I tried to use this instead of a sleeping bag to see how warm it keeps you.
This post was prompted by roadtorques suggestion of what to use as insulation in the summer.
Road torque mentioned the idea of using just a silk bag liner and carrying the bivvy for emergencies. I tried this combination out but since it is not summer (it was 44 degrees) I thought should start a new thread rather than hijack the other one which I still hope gives more ideas of light summer top insulation ideas.
In any case, I was doubtful that it would work. I used my hudson river as bottom insulation and first just tried the silk liner alone....not a chance!
So I added the Adventure medical kits thermolite 2 bivvy and then fell asleep promptly. It was 46 when i fell asleep and 44 when I awoke. I was warm. I did not want to get up. I forced myself because I wanted to compare it my REI 55 degree rated bag that I usually use in summer. I noticed when I took off the bivvy and the silk that I was actually sweating on my legs. This of course made me immediately cold as well. THe REI sack did not warm me up....it might have but I had no patience so I added the silk right away. I still did not warm up quickly so I replaced the bivvy as a top cover (not getting in or even tucking it around me just loosely laying across the top of the travel sack) temporarily thinking I will remove it once I'm warm and see if I stay warm at that point but...i fell asleep again hehe.
silk liner weight 4 5/8 0z 55 dollars size of of frozen lemonade can
thermolite bivy weight 7 1/4 oz I bought used for 15 dollars size of nalgene bottle
rei 55 travel sack weight 1lb 10 1/2 oz paid 49 for it 10 years ago size of nerf football but compresses to sizer of softball
so.....in terms of functional usage and low weight....I am now a believer that the key here was the thermolite bivvy even to the mid 40's.
the travel sack is not worth its weight in functional usage it seems but it is comfortable and better built than the bivvy which seems is almost a limited use item in its almost disposable type construction.
It seems much more effective per ounce than the silk but the silk helped and is comfortable and packs very small so it has a role.
A good sleeping bag is more durable. You could probably use the thermolite till it shreds and then just buy a new one though and you don't have to worry about your down or expensive investment.
Also about my legs sweating. I know that means I was warm which is great, with no bag and just a silk liner. I don't know what that means in terms of "breathability" of the bivvy, but I can tell you that when I use a bag that is overrated for the temp, I sweat in the past too, so I can't say this is a fault of the bag but could just be a sign of its effectiveness.
Just some observations on an inexpensive piece of gear.....