How much warmth does a silk sleeping bag liner add? – TESTED

I have used a silk sleeping bag liner for many years with the twin aims of adding some extra warmth and protecting the sleeping bag from grime and dirt (it being far easier to wash a liner than a bag, especially a down one). However it’s pretty hard to get a clear view on how much warmth a liner actually adds. Some manufacturers claim a full season (pretty much 5+ deg C) others 1-2 deg C (Rab). Where does the truth lie??

So using the same method I have used for my initial wool versus fleece insulation face off I have run a few tests on how much additional is added by a silk liner.

The testing method

As with previous tests the method is fairly straightforward to implement. A single layer of the test material is used to insulate a cup containing 200ml of warm water. This is then allowed to cool, with temperature measurements taken periodically and heat loss calculated. By running the test alongside a control (in this case an army surplus wool blanket) it’s possible to apply a bit of scientific rigour and gainer a better view of what the liner adds.

So this test looked at layering the silk liner with the surplus blanket, versus the plain blanket. By testing against other insulation options the aim is to develop a league table of camping and outdoor insulation materials to get a clear view of what delivers the best bang for the buck.

I am developing a plastic free wild camping setup to reduce my plastic use on the mountains in general and maximising insulation whilst minimising weight is a core aim for that.

The results

So did the silk liner + blanket insulate better than the plain blanket?? The simple answer is yes. However the level of added insulation was pretty modest. I the order of 1-2%. So if you want to go with a manufacturers claim Rab with their 1-2 deg seems closer to reality than the full season, albeit still probably more than you could realistically expect.

As ever there are a few caveats. The method I have used wraps the material tightly around the cup to ensure consistent comparison. However in the real life use of layers, layering is not quite so tight so the air gap would potentially provide slightly better insultation. Additionally whilst silk is a pretty consistent material at a chemical level the weave of the sik etc. could of course also play a role.

So will I keep using a silk liner? Yes. Whilst the insulation added seems fairly low, the keeping the bag clean benefit, comfort of silk, and additional draft exclusion all add sufficient benefit to be worth the slight extra pack weight in in my opinion. Additionally on those hot still nights, sometimes the liner itself is a great lightweight option on itself.

The insulation league table so far

So where are we so far in terms of keeping warm? Here are the ranked results so far from warmest to coldest with percentage comparisons versus the single layer standard wool blanket.

Material	Description	Added Warmth* (relative energy saved)
Piled fleece	Heavy pile fleece with microfleece liner	+12%
Double Wool	Doubled Army surplus wool blanket	+8%
Single Wool + Liner	Army surplus blanket plus silk liner	+1.5%
Single Wool	Army surplus blanket	Control
Standard fleece	Standard weight polyester fleece	-8%
None	nil, nada, zilch insulation on cup	-27%

There are a lot more variants to test including down and other wool variants. I have generally thought of the army surplus blanket as being equivalent of a light 1 season sleeping bag in terms of warmth so will also test that, using my 1 season Snugpak Jungle Bag which has a comfort rating of 7 C.