soggy bivy nights?

SK,

Regarding 1, "it would close" should have read "it will be close". 2. "Part clo" refers to the clo of the part taken by itself. For example, the GG pad has an R value of 2.27 which is equal to a clo value of 2.59. The pad part and the bag part make a sleeping system which has its own clo value. This system clo value is calculated knowing the clo values of the constituent parts and the sleeping position of the sleeper. In this model, I assumed he was sleeping on his back. That is the reason for the label "Pad +Bag (back sleep)".

Thank you for your valuable comments. I will use them to update the original post for clarification.

Thanks for building the model, Richard. This is fascinating stuff!

The Havenith study you mentioned looks very interesting. I was not familiar with it. The subject definitely needs more research.

I have a couple of comments and questions on your model:

1) As you mentioned, there a many other variables that could be built in that *may* improve the model quality and *will* increase its complexity. :-) Nevertheless, it would be interesting to see the effect of increased air exchange due to the higher air permeability and wind of the "bivy-under" scenario. I speculate that this would reduce the RH in the system, move the dewpoint outwards, and reduce the total amount of moisture subject to condensation in the layers.

2) Your chart of the "bivy-covered" scenario implies that after condensation to liquid at the dewpoint, the moisture will re-evaporate and escape. Why wouldn't any re-evaporated (or wicked as liquid) moisture freeze as it crossed the freezing point? Note if it did freeze, it appears that it would be on the bivy in this case and not inside the bag, which makes it much easier to eliminate in the morning.

3) The original poster said that he was at "about 30 degrees". This is a tricky temperature at which to stay dry in any sleep system. Also, at exactly 30 degrees, your models produce freezing condensation in the bivy-under case and liquid in the bivy-covered case. But a few degrees in either direction, such as might be influenced by the use of a tarp, would change that outcome completely.

4) In my experience, there really is no single "dewpoint" in a typical sleep system. It's smeared into more of a "dew gradient" by the temperature gradient, humidity gradient (which is affected by air exchange), and wicking within the insulation.

5) I usually suggest something like the bivy-covered system for breezy conditions and the bivy-under case for still air conditions — in effect, the opposite of cases you've shown. How would your models behave in these cases?

Best Regards,

-Mike

Mike,

Thanks for your comments. I will reference relevant parts of the Havenith study to address some of the issues you raised. His test environment was a 20 F environmental chamber with negligible wind (.2 m/s). 150 grams of induced clothing moisture plus insensible perspiration were evaporated in the bags each night.

1. The Havenith study showed no difference in bag moisture accumulation between using no bivy (bag under) and a breathable bivy (bag over) in still conditions. Since the bivy variable can probably be eliminated for still conditions, a simpler problem to model would be "will a sleeping bag used in the wind accumulate less moisture?" Can you come up with a model for this so I can use yours (smile). If not, I will look into this when I have a block of free time.

2. The moisture will freeze as it crosses the freezing point. In the still air “bivy over" model, this point theoretically occurs in the boundary layer above the bivy. Note the model shows 39F at the fabric and 30F approximately 12-15 mm into the ambient environment.

3. I agree that the original BPL post defined a fairly complicated border line situation. Havenith showed that even at 20F, there was no difference in moisture accumulation between breathable "bag over" or "bag under" after 6 days of use with damp clothing. It is interesting to me that the Havenith study used a similar bag to the original BPL post. For a sleeping pad he used .6" closed cell. Obviously no one subscribing to BPL would carry that heavy of a pad (smile).

4. I think you are correct on this point.

5. Again Havenith showed no moisture accumulation difference between "bag over" and "bag under" in non-windy situations. As I mentioned in point 1, we need a model we can agree on for the sleeping bag in the wind case. Hopefully I can use yours (smile).

Thanks again for your comments,

Richard

I agree, modeling the effects of wind beyond stripping the still-air boundary layer is no easy task. I don't believe I can build any kind of suitable model anytime soon… Maybe we can convince Roger to come up with something. :-D

Ahh, good point about #2. I missed that the freeze point was outside the bivy layer.

Cheers,

-Mike

> 2. The moisture will freeze as it crosses the freezing point. In the still air “bivy over" model, this point theoretically occurs in the boundary layer above the bivy. Note the model shows 39F at the fabric and 30F approximately 12-15 mm into the ambient environment.
Yeah, this highlights one of the seriously important things about camping in sub-zero weather: air movement and boundary layers. Just blocking the wind with a double-skin tent makes so much difference.

Must read the Havenith paper. Have to find it.