on average, what do you think they add to a sleeping bags rating? 3-5 degrees?

You already own the best thing to increase a bags temp rating- an exped pad. Adds at least five degrees if not more.

My bag is getting old and not as warm as it once was. I wear merino long johns when the temps are down, along with a watch cap and heavy socks. Those add a good 10 degrees onto the otherwise marginal warm weather bag.

I wear a pair of long johns and a lightweight watch cap, but I think it's the cap that adds the most to my warmth and comfort, although I'd be pressed to say just how many degrees. I don't have to close the bag up tight unless it's below freezing.

I would say Beaverjack is closer. I also wear merino wool tops and bottoms on cold nights and I think it adds closer to 10 degrees of warmth.

Like boonie, I also wear a thin merino wool cap on really cold nights.

Another means of extending the temperature coping skills of the bag is sleep with a soft fleece or sweater inside the bag on top of your chest, increases loft and acts as an extra baffle to prevent warm air from escaping (without the degree of claustrophobia achieved by cinching down the baffle).

Jan, Fri. Night I wore med. weight tops and bottoms with a fleece beanie in my 15 degree BA bag and insulated pad and was warm with a 21 degree temp in the morning.

I think you are close. I always thought long johns and socks just got the bag back to the rating it is supposed to be. A stocking cap and down booties go along way as well. The pads do make a huge difference for sure.

Last monday night I was comfortable with damp Marmot 15 degree down bag, exped downmat 9, no long johns. Temp at dawn was around ten degrees.

I think it is pretty hard to quantify a comfort rating change though I think Jan is closer than some claims I have seen on bag liners.

I use a bag liner because clammy nylon makes me feel cold no matter what the temperature is. I have a Thermolite Extreme bag liner that claims that it improves the bag rating twenty five degrees.

Not. It sure is warmer but the twenty five degrees is a pipe dream for me.

IMO all you can do is try and see how long johns work for you and dont run around under the moon before and get them damp.

i only ask because the marmot i just bought has a 31 EN rating and im sure ill be seeing low to mid 20's sometime again. i figured if i went with a 15 id be sweating if it was a warmer night, not good.

If I want to stretch my bag rating I bring something like a Montbell Thermawrap jacket and pants or Patagonia Nano or Micropuff jacket and Micropuff pants. That should get you 10 degrees. There should be a lot of threads devoted to this at Backpacking light.

I am bringing a 40F down bag to AK this summer and plan on having a synthetic layer in case the temps drop.

I think fleece would work fine as well I just don't bring it very often.

quote kanoes: "i only ask because the marmot i just bought has a 31 EN rating and im sure ill be seeing low to mid 20's sometime again. i figured if i went with a 15 id be sweating if it was a warmer night, not good."

No of course sweating is not good. It sounds like the Marmot you just got is going to suit you for most of your needs and there will be the odd "too cold " trip.

I suspect you already have a hat. Have you considered a silk liner?

I am actually pretty happy with my Thermolite liner and its about the same price as good merino wool underwear though as I said the twenty five degrees is a pipe dream. I have the Extreme.. There are other models.

sleeping bag liners

I use the liner always..even when it is way too hot for the bag itself. Then I just unzip as needed. The liners don't have zippers.

Just something to think about in balance with the budget.

Mountain Hardwear used to make a semi-rectangular bag liner that was supposed to add 20F. I have one but have never used it.

I also have an old Moonstone bag that had a quilt that snapped into the top of the bag that was supposed to add 10 degrees.

quote Beaverjack: "My bag is getting old and not as warm as it once was. I wear merino long johns when the temps are down, along with a watch cap and heavy socks. Those add a good 10 degrees onto the otherwise marginal warm weather bag."

This is my standard sleeping gear for colder weather. Also helps when nature calls.