I got a PM that I have gotten a few times, so thought I'd share it and my response with everyone to both answer the question and let us discuss it openly.
"Planning my 2017 thru and was going to buy a moderately rated bag but high quality Sea to summit bag and use a bag liner for the first part of my hike as I plan to start early March.
That being said do you think my idea is completely dumb?"
I will say though that using a bag liner can have some advantage in keeping your primary bag (especially down) clean. Though it will do little more than add a few degrees for a silk weight liner.
I will also say that a VBL (vapor barrier liner) can have some interesting and great properties- but that's another topic.
So: To this point (which I once thought was a good one)- Can a Sea to Summit liner extend the temp rating for a spring start?
I would call it a dumb idea... though only because I once had the same brilliant idea before and have since not only tried it out- but learned more of the science/common sense behind why it's a dumb idea.
We've all come to agree (generally) that the Sea to Summit liners are (being generous) rated at least double their actual performance in the field. That's probably enough to discount them, but I do better understand why they end up this way since getting into sleeping gear commercially. Let's look at the main example:https://www.rei.com/product/797114/s...ummy-bag-liner
The STS Thermolite Extreme claims up to 25* of warmth added to a bag at 14 ounces.
For most of us... we get the liner and it doesn't work as advertised and we call it a day.
For some of us... we look a bit harder and see it's basically a sack made out of long underwear material (silk to thermal weight depending on the model).
We then may realize that a pair of long underwear weighs about the same as that liner... or that we'd rather carry a puffy (vest/jacket, down/synthetic) and that that does more for us per ounce than the liner ever would.
And we can wear it outside our sleeping bags.
You also might look around and realize that you could buy an ultra high quality down bag that is less than 14 ounces.
Or you could look at that SUL synthetic quilt you made for the summer time that only weighs 12 ounces and realize you could simply carry that and pile it onto your shoulder season bag and that it legitimately does add 20+ degrees to your sleep system. Compare either of them "face to face" in your hand and you'll wonder how you ever thought that a liner could equal the warmth of an actual summer weight quilt/bag.
If you wanted to nerd out a bit: you'd get into the CLO values as this is synthetic insulation- https://en.wikipedia.org/wiki/Clothing_insulation
You might see that EN ratings call a pair of midweight thermals (pants and bottom) along with socks and a matching hat (beanie) are ALREADY included in the rating system as being worn by the user.
You might also note that these layers add about .5 CLO or 3* to the system as a whole. So even a thermal weight layer adds at best 1 CLO or about 5* F depending on the testing method.
If say you were bothered by this problem and looked into it a bit more (like say you were making sleeping bags for SUL/Speed hikers)...
So you might wonder how it's physically possible that you could even attempt to call this product as adding 25* of warmth.
You'd try to find a value for Thermolite and realize it's a bit tricky to find out.
https://thermolite.com/en/Technologi...ies/DUAL-LAYER
An old BPL thread lists the CLO value as high as .598 fabrics. https://backpackinglight.com/forums/topic/42340/
Thermolite's page lists them as 25-30% higher than denim. Denim is generously about .3 CLO, so that would put you at .39 CLO if being generous again.
Even if you give them .598 CLO per 100GSM (grams per square meter or 3 oz/yard) sample, that's a bit lower than Thinsulate 100GSM insulation which posts about .79 CLO total.
This is at best (when combined with other layers and getting a boost as a liner) a 5* bump at 1CLO.
Climbashield Apex is .82 CLO per ounce. So an equivalent 100GSM is 2.46 CLO or roughly 45* (EE rates their 1.64 CLO Apex Prodigy at 50*)
Primaloft Gold is .93 CLO per ounce. So an equivalent 100GSM is 2.79 CLO or roughly 42* (EE rates their 3.28 CLO Apex Prodigy at 40*)
You would need 4-5 layers of thermolite to equal the same insulation value in CLO as either of these products.
So nerd story short...
1- It seems physically impossible for the STS liners to ever achieve anything close to the ratings they claim- even cut in half as we used to say.
2-Further when you consider the weight of the liner itself, you could do better with either a puffy jacket or an extra sleeping bag.
3-Recent innovations and some effort by many folks (Enlightened Equipment especially) have helped to show that a 45* bag laid over any other bag will add roughly 25* of warmth to a sleep system.
(I'm finding that with my personal gear... that it is safer to take about 5* off their formula)
So yar- the Sea to summit liners for warmth are dumb.
1- bring a puffy jacket you can sleep in or better yet lay over your sleeping bag to stretch it.
2- bring a summer weight quilt for roughly the same weight to achieve that level of bump in temps to your sleep system.
3- if buying new- bumping up the temp rating of your base bag (and/or using a quilt) will weigh far less than adding a liner. In a quality down bag, you're typically talking 4 ounces or less of down to jump 10-15* in rating... much less weight than any option.
Now besides the fact I plan to sell them... I prefer the quilt as there is a way to convert it to an acceptable camp puffy in combo with my wind shell... so I get the best of both.
Though for the AT spring starter specifically- I think that a quality 20* mummy bag and a summer weight synthetic quilt are the far better/cheaper system overall.
Use both at the start and have the protection of a good mummy bag in truly cold temps with a synthetic quilt over the top to protect the down layer.
As it warms up a bit- send home the summer quilt and carry the mummy. As you hit VA and things warm up, swap out for your summer quilt and then reverse that as you approach August and/or the whites.
Two bags total covering your whole trip. 20* down mummy and a 45-50* summer quilt puts you at around zero at the start of your trip for about the price of one really good zero degree down piece.
PS... I'd be pretty skeptical of the Sea to Summit Spark series too.
http://www.seatosummit.com/wordpress...9_16_FINAL.pdf