Understanding the insulation of the new Sierra Designs Backcountry

[Brien]

In looking for a new zero-degree bag I came across this one last night.



http://sierradesigns.com/sleeping-bags/bed-style

It's the new Sierra Designs Backcountry. The zipperless design is intriguing to me. Personally, I have a hard time zipping all the way up in the confined space of my mummy bag. I also toss and turn a lot at night and many times wake up with the bag in some cockamamy configuration. So here's something I can just kinda wrap around into place. When it's cold I'll sleep with my fleece jacket on top of me and kinda of wrap it around my torso similar to this.

My question is about the fill. When I look at the site the 3-season 600 fill bag is rated for cooler temperatures then the 3-season 800 fill. I'm trying to understand why. If anyone can help clarify this for me I would greatly appreciate it. Ideally I'm looking for a zero-degree bag.

[BillyBob58]

Very interesting! I wish I had a better picture of the foot vent!

The 28F rated 600fp 3 season has about 4 oz(20%) more down ( fill weight) than the 31F rated 800FP, which appears to give more loft/warmth than than the 33% boost you would expect by going from 600 to 800. Unless I am making a math error, that does not seem right. Makes me wonder if they accidentally reversed the temp rating specs? Good thing for someone to call or e-mail them about!

Either way you are a long way from zero, but if the bag is roomy enough you can boost it a lot by layering and such.

(when I say 33% boost from 800, I mean at the same weigth/amount of down, you would expect 800 FP to loft up about 33% thicker than same weight of 600. I think anyway, maybe. I also see on the WM loft to temp rating chart that going from 3"(double layer/total) loft to 4"( +33% increase) gets you a temp boost of from 40 to 30 ( 40/30=33% warmer?) so I guess you can say a 40F bag is rated 33% colder than a 30F bag? Anyway, it looks like the % of loft increase should be pretty close to the % amount of warmth increase, but in this bags case it is not.

[Brien]

I called Sierra Designs and had them explain the difference to me. I was told the difference between the 600 fill and the 800 fill had to do with the ratio of down-to-feathers. The 800 fill can accomplish a "similar" EN rating as the 600 with less fill, which is why the 800 is 4 ounces lighter. Weight isn't so much of a concern in this situation.

The bottom line is the 600 fill 3-season is the warmest bag in this series. Which is good since I think $300 is the max I'd want to spend anyway.