Two questions...

[Happy]

I just ordered a jagbag mummy liner for my down bag...for those with experience...how much increase in warmth will this provide...i've heard from 5-10 degrees...what is your experience?

Second, I have a vapior barrier liner that I ordered from Wanderlust with my Nomad-lite tent. I seamsealed it for exclusive use as a VBL and as a pack liner. If I continue to use it as a pack liner could it double as a bivy sack for sleeping in shelters with bad weather with my down bag and "sleeping under the stars in the late Spring, Summer and early Fall? Or would topside breathability create a problem?

Thanks for all opinions!

[MedicineMan]

It will become a problem not the first night but subsequent nights unless you are provided with a warm sunny day to dry out the bag....of course a synthetic bag less so than down...I too have the VBL from Wanderlust and use it as a pack liner...albeit a large one...and think the extra ounce is good insurance for those things you mentioned like emergency bivy for you or someone else.....for use as a VBL the odds are small considering VBL's dont show effectiveness until the temp drops below 10-15F (varies according to source of course).....
I use/used a silk mummy liner for a couple of hikes---its just hard to quantitate that 5-7F increase...La AquaNa uses hers almot religeously....me personally, I would rather put that weight into down if I'm really thinking about warmth...but they do keep the bags clean

[Kerosene]

I have a silk liner that I find probably adds 5-7 degrees. However, I actually find it most useful to keep the inevitables little drafts at bay when it's not so cold that you have to totally mummify yourself. With my 15-degree Marmot Pinnacle, I get too warm if I try to seal it all up below 40 degrees (I tend to hike in the shoulder seasons where you get a wide range of nighttime temps). In this case, I wrap myself in the silk liner and don't bother to zip the bag up all the way.