The Importance of a Bivi Bag in Dry Weather?

[d45yth]

Hi, I was going to rush out and buy a bivi bag for a short trip tonight (to save carrying my tent). The weather is supposed to stay fine though, so I wondered if I'd be fine without one? It's a down bag I've got and had thought of just putting a polythene sheet on the ground to keep things clean (I'd be on my air mat too).

It may seem like a daft question, but I'm not sure how damp my bag would get from what's in the air, compared to condensation within a bivi bag?

Cheers.

EDIT: I should add that I'd be carrying an emergency bivi in case the weather turns.

[Chew]

Dampness of you bag will probably* be the same reguardless of a bivi. If you have conditions where mist/dew could form you're more likely to get condensation as well.

The main thing will be rain, where a tarp will be more use

*If you die, first dibs on you bike

[d45yth]

Thanks, I thought it would get damp whatever I decided on. I think I'll manage without one then.

P.S. If I've time, I'll nip to the solicitors and get you put in my will. Don't think there will be much chance of me dying at this time of year though. :)