cocoon mummy liners

My wife & I have a pair of silk Cocoons that we’re both happy with. Small, light, feels nice, handy for temp regulation (bag pushed down, cocoon pulled up), surprisingly strong, has a footbox, keeps the bag clean. I don’t really think it adds 9 degrees though. Maybe you’d be 9 deg warmer with it than with nothing, but it’s probably more like half of that if you’re already in a sleeping bag. You could be much warmer using it if it lets you sleep in dirty clothes that you wouldn’t otherwise.
Cons:
It tends to get twisted up if you roll over much.
White stains easily (I don’t care, but my wife does so maybe yours would too “it always looks dirty”).
It doesn’t really have a hood. The top part is only one layer thick. You can use one of the draw strings to scrunch it down around your head (the other is fixed), but you end up with several feet of cord flopping around & it makes it a little too short for me (6′ tall).
Campmor has them for $40.

Thanks for the insight, I’ve gone ahead and ordered a silk liner. I really didn’t expect to get 9 degrees out of it but I’m counting on a little mental edge to go with a bag that is already warm enough.
Browsing their stuff they make a lot of liners and overbags.

I use the silk / cotton blend Cocoon liner and love it. I use it alone in summer for hot nights, w/fleece <60 deg, and w/ my 20 deg bag to extend it's range by 4-5 deg. Also it is more easily washed than my bag.

Warmth is obviously subjective, but for me, a silk liner will boost temp by about 3F, and a fleece liner maybe 5F. I once fell for the hype that adding a liner (e.g. Sea to Summit’s Thermolite liner) can boost temp by 10 or even 15F. Don’t fall for it.

Also, be careful about giving your wife a farcical “mental edge” because it can easily backfire on you…

Ditto Egads. We took these to Philmont, where showers were few and far between, a big feature as they are significantly easier to clean than a bag. I left mine in the bag, the few nights it got really cold, it was a nice difference you could feel.

Our 10 F degree cocoons were good for a realistic 5-6 F more comfort in 35 – 40F weather. At 70 degrees, you might get 10 degrees with it vs. 80 without.

As such, I don’t believe the ratings are linear: If you get 10 degrees at 70 degrees, lunch would wager you would not get 10 degrees at -40 degrees.

MikeB