I used the setup this last weekend but I didn't want to use the 12oz space blanket so I bought a $2 one from Walmart. Just the cheap one and it weights nothing. I put a thermometer on my ridgeline and the outside sensor outside my winter tarp and there was a 14* difference and when temps dropped, it showed 10* difference. Could it the space blanket that is reflecting all the heat back?
I would recommend looking into a hammock sock as well. I did a hang this past weekend with Out&Back where the temps got down to -3 w/ 20mph winds and all I had under me was a hammockgear 40* Phoenix with 2oz overstuff and a Molly Mac hammock sock with the IX lining on the bottom. I wasn't toasty warm but I was far from freezing. Not bad for using gear that is rated for somewhere around 35* higher than the actual temps.
You will have to expreriment with the sock and do your research to figure out condensation issues but there are some great threads on here. just my $0.02
I just use 4 clothes pins, the spring type. to clip the space blanket to the under quilt supports. really makes a difference!
I have found the effectiveness of a similar Lost River UQ to vary with how well I stop drafts. Sometimes it felt like it was giving off heat; other times not so at all, but I didn't have someone to adjust it; and I had no effective draft-stopping solution.
I am sure the loss of insulating value lost due to bad seals and pumping of air from movement of furrows exceeds what is gained by the thin mylar space blanket unless there is evaporative cooling which is stopped by the mylar. Radiation is an insufficiently large component of heat loss for the blanket to make much difference, especially after the bed of the hammock and the clothing the occupant is wearing.
How to stop that evaporative cooling (as well as drafts?) Those terrific binder clips... and some draft stoppers for the ends.
I have a foam pad that I lay inside the hammock (carry it in case I can't set up the 'mock and will spend the night on the ground). I also have some "reflectix" (lookslike silver bubble wrap) from the home improvement store that gets stuffed in the underpockets of the Clark NA.
That keeps me toasty warm even in winter.
I tried using the small ones...not strong enough. I'm going to try the med size next time.