The weight issue is very interesting when you look at it more. THe down bag I have - the Valandre Lhotse 1100 http://roberts.pl/index.php?l=en&p=_katalog&i=_lhotse (now superceded by later versions - see above) has no zip yet still weighs approx. 2kg - just over 4lb. The claimed temp range is comfort - -35c, extreme -49c. (possibly a bit optimistic imho, and these temps are assuming wearing arctic-grade down clothes inside it) However, clearly well-liked by Everest expeditions etc.
THe Wiggys bag which approximates to those temps is the Ultima THule (based on my experiences wearing just a base layer in their bags and matching their suggested temp ranges, so Comfort in just base layers at -29C). (THe Wiggys Superlight says temp of 0F = -18C, and I've slept in mine at -20C, again in base layer only) The Ultima Thule weighs 5lbs. So for the total weight disadvantage of 1lb - 450grams - you get all the advantages of a synthetic bag - warm when wet, pass-through of sweat/water vapour onto the outer skin etc.
The main difference is compression. The Valandre packs down into an incredibly small size - the Wiggys ...doesn't! But I sometimes think that the "weight" disadvantage of synthetic bags is massively overstated........
THe Wiggys bag which approximates to those temps is the Ultima THule (based on my experiences wearing just a base layer in their bags and matching their suggested temp ranges, so Comfort in just base layers at -29C). (THe Wiggys Superlight says temp of 0F = -18C, and I've slept in mine at -20C, again in base layer only) The Ultima Thule weighs 5lbs. So for the total weight disadvantage of 1lb - 450grams - you get all the advantages of a synthetic bag - warm when wet, pass-through of sweat/water vapour onto the outer skin etc.
The main difference is compression. The Valandre packs down into an incredibly small size - the Wiggys ...doesn't! But I sometimes think that the "weight" disadvantage of synthetic bags is massively overstated........