How 'dry' are dry bags?

[DoctorRad]

I'd welcome your opinions and experience as to just how dry 'dry bags' will keep your kit. Strapping dry-bagged gear behind the seat or to the handlebars seems like a great way to keep bulk down and even reduce the cost of buying separate bags. However, I'm obviously worried that I might not have a very good night's sleep in a down bag which has been in a dry bag which isn't really 'dry'.

I guess this varies from brand to brand, so comments on specific brands or models would be useful, as well as more general comments.

Thanks,

Dr. Matt...

[AZTtripper]

I have a fair bit of experience with dry bags used on river trips the key thing is getting a good seal when you wrap the top. To achieve this it is key that the bag be no more then half full. My sister did one over night guided river trip her biggest complaint was that the guides only let her use half of the bag they gave her. When she mentioned this to me I said well if you didn't care if you shit got wet you could have fill it more.

A good quality dry bag properly sealed will keep your down bag dry though on river trips when I take the down it is in two dry bags. First in a small one by itself then in side of a bigger one with other stuff.

For bikepacking I use an older river bag just for my pad and ground sheet not really to keep it dry it's just a good size for that item and I put it on the bars.

For the seat post bag I have an Epic that was dry at first but over time it has developed several small holes at different wear points. If I think it may rain I stuff the down bag in side of a plastic garbage bag.

Jacks makes the best most durable dry bags on the market but I doubt they would work for bikepacking http://www.jpwinc.com/

The bag I use on my bars is similar to these http://www.outdoorresearch.com/site/window_dry_bags.html mine predates the window.

I have heard good things about these bags http://www.seatosummit.com/products/display/1 I just can't figure out how a waterproof bag can be compressed if the air can get out how is it that water can't get in.

I see that Sea to Summit does recommend double bagging electronics I would say the same for down.

Hope that helps.

Tim

[Done]

Dry bags are very dry! Unless, of course, you punch or wear holes in them--at which point any water that gets inside will stay inside, and you'll have a very wet bag. I second the Outdoor Research recommendation--good gear in general. I've also used the lightweight Sea to Summit bags, and they are great too. Just make sure that you protect them from excessive abrasion and you should be happy.