Unlimited Podcast 002 | Sleeping Bag Temperature Ratings

Disclosure:  I am the founder and manager of Zenbivy.  We make quilts and beds.

I’m impressed by the content and accuracy of this information.  Well done in my opinion.  A few additional things to consider:

• Zenbivy is moving away from EN ratings for a reason you did not discuss:  The variability of the EN results themselves.  The results themselves from the KSU lab are not reliable enough for consumers to use to make an informed buying decision or compare products, even if KSU met their +/- 5% “normal variability”.  For instance, the test apparatus may generate a measured 7.0 clo which would equate to an EN limit of 19ºF, but in reality they only claim an accuracy of a range, (between 6.65 clo and 7.35 clo) which results in a whopping 7º EN “range” of 22ºF – 15ºF.  Not only are these results potentially misleading for buying and comparison, they are useless for product development.  I have used the exact same “ticks” (unfilled sleeping bags), one with 2 oz more down than the other, to generate FULL EN results (3 tests per bag) that showed the overstuffed bag to be 4º colder, yes colder, than the bag with less fill.  KSU chalked these up to “normal variability”, but you can see the problem.   I wanted these standardized  tests to work, and have spent thousands using them for development and marketing, only to conclude it was a mistake.  I now agree with your un-named brand and we are in the process of “de-publishing” EN information going forward.  I am happy to share all of this test data so you can make your own judgement.  But my advice to your readers is simple:  ignore EN test results.  In and of themselves, they are largely meaningless.
• So what should people do?  If you really want to compare two bags, I suggest you use the same method we do for development:  measure the loft with a ruler.  Basically, lay the bag out in a controlled environment and let it “puff up” for 24 hours (prep it like the en test, bags hanging in a store are “pre-prepped”).  Then lay the bag on a flat surface and gently lay a piece of paper on the bag to generate a flat measuring surface.  Then measure.  All other things being generally equal, the thicker bag will be always be warmer.  To get the rating, we use this measurement and apply it to a rating scale.  It provides a far more accurate result than an EN test, believe me.    You would think that the design of the bag would have a big impact on the measured EN result, increasing its accuracy.  But you would be incorrect; the design of a particular bag has little impact on the EN test result.  As you said, there is very little air movement in the test protocol, so its easy for any design to “seal out drafts” when the mannequin does not move. Body movement is the only way to test or compare the practical warmth of different quilt/bed/bag designs, and the EN test does not account for this.  Thus, I believe it is a provable fact that simply measuring loft is a more accurate estimate of a bags warmth than the $600 EN test.
• This is especially true when you account for the HUGE range of other factors that dramatically impact the warmth of a sleep system in practical use in the field:
  • People move around.  LOTS.  And sleep in different positions.  And move from position to position.   The positions and movements vary widely from person to person.  This “sleep style” is the biggest factor in determining how warm a bag you need; far more important than metabolism, more important than mass/surface area/metabolism co-efficient, more important than the measured loft or the temp rating.
  • The warmth of the mattress, and how it (and your pillow + clothing) interacts with the sleeping bag is KEY; way more important than small loft differences in your blanket.  In the last 20 years the average sleeping bag sold has gotten warmer and warmer while the average mattress has gotten colder and colder; it’s not a coincidence.  People are needing warmer, heavier bags to pair with their new colder, lighter mattresses in order to stay warm even in “2 season” conditions.  This is a fact that the industry won’t accept:  If a 15-30º (en limit)  sleeping bag is a “3 season” , then a “3 season mattress” is  somewhere in the neighborhood of R5, especially if you use a quilt.  So people routinely by 15º bags and pair them with R3 mattresses, and they wonder why they are still cold at 30º.
  • One of the biggest variations in temp performance in the field is the internal size of a mummy bag:  a larger “bag” is colder than a smaller one (to a point) because of additional dead air space.  It’s one of the biggest advantages of quilts and one of the least heralded:  a quilt is not a bag, and is free to wrap snugly around you to eliminate dead air when it gets really cold, or open up for a more natural sleep position, or anywhere in between.  You do this naturally with your covers in bed every night.  So as a practical measure, a quilt (with head insulation as you mention) will generally be warmer than a mummy bag of the same loft, as long as the quilt design and your sleep style allow you to effectively seal out drafts.

I hope you find those comments additive and thanks again for producing such thoughtful content.

I gain so much dray warmth laying out my puffy jacket inside a quilt with arms splayed out foot to neck on the windward side than I do when wearing it to sleep which gets sweaty anyway  . I agree with above. Functionally removing drafts are key.