How do i calculate the temp rating of two bags combined??

skin temperature is about 90 F

theoretically, if you had two 50 F bags, each one gives 40 F difference, together 80 F difference, so that would be a 10 F system

you can see that from Richard N's table of CLO vs temp (the X axis intercept is at about 90 F and they're straight lines), or from BPL's position statement on sleeping bag temp ratings

inner bag would get compressed some, especially if it was down, so it wouldn't be as good

x -(70 – y)/2 = z

x = first bag (higher rated/lower degree)
y = second bag (lower rated/higher degree)
z = rating of doubled bags

**I take no credit for this equation. Had it written down from years ago.
Where? ->?

Michael

Assuming the bag or quilt is accurately rated

Add 15-20F for a summer (40F) quilt or bag to the base bag

Its rough … But it works

thanks all!
so using all the mentioned I get for my MLD xp2.5 (which i think is rated at 40) and my light down bag which is 45: 15F~30F well thats quite a range but i guess it varies with compression and draft/coverage issues.
will have my jacket to help me out :)
M

Michael

Put the synth on top if you can and you should have a very condensation resistant combo

What everyone else has said is spot on (the equation is nice too!). Just be sure that your overbag isn't too small so that it compresses the insulation of the inner bag.

I would argue that the down should be outer – synth outer would compress the down inner

At least for me where I don't spend many days of very cold in which case accumulated condensation isn't an issue