Layer to bring 30 degree bag to 20?

A VBL maybe? Lighter than a space blanket and packs up smaller, though it's more expensive.

It also depends on how your 30 degree bag suffices for you in general. I don't think the Flash Vest and R1 will get you 10 extra degrees out of a system, maybe five though.

One key aspect, I think, will be your feet. Down Booties are one option, while VBL socks (bread bags work) are another.

I would add some insulated clothing for the same part of the trip. If you are cold when sleeping, it will be cold in camp and on breaks too. As easy to ship and versatile.

IMO I don't think you need anything extra. I've taken my 30 deg. quilt to 20 many times just with a hooded puffy jacket and a hot water bottle in my bag (makes a big difference). If you're in an enclosed shelter as well, I think you'll be fine with the R1 and vest unless you sleep really cold (provided you have something other than the R1 hood on your head). I usually wear my baselayer plus hiking pants, but that's usually in the dry desert. If you can't do that because they might be wet, maybe add some puffy pants or powerstretch tights for your legs.

Personally I would use something like a Down Sweater equivalent. As another poster said, the additional warmth you will need for the Bag will also be needed around camp, so a jacket like that has dual purpose. They are certainly light enough at around 9oz (my Patagonia one is). I would also make sure you have some nice thick wool socks for sleeping only. Helps dry your feet out at night and give you some nice warmth.

Agreed; Id probably replace the down vest and R1 with a hooded puffy jacket. You could be different but I find the R1 way too hot to hike in unless it's very cold.

1) down booties
2) puffy jacket with sleeves
3) warm hat or hood

You might be a bit cool near dawn, but usually only for a couple hrs at most.

I agree with insulated jacket or sweater and pants, as long as your bag is roomy enough for them without compressing loft from the inside. My favorites are BPL Cocoon hoody and pants (too bad we can't buy these anymore) and WM Flash/Flight jacket and pants. I bring the synthetic Cocoon stuff if I expect pretty wet weather and the down at other times. For the PCT I think the down would be my choice.

Dropping in a Silk Cocoon liner is only 5oz and has always helped me when I get chilly.

Like others I have used a WM Summerlite that works at around 32f with long wool pants ,wool T and wool hat (sometimes gloves and light socks ) down to 20f by adding a WM Flash jacket and pants and to start warm (loose) socks.
The bits I add are my camp clothes anyway so that works for me.
BTW, you will sleep warmer if clean… (at least wipe the sweat from your body before changing into night clothes)
I used to use a silk liner but don't anymore.
Franco

What are you using for a pad? Using an insulated pad instead of the standard neo-air is going to make a big difference in warmth.

I've learned a few tricks while using only a 35 degree bag on my AT NOBO thru hike last year from April – October. And let me tell ya, it got cold in Maine – some nights it got down into the teens.

My 6oz Marmot UL Down jacket is a great piece of gear. It works better if you drape it over you inside the bag or wear it backwards so all the down on your back isnt wasted.

I've used my ground cover and backpack rain cover inside the sleeping bag to create more insulation.

One thing I did that helped a lot on the really cold nights is to put myself into the sleeping bag, and then put my feet inside my backpack, creating another insulation layer to keep my feet warm all night. It does create moisture so the bag may be wet in the morning, but you can always dry it out the next day.

Last tip – fetal position!

Good luck on your trip

I have used a Montbell UL down sweater, down booties, and down beanie to get an extra 10-15 * out of my bag.

I also used a thermarest reactor bag liner. They advertise 15* drop of bag rating, but I found it more like 7-8? I think it did add some warmth, kept my bag clean, and I used it to shear my face from the wind. Worth the small weight gain?

+1 for the hot water bottle in there, helps more than you think.