How many F degrees of warmth can a silk sleeping liner and a LW down puffy add?

Can you say "flexible"?
Can you say "adaptable"?

There is no ONE solution if you are moving through varied terrain and latitudes.
You would hate a 20°F bag in a double walled tent for most of your journey.
A quilt, and a net-body tent, or a hammock, on the other hand …

Don't be afraid to make "mistakes". You won't die.
Make a "best guess" and then learn what works as you go.

Bolivian stand off.
LLamas having an argument.

The point of contention is the exact direction to the nearest tree

Brilliant Franco.

As with regards to "flexibility", I know my own limits, don't you worry. I tend to plan ahead a little too much, trying to forecast circumstancies and eventualities I don't and can't have any control on. It's part of my perfectonism. And I can be fairly rigid at times. But I'm a good egg afterall.

I also do know, though, I won't have much money left once there and I surely won't want to spend them in acquiring new gear. I'd rather just go prepared.

Possibly a 20F/15F quilt with a CCF sleeping pad and possibly a Therm-a-rest NeoAir Xlite + Moment DW or StratoSpire 1 might be the solution…?

Agreed. (I am agreeing with Ian to be clear)

Now that you have wisely changed your plans to "not camping most of the time" your best bet is to grab a 35 degree bag or so and in areas it is too cold you either:

A) stay inside (best choice)
B) buy/rent another cheap bag and use both in that region and sell/return it when you are farther North
C) Ignore that first part and buy a 20 degree bag and sell it for a cheaper bag when you get to a warmer climate (you are going to lose money here obviously)

Really don't see how it gets simpler than this.

You are going to HATE it if you are carrying a 20 degree bag and it is 60 degrees and humid at night.

A sleeping bag or quilt can be expensive and important investment. I'm not trying to be rude but by the flavor of your questions, it seems that you will benefit by some time on the trail between Mexico and Bolivia before you make that decision. It is the best teacher.

You can buy at $30 synthetic sleeping bag and won't need to even think about replacing it for a couple thousand miles. So let's say that you get to Bolivia and decide that you'd prefer to go with a modular system. Great! Cut the zipper out of the cheapie synthetic bag thereby turning it into a quilt and use that to supplement a higher quality down sleeping bag. I've traveled all over the world and in some pretty sketchy conditions. I've never found myself more than half a day away from an ATM and an internet cafe. Keep the money you're looking to spend on a sleeping bag in your savings account. It'll still be there waiting for you.

I'm not suggesting you do something I haven't done myself. I have over 100+ nights in the Panamanian jungle. You'll be fine for the first 1000 miles or so of South America.

I don't see why you can't take care of the backpack and shelter now but I'd wait on the sleep system and get the cheapest solution you can live with for the time being. You don't have to take my advice and I'm certainly not interested in being a nag but you're asking the questions and this is my suggestion.

And with that… I'm done lecturing. Good luck and I can't wait to read about your adventure.

"You can buy at $30 synthetic sleeping bag and won't need to even think about replacing it for a couple thousand miles. "

+1

That has a zipper all around, to open into a quilt when warm, and big enough for any extra layers, inside, when it gets cold.

When traveling in Colombia, Ecuador, and Peru you will encounter a very wide range of temperatures as you change altitude. In my experience between 10k and 15k in Colombia and Ecuador you will encounter daytime temps between 60F and 80F depending on the cloud cover or inclement weather (25F to 45F nights). Between 10k and 5k you are talking a very wide range of temps; between 65 and 85F for daytime highs depending on cloud cover (45F to 65F night time temps). Below 5k around the equator and you really start to heat up; around 1000m you can expect daytime temps in the 80F to 90F range (70F or so at night). This average temp estimate is appropriate for Colombia and Ecuador, I don't know about Peru but I would guess it would be fairly similar in at least the northern half if not a lot of the southern half as well. I went from Cuzco to Puerto Maldonado by bus and the mountain passes had some snow and it was chilly (maybe 20 to 25F at night), but the Amazon in Puerto Maldonado was scorching.

I guess my point is that the tropical Andes region is to be thought of in terms of verticality and not longitudinal distances. Also, you are talking about large countries that are oriented north and south (Peru, Argentina, and Chile in particular) and half terrain ranging from glaciated peaks to Amazonian tropical rainforest. The idea of an average temp for every country doesn't make any sense.

What high altitude areas are you planning on visiting? A more detailed itinerary would get you better/more thoughtful responses and would also be a very wise thing for you to do. Your gear choices will depend on where you are going. The ideal gear for Mexico down south to Peru will be quite different than Bolivia, Chile, Argentina (especially if you'll be arriving there outside of summer)

A silk bag liner will keep your bag cleaner, not warmer.

No uninsulated, breathable bag liner is worth its weight – period.

The only bag liner I'll carry is a VBL liner for very cold temps.

EDIT:
Oh, yeah, a puffy jacket – That WILL keep you warmer. It depends on how much loft it has and how much room you have in your bag to accommodate it without unduly compressing either the jacket or the bag. But you will also need to insulate your legs as well.

I agree with what Eric says.

Pretty much agree here as well.

What you basically have with a bag liner is a sheet.

There isnt any magic. If a liner was made of magic material that could add 15F warmth while being only 1/16" thick, all sleeping bags would be made of that magic material, instead of down.

Well, I did say what down jacket I have and how muc it is filled. I also said I will have fleece pants, along with long underwear.

William F has it right. In the Andean countries, average temps are meaningless. There are three main climates: Pacific coast, Andes, Amazon. Temps will be mostly determined by which side of the Andes you're on and elevation. The coast of Peru is mostly not hot, not cold, except hot in some coastal deserts.

Today's temps in Peru vary from 15F (Juliaca on Lake Titicaca south Peru) to 89F (Tarapota in the Amazon).
http://www.wunderground.com/weather-forecast/zmw:00000.1.84628?
The weather underground has current and past weather for many places in Peru, and probably other countries. You can look up most the detailed info on many places on the weather underground.

You will probably get more use out of a sheet and light fleece blanket than a sleeping bag until you're traveling in the Andes (except I know nothing about the mountainous areas of Mexico and central America.) How warm a sleeping bag you ultimately need depends on how high you go in the Andes.

But I do not want stuff for the Andes right now. I said I'll leave those aside for the moment.

Also, I do have a snthetic sleeping bag already, which is massive, is heavy, bulky and doesn't keep me warm at all. When there were +15C here in Mexico, I had to sleep, inside a house, with that and 3 more wool blankets!

Well I don't know what else can be said at this point. What have you decided on?

Alex,
If you need three wool blankets on top a sleeping bag at +15c (59f) I would suggest you don't go anywhere near snow or get something rated 20c colder then the temp you expect.
BTW, often the rating on a synth bag is way out.
The less expensive the bag, the higher the discrepancy.
(that is because the assumption is that if you buy cheap you don't intend to use it that much and most likely you don't know about bags and temps so you will be impressed by numbers rather than materials/loft/design)

Well, the synthetic sleeping bag I have was "taken" from my old house "left over" stuff, so I don't even know how much it costs or if it is any good. I give it a crap+ as a score.

I said I'm a very cold sleeper, but in Playa del Carmen, the humidity makes the heat hotter and the cold colder!

I haven't decided anything as yet, all I have done was printing this thread out and I will go through it properly tomorrow, then, hopefully, make a decision.

I need to get everything by the 11th of June at the very latest! God helps me make decisions!!!!!!!!!!

In my experience the best (current) synthetic bag fill is Climashield. It resists loss of loft from being compressed in stuff sacks better than the other synthetic fills.

Get a good bag rated to 20 F. with that fill and be done with it.

OR… if you are independently wealthy, find a down bag manufacturer that uses one of the new down water resistant treatments like Dri Down or Down Tek. Then re-spray the bag's shell with a good DWR like REVIVEX.