Zone insulation refers to the intentional placement of down in the parts of the sleeping bag where it is needed most. The purpose of zoned insulation is to maximize the thermal properties of the sleeping bag while reducing weight. In simple terms, this means that there will be less down on the bottom of your Therm-a-Rest sleeping bag than on the top because any insulation that is under you gets compressed by your body weight, thereby eliminating the insulating properties of the down in that part of the sleeping bag.
Another way to observe this is to lean against a cold wall while wearing a down jacket. You will find that the part of you in contact with the wall gets cold really fast, almost as if the down isn’t there. When you're sleeping in a sleeping bag, your mattress provides insulation from conductive heat lost to the ground, so as long as your mattress is rated as having an R-value of 4.7 or higher, your back (or side if you are a side sleeper) should be warm at the advertised temperature rating of the bag.
By diverting insulation away from the bottom of the bag where it is not useful to other parts of the sleeping bag that lose heat faster (i.e. the near the head and feet) we are able to create a better sleeping experience and provided added weight saving and compressibility to the customer.
For more information about zoned insulation check out this post on our blog:
https://cascadedesigns.com/blogs/thermarest-gear/lighter-warmer-zoned-zoned-insulation-explained