Questions about the fill weight and zipper compatibility of the REI Co-op Magma sleeping bags.

[jack1]

A few years back (2016?), I bought a Magma bag, mens regular.  At that time, they didn't come in "15" and "30" sub-models, just Magma.

Now I want to match (with other zipper side) for wife.  My old Magma has, according to the tag, 1-lb, 11-oz net fill (that's 27-oz).  Curiously, your website says the fill for the new bags are 9.7-oz and 17.3-oz.  

Something is not adding up here!  Can you look into this fill numbers, and let me know?  Also, will zippers still match up?

[REI-JohnJ]

@jack1 Thanks for reaching out!

There are a couple of reasons that there is a difference in the fill weight for the different models of Magma sleeping bags:

1. The older Magma sleeping bag used a 700 fill duck down and the current models of Magma sleeping bags us an 850 fill goose down. Those numbers indicate the volume, in cubic inches, that one ounce of down fills. So the goose down in the current Magma has more loft than the duck down in the older model and is therefore significantly more insulating per ounce of weight.
2. The temperature rating of the older Magma sleeping bag is 10° F, whereas the current models are 15° F and 30° F. The lower temperature rating means there will be more down in the older model.
3. The current models of Magma sleeping bag use a different baffling style, called 'Variable Baffle Spacing', which stabilizes the down and allows for better thermal efficiency, ultimately requiring less down for similar insulation levels.

In the end, these add up to more efficient use of higher quality down, resulting in a decrease in the overall fill weight of down.

Unfortunately, while the zippers on the magma should be compatible, due to the way the zipper comes up and across the chest in the newer model of the Magma and that it is not a full length zipper, you will lose a significant amount of it's warmth if you connect the two sleeping bags. 

Hopefully this helps, thanks!