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I read a few blog reviews and don't recall off hand which one was which, but one was tested down to around 28F. That guy initially tried the blanket on its own, but it wasn't cutting it. He then lined his spring bag with one to see if it would work in a pinch. Another was tested close to OP's temp of around 40F. I have a feeling it was the colder temp one, as that guy sealed it over his head. Said his clothing was damp when he crawled out.What was the outside temperature at the time?
I would think that installing one the these blankets inside a sleeping bag would prove more of an issue as condensation would buildup inside your sleeping bag... kind of like a cheap tent that doesn't breathe and waking up to rain drops inside.
I've used a mylar blanket several times to supplement a 40 degree bag in temps in the 20's but kept the blanket outside of the bag. They trap every bit of moisture and by morning the outside of the bag was very damp. Pull the foil over your head and you'll have cold drops of water in your face by 3am so I keep the foil over the bag and wear a wool hat. I was out west in high desert so getting the bag dry for the next night wasn't an issue but something to consider in other situations.I've read a few reviews of those foil bivy products that all mention this issue. One reviewer said he was soaked from condensation after a night in one. It did maintain a livable temperature, but if completely sealed-in including your head, your own breath is enough to cause dripping condensation after a few hours. Your body also gives off moisture as well.