Down insulation vs. Synthetic for Sleeping Bags

I have a Down bag LaFuma 800 that weighs 28.2 ounces. What would a quality synthetic bag weigh that provided the same insulation? In otherwords, is there a ratio or percentage that one might use? For example, are, in general, synthetic bags 25% heavier?

I honestly don't think you can get a number like that. Besides the simple fact that there are many different synthetic materials bags are made with, but even down bags of the same rating can weigh different.

There's not a number or ratio like that that I know of (I'm not a VERY experienced gearhead, but just bought a new bag recently so I did a lot of research on them). If you're asking because you are or are thinking of buying a new bag, a better strategy might be to decide on one or the other insulation and find the lightest option you can afford.

The answer depends on the quality of the down and the type of synthetic insulation you have in mind. Your LaFuma 800 uses 650 fill power down. With higher fill power down your bag could weigh less and still have the same rating (I notice your bag is rated at 30 degrees) and the converse for lower fill power down. Temperature ratings are unreliable indicators of comfort but they are at least somewhat consistent between bags. So you should compare your bag to a 30 degree synthetic bag. The LaFuma Extreme 1000 is a synthetic 30 degree bag. It weighs 36 ounces. So that should give you some idea. Of cours the comparison is not really fair because the two bags aren't exactly the same length.

I don't know of any solid numbers, but you can use my synthetic bag for comparison to yours: It's an REI Nooksack UL, and has a nearly identical weight of 28 oz. (This is the manufacturer's number; I don't have a sensitive scale.) It's rated at 35 degrees. I just got it as a present and haven't used it yet.

Heber:
That makes a lot of sense. So at least withing that brand, LaFuma, the 20 degree Down bag is about 8 ounces lighter than the equally rated Synthetic. That is really what you have to do, stay with one brand/design, to make comparisons so quality and design don't differ too much. I did a lot of work (many hours) on the statistics from Backpacker Magazine's data base on (126)sleeping bags and it made no sense whatsoever. Some bags with the same fill (say 800 down) and same rating varied considerably in weight when you look at different manufacturers. I'm beginning to disbelieve the mfg. temperature ratings entirely. Probably my conclusion is that the only good source of data is reviews where the reviewer actually sleeps in the bag at various temps, reviews more than one bag, and more than one reviewer's opinion would be even better. At least in one case Backpacker Magazine did just that.

I think you are right about mfg. ratings. No connection to reality whatsoever for the most part. However . . .

Actually, there is a "Gold Standard' to work from. Western Mountaineering has been recognized to be highly reliable and conservative in their ratings. Where another mfg has fill weight for down, you can compare to WM. If they don't have at least as much down of the same fill power (more down if less FP), then they are clearly lying through their teeth.

Disclaimer: I am not claiming that there are not other good brands -- and perhaps in some respects, even superior products. But WM is the gold standard. If another brand is better, it is likely to be described (informally) as "better than WM". I personally have several different brands but mostly WM

I just took my new Mountain Hardware Sunrise 50'F, 1 lb 7oz, Reg., Down insulated out into the Superstition Wilderness for New Years eve. I haven't verified the actual weight.

I was expecting temps in the low 40's, so I thought I'd make up the difference between actual and the bag rating by wearing a wool sweater, light weight thermals, pants etc. in the bag.

It got to freezing, I am certain, and I froze my fanny off.

If my feet hadn't stayed so cold I think I'd been alright for the most part and a heavy pair of wool socks, that I didn't bring, might have made all the difference in my comfort level. I was tempted to try another night but decided against it.

I think the bag works as advertised and will do well this summer.

I did survive a freezing night in it, albeit with little wind, so I know I can.

added: I just weighed a bag. I used the Reg. length camping because it came in first. I also bought a Long which came today. The Long weighs 660g (1# 7 1/4oz)
Let me add that I am 5' 11" and the Reg is plenty long for me, but just a little to narrow in the chest, however I'm 30# overweight, but I did like how the bag moved with me when I rolled on my side. I'll be using the Long in the future & my kids the Reg's.

Its a good idea to bring a thermometer to get more out of your experience.

It seems to make a difference if the temperature is dropping or rising. I think a night that starts at say 40F and drops to 30F is alot colder than a night that starts at 30F, even if it doesn't rise much from there. I think it has to do with relative humidity, and also why the temperature is dropping, such as a clear night sky, or a cold air outbreak. Of course there could be other factors, like how warm or cold your skin and clothes are already when you go to bed, and how long you sleep, and how much food you are digesting, and of what type. I understanding when sleeping we produce about 100kcal when digesting vs 80kcal when not, but I wonder what a mix of carbs, fats, and protiens might be best. There is another factor which interests me, which if more or less heat is produced during sleep if the body is doing stuff like repairing tissue and storing glycogen back into the muscle. It seems to me that any activity which takes some energy will also produce some heat, but I think we are capable of producing more heat on a cold night if we are more rested going into it, rather than less. I would like to know more about how the body produces heat during sleep. I think we wake up if we start to shiver, but I think its also possible to suffer hypothermia or frostbite without shivering. I think its good to be acclimatized to colder skin temperatures, but perhaps not to make too much use of it on a regular basis.

Me too, Pliny. Fortunately, there in now a more objective way to compare ratings. Bags rated by the European Outdoor Group have to undergo standardized testing on copper manikins in climate-controlled chambers. So it's now much easier to compare ratings both within and across brands.

Here is a partial list of those ratings. The numbers you'll be most interested in are the ones in parentheses on the yellow stripes. These are the Lower Limits expressed in Fahrenheit. For example, if you scroll down you'll notice that both the synthetic fill TNF Cats Meow and the 600 fill TNF Kilo received a 25F Lower Limit rating. The Cats Meow weighs 2 lb. 10 oz. compared to the Kilo's 2 lb. 3 oz. giving you a 7 oz. difference between the two.

I found in an old Exercise Physiology text (by william d. McArdle)quite a bit on "Human Energy Expenditure During rest and Physical Activity." World class athletes nearly double their daily caloric outputs as a result of 4 hours of training. People in tropical climates have a resting metabolism up to 20% higher than those in temperate areas. Food has a stimulating effect on the metabolism. Specific Dynamic Action (SDA) effect reaches a maximum within 1 hour after a meal. The SDA is different for different meals. High protein meals can use up 17% of that food for digestion, absorbing and assimulating, while high carbo meals use up only 9% (all according to McArdle). Body surface area Is a predicter of BMR (and is directly proportional). Also heart rate is directly proportional. Interesting, also complicated.

While it wasn’t really the original subject of this thread, the subject of temperature ratings of sleeping bags has come up, and it is an interesting one, and some very salient points have been brought up.
Many inside and outside factors, such as sleeping bag design, shell material, mattress, tent, humidity, individual metabolic rate and so on, effect (or affect, help me, I never seem to get that right) the warmth of a sleeping bag. Yet given the same quality and quantity of down there is a limit to how cool (that is, how good) the temperature rating can be. Someone suggested, and rightly so, that if an excellent quality company like Western Mountaineering is rating a bag at a certain rating, then it is highly unlikely that another brand with the same down fill power and weight of fill is going to be any better. That is to say, Western Mountaineering does a good job at designing and rating their bags and if you see another brand (given the same quality and quantity of fill) that has a lower rating it probably is advertising hype.

Having said all that, the temperature ratings are still relative and relative to all the factors mentioned above. The free loft of the bag is probably a better indication of heat retaining efficiency than the manufacturer’s temperature rating because it is affected by not only the quality and quantity of the down, but also the design of the bag. The experience of others (reviews and friends) is valuable, but your own individual experience in your own situation and climate is the only proof of the pudding.
Thanks to all that contributed to subject here.