Originally Posted by tjm
When winter camping, I nest a down bag inside of a larger synthetic bag with very good results.
The outer syn bag actually works better in the combined arrgt when it does on its own.
inner - rated 15 deg F down bag - a true 15 deg F for me
outer - rated 15 deg F syn bag - but really only good to about 25-30 deg F for me because it is a wide cut with lots of dead air space
I have used the combined set down to -5 deg F and feel I still have a tiny bit margin.
I think the key is that the bags fit together extremely well so that the inner down bag gets full loft and there is very little dead air space between the bags and around me.
Of course, nutrition, hydration and fatigue play huge roles as well since it is the heat from my metabolism that heats the air trapped by the bags
I'm on board with Tim
The difference in ratings from mfg are all over the board so the bottom line is the true loft of the insulation
Not allowing for varence in metabolisem the bottom line it all has to do with LOFT !!
The more loft = more insulation
Comprised insulation , less loft reduction in insulation= colder
The other factor is your pad or under insulation , its a third of your bag or more.
So when in a hammock the heat lose from below is a major factor.
When using two bags always make sure the over bag is larger then the inner
and insulation is not compressed.
A good wool sweater will add 5-10 degrees or more to a bag.
If you have cold feet its better to have a down foot sack that one can place both feet to gether then down booties.
To me there is down and there is every thing else !!