Sorry about the slow reply.

I use Reflectix brand bubble foil insulation. They make "Double Layer" and "Single Layer" (of bubbles) and I've used both. I ended up using the single layer because it's folds and rolls into a much smaller parcel and there's not a huge difference in performance that I noticed.

You can get it with either one or both side made of the reflective material and I've used them both as well. I placed it so the reflective side was towards my sleeping bag and I didn't notice any real difference, they both worked well.

I really don't think you can do much better than the bubble foil. It's light, durable, waterproof, and multi-purpose. You can fold it so it's several inches thick and it make a great pad to sit on.

I am a huge fan of the "SuperShelter" design and the small "Baker's Tent" style I used for my tent. I made the Baker's tent because the design was simple and easy but I've grown to love the design for it's comfort and ease of getting in and out of.

And honestly, used with campfire that design is perfect. It's what a Baker's Tent was designed for so even without reflective material they'll still warm up inside. But you can use a lot smaller fire and a lot less fuel and get them a lot warmer when you use the reflective material.

Without a campfire I wouldn't say my tent gets warm with just me in it, but I have sat with two friends in it on cold mornings with no fire and it warms up pretty nice inside then. And it will warm up fast with just me and a stove going inside too.

I would encourage anyone who does cold weather camping to make one and give it a shot. You won't be disappointed. In fact, you'll probably be amazed. I was and still am.