Estimate the comfort rating of a layered sleeping system

[Stephane G]

Working on a layered sleeping system. The idea is to combine a liner made of Alpha Direct 90 + an overbag in Polartec Hard Face Thermal Pro. The liner will probably be a cocoon whereas the shell will be zippered — left open and draped over the bag (40F down) or closed to encase the whole enchilada.

I very naïvely estimate that both the liner and the shell are comparable if not warmer than the S2S fleece liner  so the total warmth added by the liner + shell combo would be 50F+ . If this were to hold, I could sleep in sub-zero temperatures with my 40F bag. Perhaps optimistic…

Is there a better way to estimate the rating from my desk? Or a systematic way to sleep in the backyard and take careful temperature measurements? Or a cheap approximation of an ISO test that we can do at home?

[Jerry Adams]

You could measure the loft.  For synthetic there is about 25 F per inch (kind of a spitball number).  Your skin temperature will be about 95 F.  So, with 1 inch of loft, you would be comfortable down to (95 – 25) = 70 F.  With 2 inches of loft you would be good down to about (95 – 50) = 45 F.

I suspect this will give you a comfort rating that is disappointingly high.  Maybe something larger than 25 F per inch is more accurate.  There are factors other than loft that can affect comfort rating.

Sleeping in the backyard is more accurate.  Note what temperature it is.  Normally, it’s coldest just before sunrise.  If you were comfortable then your comfort rating is less than that temperature.  You could estimate that since you were a bit uncomfortable the comfort rating must be a few degrees warmer, or if you were very comfortable the comfort rating must be a few degrees colder, etc.

I tried using synthetic insulation for a sleeping bag but found that it was too heavy and to bulky in my pack so I switched to down.  But that takes more work.

Down is best, synthetic is half as good as down (half the warmth for the same weight), fleece is 1/4 as good as down (or worse), layers of fabric are even worse.

 

[Stephane G]

Thanks for the formula. Made me look for loft data and comfort ratings. I find something fairly close on EE (20F per inch, but with a different intercept, i.e. 0 loft -> comfortable at 70F). Interestingly their scale suggests an efficiency of 40F per inch for APEX (!?).

[Jerry Adams]

70 F is way too low, 92 F might be better, En13537 assumes something like 92 F – just eyeballing the y axis intercept of a graph

what it is, is the skin temperature when you’re sleeping in your sleeping bag at a comfort rating level of comfortableness

of course, you could use a lower intercept and a lower slope and get the same comfort rating at a particular loft.  and it’s not really linear so if you go too far away from nominal this falls apart – that is, at much warmer or colder temps the 92 F, 25 F/inch aren’t as accurate

EE is a respectable source of information, maybe their numbers are better than mine

[Jerry Adams]

maybe measure your loft and find an EE product with the same loft and use its comfort rating

[Stephane G]

I wasn’t commenting on the merits of the estimates, other than the fact that the gradient was close.

1. My impression is that backyard experiments will/would be useful. I think that next summer I’ll carry my usual kit just to be safe, and will see how often I can sleep with the liner + over bag and at what temperature. Fall is approaching — hopefully I’ll have the time to get the liner/over bag done in time to sleep some tests.

2. WRT the intercept, I am ambivalent. A bed sheet (i.e. negligible loft) is usually enough in a bedroom at 70F, the idea probably being that the sheet keeps body warmth from escaping into the room. But thanks for pointing out the ISO norm. What graph were you referring to?

[Jerry Adams]

Mammut_Sleep_well_pt1_E.pdf which used to be on the mammut web site, but for some reason they removed it

you can find it somewhere

I’m sure there are other documents about en13537

[Stephane G]

(probably) found it here.

BTW — I am very impressed by the quality of the conversations on BPL.

[Jerry Adams]

Yeah, that’s it.  It’s a quite good discussion of the subject.

[Bill Budney]

The S2S estimate of 32F increase in temperature range is absurdly high. A more reliable estimate from a real user is 5 to 10F. Your two-layer system will likely be warmer, but nowhere near 40F.

Fleece varieties vary more in weight and factors other than warmth, so you can get a quick estimate by adding $10 fleece blankets until you get approximately the same thickness. Then test them yourself. Maybe even make a prototype of your design using cheap blankets.

In other words, Alpha Direct is remarkable for its weight and breathability, but its warmth is approximately the same as any old fleece with the same thickness. The extra price that you pay for AD is mostly for saving weight.

For example, if you are comfortable in a sheet at 70F, and comfortable at 55F with two layers of inexpensive blanket, then your two-layer system may give you a similar 15F boost (depends on the blankets you choose for your prototype).

The EE chart looks pretty good.

That said, everyone sleeps differently, and even individuals vary with season, humidity, tiredness, and other factors. Better to pack an extra layer rather than shiver all night long.

As Jerry says, fleece tends to be bulky compared to down or puffy synthetics. But it sure is easy and inexpensive to use for prototypes and experiments.

[Jerry Adams]

Fleece is good because it maintains loft when wet.  Good if you’re exercising vigorously and sweating.

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