MYOG bivy question

I don't think the reflective barrier under you will provide any meaningful radiant benefit. Radiant barriers provide the most benefit when there's a big differential between where the heat is radiating from and to.

The sun to your body has a 6,000C temperature differential- reflective barrier works great as a sunshade.

Your body to a clear night sky (absolute zero?) has a 300 C temperature differential- Reflective barrier could work well.

Your body to snow has 37 C temperature differential- Reflective barrier not so effective. Also consider that the mass of your pad will block radiant energy loss as well. Radiant barriers really need to be suspended, not in contact with the two masses that you're trying to block heat transfer between.

The thin mylar space blanket is cheap, light, and fairly tough for the weight so it may be good for a bivy bottom as long as you are willing to replace it when it wears out. Just don't expect miracles in heat retention.

Adam,

You need an air gap for a reflective barrier to achieve its potential. You will receive no additional heat retention.

Once a small tear appears (& it will)…watch out!

They aren't durable.

You could use frost king window insul film. That is what everybody uses for UL ground sheets.
I bought 2 kits for sliders and it will probably last me a life time.

I tried to glue some with mcnett aqua seal which is very very tough glue. It will hold and is strong, but enough pressure and the glue will fail. It will however glue any other cloth to any other cloth including tyvek, cuben, sil nylon dwr nylon etc. With any of those the material will fail before the glue.

A better material would be a cuben floor.

1.25oz tyvek cost like $5 a yard from quest.

Thanks for your feedback everyone, sounds like the mylar material would have some durability issues and not help with heat retention. I will probably just stick with silnylon for the floor and use a groundsheet.