A bivvy-bag or bivi is not just a way of saving money but also of having new, intense outdoor experiences and hopefully some fun; so here some basic tips to avoid too much hardship! For moisture to pass through the breathable membrane of your bivvy bag efficiently, avoiding a build up of condensation, the conditions need to be:
- warmer on the inside of the bag than the outside
- more humid on the inside than the outside
- have a movement of air across the outside
These conditions normally prevail (unless you have either died or been inundated by rising water.) However there are a few balancing factors to take into account.
Warmth The best sleeping bags for bivvying are probably lightweight, one season and synthetic filled, which dry very quickly if you should experience some condensation. A bivi bag will usually raise the performance levels of a sleeping bag by 5 -10 degrees, depending on wind factors. It seems to make sense to carry a light sleeping bag (800-950grms) with a contingency of warm clothes such as fleece jacket and trousers that can be worn in the bag if necessary, rather than a 1.5 kilo bag. The clothes can be worn to sit around in and so allow more versatility. However when deciding what to wear in the bag don’t put on more than conditions warrant and get too hot as this will only lead to sweating and a clammy atmosphere. Clear starry nights are the cold ones when you will need more warmth. On hot summer nights especially in warm climates at low altitude you could probably do without the sleeping bag altogether and just wear some warm clothes in the bivvy.
Wind A bivvy bag isn’t going to blow down but if the wind is strong enough to mould it flat against parts of your body then you will lose heat in those places. So, in situations where cold is an issue, choose a site with some shelter from strong wind, such as a bush or rock.
Rain. Close up the foot end drawstring into a small aperture and tuck it under the foot of the bag or pointing downwards. Close the head end drawstrings into a small aperture you are comfortable with and arrange so your mouth is near the opening which is pointing downwards to prevent ingress of rain. Wearing a hat with a substantial brim or peak, such as a baseball cap, will keep the fabric off your face while sleeping and do much to prevent the over enclosed feelings that some people feel when completely inside a bivvy bag.
Midges. Closing up the bag to a small breathing hole over your mouth seems to deter them, or wear a midge proof head bag, or sleep above 600m- 2000ft where they aren’t usually around.
Comfort. Dry, dead grasses and heather are the best for warmth and softness. Flat is good but if your feet are swollen and sore sleeping with them slightly raised can help. Do lay on the ground before committing yourself to sleeping on it as there may be hidden stones. Unless its very windy you will lose more heat into the ground than the sky so a mat of some description is a good idea. The ¾ length, lightweight, inflatable thermarest type are very useful. The closed cell foam rolls are also O.K. and cheaper and lighter (cheaper and lighter still if you cut one in half between two people). I put my mat inside the bivvy as I find it helps to stop me rolling off it in the night
Care of the Bivy Bag
- don’t use it as a ground sheet over thorns or sharp stones.
- don’t store it wet for longer than a day. It will not take long to dry.
- Roll it up, rather than crunch it, and store in a stuff bag.
- If it gets dirty wash with ordinary soap or one of the special products,
- such as marketed by nikwax, not detergent of any kind.
- Reproof with a spray for breathable fabrics, such as nikwax tx or grangers supapruf
- Finally, for anyone who has never slept outside, in the dark, in remote, high or wild locations, by all means take a friend the first time, but for the full primal thrill of it make sure you do it alone some time.