flies through every seam. Every breeze sends cold air swirlign around my arms. And the fleece breathes way too much for me. The jacket keeps my back toasty and my head warm while my arms, armpits and shoulders freeze. Its an odd combination. The Atom must be meant for hot blooded people who need to dump lots of heat. The Atom would give them a sweaty back though, but they wouldnt be chilled by it."
When I throw on my insulation layer at rest stops, I want instant warmth. If I need to vent, I open the front zipper. Mine is sized up one size to go over shell and all.