I have a 3 season down sleeping bag that I use mainly during the summer months, but was thinking of a late summer/early autumn bike packing trip. The question if have is that when you go outside in the morning at this time of year the grass is damp and I suspect a tent would end up getting pretty damp too, especially, with condensation.
Do down sleeping bags absorb atmospheric moisture, or do they physically have to come into contact with something damp / wet before they start to lose their warmth?
Aside from an obvious soaking you should be fine. I never use anything other than down regardless of the weather. If your tent is getting damp then open the door up and let a breeze in.
Yes, they absorb atmospheric moisture including your breath but this is generally so minimal that it's unlikely to affect the performance of the bag.
The moisture from your body is actually more of a problem. As long as the bag is warm it isn't an issue but if you allow it to cool down, moisture will condense inside the baffles and make it damp. The advice is to roll your down bag as soon as you get up so as to push out the air while it's still warm.
So I needn't worry 🙂 Thanks Chaps
Sleep under low tree cover where the dew doesn't form and all will be ok. A good 3-season (6-8 deg rating approx) on a decent mat will be ok into late autumn ime. What you sleep on, how exposed, wind etc make a big difference, so look for added natural shelter and warmer drier ground like leaf litter etc.
I've only got a down bag, never noticed it get damp unless my feet are touching the edge of the tent.
Irritatingly this kinda puts me off using my 1 man tent in winter when it should be the most usefull as it's barely bigger than a bivi bag.
If you're worried, get a goretex (or similar) bivi bag, I've woken up with my feet in puddles in my ex-army one, inside the bags always been dry. As an asside, it easily adds one seasons warmth to the bag, probably heavier than an extra seasons sleeping bag (1vs 3, or 3 vs 4), but was cheaper than an extra bag for occasional winter camping.
If atmospheric dampening is going to occur - ventilation is key - so maybe pitch your tent in the wind - base layers. can also help trap moisture leaving the body ... Personally I like an oversized 1 season synthetic bag with according down clothing inside the bag - best sleep system for UK conditions - unless it's sub zero then the down bag comes out but then moisture isn't so much of an issue ...