Instead of this just dress warmer. Don't just throw on some sweat pants or something. Put on a very thin layer under just about any pants. What your looking to do is contain heat, you don't need to generate more of it. Having a thin layer under some normal clothes will trap air to be warmed between them while also providing your skin with direct means of warming. Just having one or the other does not generate this effect. It almost doesnt matter how thin the bottom layer and how thick the top layer is.

If you have to be outside a wind breaker with a light sweat shirt is better than just a shirt and a large jacket. Unless you live in the arctic, your not going to need much more than this.

Use your body heat in your favor.