UKC

is it true that doing this helps to reduce the condensation getting on the sleeping bag, or will it makes the condensation worse???

I read this on a website and wondered whether anyone has any experience in this area, or any better methods.

Thanks,

ADJC

In reply to ADJC:

You will have a wet soggy liner to carry round !! and a dirty sleeping bag inside

In reply to nz Cragrat:

so not a good idea? keep the silk liner on the inner?

In reply to ADJC:

Well I have never heard otherwise and I have been tromping about the hills for 30 years

In reply to nz Cragrat:

much experience of bivi's?

In reply to ADJC:
You get some condensation in bivis... that's a given. You keep the condesation down by having an appropriate bag inside it, too warm, and you get a lot of condensation. If you think condensation is going to be a problem, you use a synthetic bag.

Saying this, when I'm asleep I don't sweat much at all, but one of my friends seems to sweat like a blind lesbian in a fishmongers the second he gets in a sleeping bag. I don't think he's found a solution to condensation in a bivi...

But the silk liner is going to be too small to cover a sleeping bag, and just get wet anyway.

So I take it you are talking about a s/bag inside a bivy bag?

I have never had much of an issue but a good idea with a down bag is to get a shell thats semi water resistant.

I have a great Wild Country Alpine Bivy bag that I got 20 years ago and has generally been great in that respect.

Definitely not had many probs and I would be in snow caves a bit over the winter - when i might not actually use a bivy bag come to think but usually do as I feel the cold.

So depends on the situation often.

In reply to ADJC:

I don't think it will work, unless it's a water resistant lining (like treated pertex) rather than a silk lining. I’ll explain why.

I work in building design and I have to do condensation risk analysis for houses. I think the same design theories can be applied to camping in tent/bivis.

In a modern house the building fabrics (e.g. external wall) will be made of various layers which include internal lining, insulation, breather membrane, airgap and external cladding. In a tent the equivalents will be silk lining, sleeping bag, inner tent, airgap and flysheet, while in bivis the roles of inner tent and flysheet will be replaced by bivi bag and any forms of shelters (tarps, trees....).

So where does condensation comes from? It occurs when there is a high air moisture content and a sudden drop in material temperature. In a tent your body will evaporate water vapour, which pushes its way through the breathable silk lining, sleeping bag and leave a lot of moisture inside the tent/bivi bags. The inner tent/bivi bag surface will have a lower temperature than the moist air and they are not very air permeable, therefore water tends to condense on the inner surfaces.

The way to reduce the problem is either:
1. Provide better ventilation on the shell (air vents, goretex tent/bivi bag material) to encourage the moisture to escape.
2. Reduce problem caused by dripping condensation, e.g. water resistant sleeping bag shell. In fact most mountain tents have water resistant but breathable inner tent and a non-breathable fly. Therefore any condensation trapped inside the fly will not drip back inside the tent.
3. Stop any moisture coming through the sleeping bag – in extreme conditions some people use a vapour control layer, basically a plastic bag inside you sleeping bag. You’ll probably sweet like hell inside but at lease your sleeping bag and tents can stay dry.

To sum it up water vapour wants to find its way out and you want to let it out in your desired way (ventilation) not stopping it on surfaces you want to keep dry.

The fundamental use of silk lining is to keep your bag clean and trap the warmth. Since silk is more permeable than sleeping bag material (e.g. pertex) it will not stop water vapour. However, arguably a sleeping bag lining might absorb a small amount of moisture and keep your skin drier.

Comparable to old stone, mud or timber log houses the way traditional outdoor fabrics like wool work in a different manner. Instead of trying to provide a tight waterproof shell you allow the outer surface to get wet. Since the inner temperature is higher water wants to push its way out. Without a waterproof coating these material tends to get wet quickly but also dry up quickly. Besides, the thickness of these material allows a lot of moisture to be absorbed into the material and keep the inside of the house/ your skin dry.

In reply to ADJC:

The other issue to consider is: is the 'liner' big enough to contain the sleeping bag without restricting the loft?

In reply to ADJC:

I'd buy a sleeping bag with a water-resistant outer shell, eg Pertex Endurance such as Rab use.