Guys, I'm planning a backpacking trip to a 4200 masl basecamp in summer (South America). Low probability of snow but some times it is really cold arround -15C.

I do not own a sleeping bag -18C (or similar) so I want to use the following configuration to sleep only with 1st layer on a -15C night. Opinions?

Saco de dormir: Marmot de pluma 850 -9C (http://marmot.com/products/details/helium-new)
Comfort: 27.7°F / -2.4°C | Lower Limit: 16.3°F / -8.7°C | Extreme: -16.6°F / -27.0°C
Upper Limit The highest air temperature at which an average man can sleep comfortably.
Comfort The lowest air temperature at which an average woman can sleep comfortably.
Lower Limit The lowest air temperature at which an average man can sleep comfortably.

Liner: Thermolite Reactor Extream (http://www.seatosummit.com.au/products/liners/thermolite-reactor-extreme-liner/)
The Thermolite® Reactor Extreme increases sleeping bag performance by up to 15°C with its super insulating 110g/m² Thermolite® fabric. Spacious design allows for a comfortable night’s sleep, even when the mercury plummets. Thermolite® is a fibre with a hollow core that provides extraordinary warmth for its weight while remaining extremely breathable. The Thermolite® Reactor Extreme adds warmth to a sleeping bag or, used alone, is ideal as a warm weather bag. With the stretchy knit fabric that moves with you and a draw cord hood, this liner is perfect for wrigglers and rollers or anyone who needs some extra warmth.

Marmot Alpinist Bivy (http://www.backcountry.com/marmot-alpinist-bivy)

Sea to Summit Duck Down Sock

Thanks in advance

Cris

@cbobadil

I see no problem there. You won't be sleeping naked in your sleeping bag. People usually sleep in all their clothes while sleeping on a climb. So taken this in regard you will do just fine with even lighter sleeping bag.

I have slept with +2°c sleeping bag on -15°c (BUT it was in a tent, not a bivy) fully clothed, and I was sweating through the night.

In a book Extreme Alpinism by Mark Twight there is a good explanation about this very issue. And saving weight by having lighter sleeping bag.

If you cannot cannot afford to buy a book, there are ways of downloading it via torrents from websites like torrentscan. But I advise you against it, because it is illegal and immoral. After all, it's a theft of intelectual property.