The Heatsheets Bivy is not made of Thermolite.
The Tlite is a heavier and feels more like a fabric. The bivy takes up about the size of 3 or 4 HS bivies. The Tlite has a general "sleeping bag" rating of 50F which means it will keep you comfortable above 50F but it will HELP keep you alive WAY below that. Think; Heatsheets-summer, temperate weather, "just in case".
The Tlite is more-colder weather, very possibly overnight, UL-sleeping bag.
Bill