We all know the benefits of dressing in layers. I've been attempting to put the same logic to a home brew sleep system. By layering up a sleeping bag, vapor barrier, heat reflector, clothing, sleep pad and bivy sack one should be able to create a modular system which can cover a wide range of temperatures.

What I can't figure out (on paper anyhow) is what approximate rating the sum of the parts will be (in various combinations). I don't expect the whole to be the sum of the parts.

For example my sleeping bag is rated to 20F/-7C. It's going to have a heat reflective vapor barrier liner inside it and the bag will be inside an REI Minimalist Bivy...add some light clothing and a Thermarest Ridge Rest and it should be good to a rating of...well I have no idea beyond 'damn cold...I think'.

Do any of you know of any resources where there is information on this sort of thing? Or do any of you have information/opinions? I'd like to be able to figure out just how low I can go (theoretically) before I learn the hard way that it's not enough.

I do plan to test the system before relying on it...but I'm not going to wait for the coldest day of the year to do so...that could be the day I need it afterall!