On one of the nights I was out recently it rained heavily. My tarp was set up in a half pyramid so my bottom half was out in the rain. I have an Alpkit hunka which I have never had a problem with before but this time woke to find the foot box area quite damp. I sleep on a 3/4 mat so the bottom half of my legs are on the ground, would the condensation that formed inside the bag be due to the temperature difference inside the bag and the ground and would a full length mat have avoided this?

It's more than likely down to the rain creating a film of water over the exposed sections which prevents moisture escaping. A DWR coating on breathable fabrics helps rain the 'bead' off and prevent this from happening, once any waterproof fabric has wetted out breathability becomes an issue.

Personally I prefer a full coverage tarp, with only a water resistant bivvybag. It keeps weight down and condensation is much less as the water resistant bags tend to breath a lot better.

By water resistant I still mean a HH of 10,000mm or there abouts.

Anthony wrote:By water resistant I still mean a HH of 10,000mm or there abouts.

Is that typo? A HH of 10000 is listed for a Hunka, and many would say is waterproof. I use a PHD drishell sleeping bag cover, it has HH of 1500, that's water resistant.

The most breathable part of your bag is the head area, the big hole lets all the heat escape. The toe box is the least breathable, hence why it's damp, the heat can't escape anywhere. A little through the fabric but no-where near as good as the top end. It happens with waterproof boots, damp toes and dry heals (and most people will think their boots are leaking when they're not (well, not all the time!))

I've got an alpkit bag and on the WRT13, my bag had condensation on the inside on a clear night with a full sleep Matt. They're very waterproof but it didn't excel at being breathable

Not sure if my ramblings help you though

Simon, that has cleared up a problem with the walking boots!! Theres me thinking the goretex has broken down

As Anthony says Ray, if the outside of the bag gets wet then it won't breath as well as it could. A still / calm night will also make the problem worse. If you're overly warm during the night the problem will also increase.

I keep thinking that I might add a small mesh vent to the foot end of a bivvy bag. I'm sure it could be done without losing much protection and would hopefully help with condensation troubles.

I think Anthony has it right then as the outside of the foot area was all wet so stopping breath ability. If that's the case then I'll have to go back to using a frame style when it's wet. Only used half pyramid that night due to lack of space/sloping ground. Biviing close to Gretna did limit my options somewhat, lesson learned.

s8tannorm wrote:I keep thinking that I might add a small mesh vent to the foot end of a bivvy bag. I'm sure it could be done without losing much protection and would hopefully help with condensation troubles.

I was thinking the same. There must be a place to have a small vent, say 0.25inch square, to allow ventilation but wouldnt be an issue if it rained?