SgtFruitcake has tried to compare apples to oranges. He is comparing a what appears to be an excellent but,(as you point out) small single down bag rated for -5 or so with a full size double bag rated to -30. His silk liner and WATERPROOF overbag will not bring it down that much (5 or so at the most). He also appears to be recommending an overbag that will make his excellent down bag (or any other) a frozen lump in only a few days. Coated nylon vapor barriers are used by some (not me) as liners, but the half liter or so of sweat every night will soak the bag then in the day it will freeze up. NOT for me, thanks.
Actually in the video SgtFruitcake was demonstrating a Snugpak Osprey 12 (weighs 2 Kg) and has a Comfort rating of -10C (EN13537 Specification) The Osprey is a top end Synthetic bag not a down bag. It is also issued to Royal Marine Commandos for Norwegian Arctic Operations. The bag will typically cost between £100-130.
A Snugpak Expanda winter panel will add another 0.6Kg.
The Snugpak Bivi bag cover is waterproof a Hydro Static Head in Excess of 5000mm (Goretex I think is rated to 10,000mmm). The Snugpak bivi bag cover is also breathable (will transmit upto 7.5 litres of moisture per m2 per 24 hours (Goretex being slightly better depending on the specific fabric). It is not a vapour barrier product. The Snugpak Bivi bag weighs around 400gm, i.e. less than half the weight of a military Goretex bag. I am not sure that is currently military issue though.
The silk liner a bivi cover will probably add another 5 degree Celsius. A Thermolite Reactor extreme liner would probably be even better than the silk liner.
The army issue reversible Softie tops and bottoms SgtFruitcake was wearing will easily add another 10-12 Degree Celsius to the sleep system. Cost is around £50 for the Softie suit.
http://www.strikeforcesupplies.co.uk/index.php?method=cat&id=79Military Issue Sleep Bags such as the US Modular Sleep System are generally rated for lower temperatures than civilian specification bags (military personnel are expected to cope with colder temperatures) for similar technology and weight and are not included in the Civilian EN13537 specification. They are available in the UK.
http://www.strikeforcesupplies.co.uk/index.php?method=stock&id=3006&from=6Even at £150, they do represent excellent value for money (especially considering the addition of a Goretex Bivi Bag) if you can handle the weight (4.4Kg) and the bulk. You will still need a full 5 season full length sleeping Pad to get the best out of the ECWSS as well.
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