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Improve your bag's performance.
The Western Mountaineering Hotsac Vapor Barrier Liner is your most important companion for those technical winter trips. The liner works by stopping heat loss from evaporative cooling, and it also prevents your body vapor from degrading your insulation. This is most important during extended stays inside your bag. This Hotsac VBL goes one step further by stopping radiant heat loss with its reflective metal coating, while an elastic drawcord at the top of the Hotsac cinches around your neck to prevent convective heat loss (you're welcome for the thermodynamics lesson). Plus, the new Hotsac weighs only 4.5 ounces and is light enough to carry on day trips as an emergency bivy.
- Prevents heat loss and condensation build-up in your bag
- Ultralight ripstop fabric with reflective lining
- Elastic drawcord at neck for snug fit
- Western Mountaineering: American-made quality and craftsmanship since 1976
- Item #WES000W
- Q & A
Always carry just in case
- Familiarity: I've used it several times
Sometimes it gets colder than expected when we sleep outside. Having an extra lightweight emergency bivy is something I make room for in my pack. They say that you pack your fears into you backpack. I dislike being cold so I don’t mind the added weight of a bivy sack which I mostly don’t wind up using. You can get one from SOL for about $20 but it will rip after the first or second use, guaranteed. WM’s bivy has nice tough material that will last. If you want to save weight and carry a lighter sleeping bag then you can pair it with this... just expect to wake up soaked in your own sweat. Bivy sacks are little saunas with zero breathability. Sleeping bags are generally not something you want to cut weight on anyway, unless you are dead set on becoming the ultimate ultralight bushcrafter.
Is this breathable?
Kevin, I am so sorry for the delay. I missed your question earlier.
I reached out to Western Mountaineering and received a detailed email from Gary, the product development manager. Here is his lengthy, but thorough reply:
It is not breathable, so it is not something that would be comfortable to use at temperatures above zero in my opinion. Some people say its fine to use in temps below freezing but still above zero F, but it depends on the person. At temperatures that cold it is possible to balance your layers and body heat so that you aren’t sweating but just sleeping in a warm microclimate with a slight humidity. If you don’t shed layers when you start overheating, or if you use one of these in warmer temperatures it can lead to a bad situation where you are sweating too much and so we only recommend using it when it is cold enough to use without feeling uncomfortably clammy. When used correctly in cold enough temperatures it will help retain the loft of a bag (usually used with bags that have a laminated outer shell, which can trap some moisture that is evaporating off the body) by protecting the down from internal moisture (i.e. from evaporation of the person sleeping inside) while the Gore Windstopper outer shell protects the insulation from external moisture. On longer trips in colder weather it is a good way to prevent depreciation of loft over time from evaporative moisture that can accumulate at a far quicker pace than moisture vapor can escape through the laminated Gore fabric. Basically the outer shell is breathable but not breathable enough to keep up with someone breathing inside their bag, or sweating because they haven’t shed a layer soon enough, etc. etc. so it builds up unless you can make a conscious effort to breath outside the bag, layer only as much as needed but not too much so that it causes your body to sweat, air out the bag in the morning, etc. Using a VBL is a method of retaining some of the moisture inside & away from the down, so you sleep a touch humid in an effort to preserve loft when using a bag with questionably breathable laminated shell fabric. I’ve known people who prefer not to use them but still bring them along and sleep with a VBL on alternate nights to preserve loft for a multi week winter trip or expedition and just use other preventative measures (breathing outside of the bag, layer so that you aren’t sweating, air out the bag in the morning long enough to dry out any moisture, etc.) on the nights where they don’t sleep in the VBL.
Kevin, I hope that helps!
Western Mountaineering Rep