Check out this article. Good information.
Thermoregulation: An Overview of Heat Loss Mechanisms and Practical Guidelines for Staying Warm with Lightweight Gear @ Backpacking Light
I've done a lot of winter camping. Most important thing is to know the 5 methods of heat loss.
Conduction
This is where your pad comes into play. I have an insulated Big Agnes as well. If it's cold I'll bring a closed cell foam pad also. Cold ground will suck away your heat.
Convection
Wind AKA the devil. Your shelter must be windproof. Not waterproof so much. Generally in winter it's not raining. Gore-tex is the most over hyped product in the outdoor industry. It cannot breathe as much as you can sweat. Sweat destroys the insulating value of your gear and makes you more vulnerable to evaporative heat loss. Sil nylon has a DWR coating and will breath better while shedding water.
Radiation
Space blankets are good for keeping heat from radiating away.
Evaporation
Goes hand in hand with convection I find. You sweat, it evaporates, you freeze. Regulating your temperature is crucial to minimize evaporative heat loss.
Respiration
Don't be a mouth breather.
So what I consider when winter camping are the following. Does my shelter keep the wind out? Does my pad keep the ground from sucking my heat away. Does my sleeping bag have enough loft to keep me warm? Does my shelter stay dry?
A couple other things... Don't wear a lot of clothes to bed. They constrict too much and compress the loft of your bag. I usually sleep with light fleece pants a long sleeve synthetic shirt and a merino wool hat.
Bring a pee bottle. Always piss before bed. Your body wastes a lot of energy keeping the contents of your bladder warm. No way in hell I get out of the tent to piss when it's -20 with wind chill. Get an old water bottle and mark it with skull and bones. Have some sort of tactile identification so you know you're not pissing in your water.
Eat before bed. Give your body some energy it can convert into heat. Don't drink, don't smoke.
Toss a handwarmer or hot water bottle in your sleeping bag.
The most extreme alpine and arctic explorers use vapor barriers. It's the only way to go when it's really cold. You have a light layer against your body, followed by the vapor barrier, insulation on top. Your sweat stays with you, it can be rather unpleasant, but you stay warm. your insulation stays dry because it cannot touch your sweat. Remember putting bread bags in your shoes as a kid? Same idea. Just change your socks often or get trenchfoot.
Not to mention your Land Cruiser!!! They sell Mt Bikes, but give $$ to Environmental Groups who fight to keep mt bikes out of the woods!!! They will take your money then pay someone to stab you in the back...I say screw REI.
Not trying to bash you at all just telling the real story. I really hate the Sierra Club.
