Sleeping Bag wet from condensation.

All I want to know is why it happens.
I am sure others are in the same situation.

I am inside a canvas tent.

What is the road less travelled?

Hi Humphrey57 ....

"All I want to know is why it happens."

Simple thermal physics. Your body warms the inside - so the cold air forms condensation on the outside.

SB's with better external waterproofing will form more condensate than those that are less efficient. The less efficient ones will absorb the condensate and you may not notice it.

Ok. At first my bag feels wet on the outside.
Then progressivlly gets wetter until it is wet right through.

The outside is not waterproof. Am I correct?
So sleeping under a tarp is a good idea?

.

Yes and No.

Contrary to my previous reply, the condensation that may be occuring on your sleeping bag is likely to be from the moisture in the night air because, as you said, the more it is ventilated the worse it gets. Putting a tarp over yourself will help, but if your head is under the tarp you will get condensation on the inside of the tarp for the reason I described above.

R.

Short answer:
Keep your head outside the tent.

Long answer:
To understand why it happens you must understand why we get rain and storms.

Air can only hold so much moisture. Warm air hold more moisture than cold air. Say for example Air @30 deg C can hold hold max 10ml of water per cubic metre, while air @10 deg C can hold only 5ml of water.

So, if your air @ 30degC holds 5ml of water it is said to have 50% "relative humidity." If that air is cooled to 10degC it still has the 5ml of water in it but the relative humidity is now 100%. If the air temperature drops below this temperature, the air will need to release some of the water vapour and form water droplets. So if the air is cooled to say 2degC it may only hold 2ml of water and the remaining 3ml will be in the liquid form and will fall (out of the sky.)

When you are in your tent sleeping, the air you breath out is warm and full of moisture. When this warm air passes by something that is cold, like tent poles, tent lining etc, its temparature drops and water droplets are released.

What could possibly be happening in your case is condensation is occuring on the inside of your tent ceiling and dropping on you. I think it is unlikely that condensation would occur on the sleeping bag itself.

I heven't heard of any great ways to prevent this other than raising the temperature of the air in the tent and therefore the tent itself. Try putting a blanket over your tent above the area where you are sleeping to increase the temparature of it in that spot. Else, stop putting warm, moist air into it.

Hope that's clear enough.

R.

Thank Rob. Now I know why I can try a few different things.

It seems to be worse in bigger tents with more ventilation.

I was in a 3x3 mtr Serengheti on the week end and in the past a 14x14 marquee.

When in my caravan I just have sheets and a blanket.

Cheers Phil

There are now water-resistant breathable bags you can slip your sleeping bag into to increase warmth and reduce water absorption.

If you want to spend up big, there are also sleeping bags with shells of this material that does the same job.

Maybe there is a market for a goretex blanket.

Vapour transmission with Goretex and the like depends on the pressure difference - warm humid inside, colder and dryer outside.

What's happening here as posted is condensation on the bag from moist cool air.

Another option would be to spray some waterproofing stuff on the bag shell; that'll reduce moisture absorption.

That's a great idea Sigmund. I will track some down'

I hope this has helped some others

Happy to help.

I've got 4 down and 1 synth bags, for sub-zero to mild nights.

Some condensation is inevitable. My old Paddy Pallin bag, japara covered, looks like I've been incontinent :-{

I've always been in small hike tents. My take is that it's water vapour off the body that condenses when it breathes through to the shell; but many mornings the foot of the bag is against the tent when I wake up, and is pretty damp, so there are other things going on as well.

Anway, re a proofing treatment, speak to Roman or a decent bushwalking shop since there are options. Even one where you wash the whole bag in it.

Good luck.