Don and his son tried out two of the large side zip bivysacks on subzero outings this past year.
One was made with the mil-spec 30d nylon with a Durable Water Repellent finish. Not a waterproof fabric, it has a hydrostatic head (measurement of water pressure at the point of leakage) of over 400 mm.
The other fabric was a 70d ripstop nylon with a pTFE laminate on the underside. This is considered a waterproof and breathable fabric with a hydrostatic head of 3000 mm.
Both sacks used a silicone coated nylon bottom.
I asked Don to let me know about condensation and other considerations he discovered on using the two bivys.
Here is his report.
OWARE BIVYSACKS FOR WINTER USE
I tested two of the OWARE large size winter bivysacks, one with the High Vapor Perm PTFE fabric and the other in Durable Water Repellent ripstop. fabric.
One night the temperature was a few degrees below zero Fahrenheit. My son and I used the two biviys directly on the snow after digging and firming up by snowshoe compression the base. The bivy sacks’ large size easily accommodated winter weight mummy bags and double sleeping pads. They were great for fitting in clothing and the many items such as winter mukluks or boots, clothing, etc that are needed close at hand on a winter bivouac. I was particularly interested in comparing vapor transport between the two bivy’s.
I found that there was very little if any appreciable difference in frost build-up between the HVP bag and the DWR bag. I think that vapor transport in far below freezing temperatures ceases to take place if the fabric isn’t close to a warm body, in this case the sleeping bag. When the vapor left the sleeping bag outer fabric and hit the cold bivy fabric, it turned into frost,.
I found that the black , textured PTFE fabric tended to harbor frost and snow within the fuzzy nap of the fabric on the top cover and was hard to thoroughly brush off.
I decided to choose the olive ripstop , very slick bivy as my winter bivy of choice.
The insect netting at the head was not just unnecessary for winter use, but actually a nuisance, as the netting accumulated frost from my breath and rained it down on my exposed face and sleeping bag. Much less of a problem (though still present) with a head opening lacking netting. I will probably remove the netting in the future since the Oware Large Bivy will be used by me for only winter use.
Another factor to be considered in purchasing gear is the sewing. The seams were all very tight and with no wander. All high quality zippers and material and made in America!
Buy them here