Please check my math
Display Avatars Sort By:
Hobbes W
(Hobbesatronic) - F

Locale: SoCal
Please check my math on 12/19/2011 09:37:57 MST Print View

I'm considering making a new down quilt to replace an existing one made with APEX. I'm familiar with the ongoing difficulties trying to compare CLO, loft, etc, between the two, so I've attempted to stick to just dimensions & weight. The differences I'm deriving between down & Climashield are enough to question my math, so I thought it would be best to throw myself to the wolves.

Anyway, here goes:
1. The quilt is 7' (84") x 4.5' (54") [neck] tapering down to 3.5' (42") at the foot. If I begin the taper at the half-way point, then in essence the quilt measures 7'x4' or 28 sq ft.

2. Between my own experience, BPL guidelines and RN's clo charts, one layer of 5.0 oz APEX + 2.5 oz APEX or 7.5 oz APEX (total 1.8" loft, 6.2 clo) is good for around 30 degrees.

3. If I divide the 7.5 oz/yd2 by 9, I get .83 oz per sq ft. Multiplying 28 sq ft x .83 yields a total insulation weight of 23.25 ounces.

4. For down, I convert square feet to square inches my multiplying 28' X 144 to derive 4,032 cubic inches. Since I want approximately 2" of loft to also get in the range of a 30 degree bag, I need to multiply the 4,032 x 2 to arrive at 8,064 cubic inches.

5. If I end up going with down, it would be 800+ fill power, so I simply divide the 8,064 cubic inches by 800 to get around 10 ounces. Using the 10-15% overage rule-of-thumb, I would actually need 11.5-12.0 ounces of 800FP. This calculation seems to bear out as the 32 degree WM SummerLite lists 10oz of 800 down fill.

6. OK, so the first impression is that APEX weighs around 11-12 ounces more to achieve a comparable warmth to weight ratio. All other things being equal (that is both would be built using M90), the total quilt weights come in at 29 oz for the APEX and 18 oz for down.

WM lists 21 oz for their 6'6" SummerLite, but as that also includes zippers and other commercial bells & whistles, it appears I'm in range for the down quilt. As for the APEX quilt, my calculations are confirmed since it's also the actual weight of my own current 30 degree quilt.

I guess my basic question is: does down really have a 11-12 oz weight advantage vs synthetic for a 30 degree quilt? That is, does this pass the smell test?

Chris W
(simplespirit) - MLife

Locale: .
Re: Please check my math on 12/19/2011 09:51:35 MST Print View

This is strictly subjective but-

My current 30ish degree down quilt uses 20d fabric and 900 fill down. It weighs a bit over 16 oz.

IMO, 5 oz climashield should be good to 30. 7.5oz is more 20ish degrees and I used that to the low teens a few years ago. I've also used 3.7 oz down to freezing. IIRC, the 7.5 oz quilt was around 24 oz and the 3.7 around 18 oz.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Please check my math on 12/19/2011 10:18:04 MST Print View

Same here - just my opinion

10-15% overage = thin spots like the recent JRB quilt thread? maybe 25% is better? I made a vest which is a little different and it's more like 40% overfill (actual loft and area measurement).

There is more shell fabric with down because of the baffles - maybe 25% more weight.

Other people have said 5 oz Apex is good for 30 F. My question is, is this wearing something inside? I think 7.5 oz might be good down to 30 F if you're just wearing a base layer. 5 oz Apex would be good down to 30 F if you're wearing a 2.5 oz Apex jacket or vest or equivalent.

Chris W
(simplespirit) - MLife

Locale: .
Re: Re: Please check my math on 12/19/2011 10:22:15 MST Print View

Jerry - I used 3.7 oz to 32 with the following:

Golite stride short + Rab Vapour-Rise pant on bottom
thin wool socks
BPL UL Merino hoody + Montbell UL Down Inner jacket on top
Patagonia R1 balaclava for my head

I was also in a TiGoat Ptarmigan bivy.

Also, the 3.7 oz quilt weighed in at 15.5 oz (went back and checked). I was on the colder side of warm, but slept ok.

Edited by simplespirit on 12/19/2011 10:34:55 MST.

Hobbes W
(Hobbesatronic) - F

Locale: SoCal
Re: Re: Please check my math on 12/19/2011 10:32:43 MST Print View

Thanx Chris, good to see I'm in the ballpark for the down calculations.

With regard to Climashield APEX, I originally built my quilt using just 5.0 (clo 4.1) assuming it would be good to around 30 degrees. Well, the first night I took it out last summer over 11k, the over-night temperatures dropped to the high 20s. (Last summer in the high Sierra never seemed to warm up.)

Suffice to say, I froze my ass off. It wasn't just sort of cold, I could feel the cold literally seeping through the quilt. Of course, I had all my clothes on, beanie, etc, but by 5am, I said screw it, and built a tiny fire to warm up a little bit. (For those that don't know, fires are prohibited over 10k.) Since I was up there specifically to fish, I was active enough by 6am to warm up and stay warm.

Now, granted, I had an open-ended tarp, so I didn't have any "tent effect" to help contain some warmth, but it was also a cloudless, windless night, so it was just still, cold air.

Anyway, when I got back home, I did some more research, and ended up re-reading many of Richard Nisely's threads, including these two:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12101

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=18950

According to RN, you need 3.88 clo to achieve an EN rating of 50 degrees. Btw, the first thread listed above got a little heated, with Roger mentioning 'dodgy claims'. After some more research, I came to the conclusion that I really did need a clo of around 6.2 to get to 30 degrees.

Since I had made my quilt in a duvet style, it was easy enough to open up one-end, flip it inside-out, and sew on another layer of 2.5 oz. Voila, 7.5oz of Climashield APEX, good enough for 30 degrees.

Being the eager beaver that I am, I got to test out the new & improved quilt the next week back up in the same general vicinity. As this was the summer that never warmed up, the first night @ the trailhead had people coming out who were describing sub-freezing temps, hail showers, etc.

Well, I got lucky as the storm cleared out, but each night the temps were definitely falling into the high 20s. Once again, I had only my tarp (no wind, dry conditions), all my clothes on, etc, so all things were equal. And the quilt? Worked like a champ - even though I was definitely feeling some cold by dawn, it was nothing one couldn't just curl up and go back to sleep.

So, based on my own (subjective) experience, combined with more formal supporting calculations, I feel that you really do need 7.5oz (1.8" loft, 6.2 clo) to achieve a 30 degree syn quilt/bag.

That being the case, at least for my own personal experience, I was wondering if an 11-12 oz difference between down & insulation is reasonable.

Chris W
(simplespirit) - MLife

Locale: .
Re: Please check my math on 12/19/2011 10:41:52 MST Print View

Yep, I know where you're coming from. When I used 7.5 oz XP to the low teens I was in MT (low humidity), on snow, in a Shangrila 6 with 5 other guys, and I was 40 lbs heavier with 3x the body fat I have now. Contrast that with my trip this weekend where I was cold in my 30 degree down quilt with a bit more clothing on but only in a bivy. It was probably mid 20s but high humidity.

Even using a standard like CLO some people will find the ratings too high and some too low. It's very hard to be objective with temp ratings.

Hobbes W
(Hobbesatronic) - F

Locale: SoCal
Re: Re: Please check my math on 12/19/2011 12:39:24 MST Print View

I went back once again to read through the comments on this old thread:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12101

Further down the list, Ayce/Paul from Thru-Hiker said this: "In my experience for sleeping bags (actually a quilt), your 800 fill down IS 1.5 to 2 times as efficient as synthetics."

Going back to my original calculations, I was coming up with an insulation weight of 23 ounces for APEX vs 12 ounces for 800 FP down. I was wondering if my math was off, or if I had made some other kind of mistake, but based on my own experiences, research, and Paul's observation, it does appear 800 FP down can be 50% lighter on a relative warmth/weight scale.

Now I just have to determine how to best seal off a small room if I want to make my own down quilt. LOL

Mark Fowler
(KramRelwof) - MLife

Locale: Namadgi
It's 29.75 sq ft on 12/19/2011 15:14:54 MST Print View

Your point 1 calculation of 28 sq ft assumes a taper from the top of the quilt. If you taper from 1/2 way you add 1.75 sq ft (3 ft x 0.5 ft) so you get 29.75 sq ft.

Dustin Short
(upalachango) - MLife
Re: It's 29.75 sq ft on 12/19/2011 18:18:31 MST Print View

I caught the same thing Mark!

Which now makes me realize why particularly strict math teachers will mark and entire problem wrong if you get part A wrong, but do the correct math for b-z...

1) You have a trapezoid starting at halfway and a rectangle the rest. So you half the length is 3.5' which gives 3.5*4.5 for the torso and 3.5*(4.5+3.5)/2 which gives Mark's number of 29.75 sqft.

Other issues you have in your math (rather the logic is flawed).

2) This is subjective as others have stated. I personally would use 5oz~30F but while a cold sleeper I also have a higher tolerance for discomfort.

5) Your oz are off as per part 1. Also you're comparing your quilt's down fill which is single layer to a WM sleeping bag, which has down on the bottom. That's not a fair comparison to either item. In the summerlite there may only be 6 oz of down above your body compared to the quilt's 10+. However being a full bag drafts are greatly reduced which helps in the subjective warmth category. Remember, with a quilt you'll need solid ground insulation in the form of CCFs or inflatable pads.

6) You're ignoring that down requires baffles which increase the weight variably depending on baffling type and frequency. So while pure down is warmer, it does have associated non-down weight penalties which explains why down makes more sense as the expected temperatures get colder and colder. Conversely you can use sewn through construction which drastically reduces perceived warmth by creating a multitude of cold spots...

As for down vs synthetics the data is still out. All of the data testing down clo is based off 1950s definition of down, which today we'd probably classify as somewhere between 500 and 650 fp down. Very little data exists on 700+fp down and there is evidence to suggest that in the laboratory higher fill power has higher clo rating. My own calculations based off two data points (pretty small set to run a regression on sadly) gave 800fp a clo/inch value of either 3.26 or 4.31. Pretty broad range, but using 3.26 as a conservative number says that roughly 2" of 800fp down is good for ~6.5clo. This falls in line with the rule of thumb of 2" (or your conclusion that 6.2 clo) is sufficient for 30F weather.

As an aside, 900fp provides 3.57 clo/in in my regression which would say that it's around 9% warmer for same loft...while also being 11% lighter than 800fp. In all that says 900fp could theoretically approach 20% total performance improvement over 800fp.