Taking a 25 degree bag down to 0

0 degrees is a little below my experience, but

if you wore fairly thick insulated jacket, pants, booties, hat, and mittens you should be able to take 25 bag down to 0

If there's enough room inside your bag

I have maybe a 45 degree bag I take down to 20 F wearing clothes inside

“Thinking about getting and overbag or bivy to augment a Montbell SS Hugger #2 Sleeping Bag (25 degree) to add another 10 degrees of warmth. Any thoughts on pros or cons of either setup along with suggestions on what to get? I expect the tent and clothing to take me down another 10 degrees to reach 0. Is that reasonable?”

First of all base layer clothing, a hat and protection from wind/elements (i.e. a tent) is a given in the EN testing process, so assuming you will get another 10 degrees of warmth from that is not likely. Wearing insulating clothing such as puffy layers to bed will increase the temps you can reach so long as the fit of your bag allows you to wear that clothing without overly compressing the insulation in your clothing or your sleeping bag. Using a bivy inside a fully enclosed tent with a sleeping bag would be somewhat redundant and would likely generate condensation issues as opposed to giving you a significant amount of warmth. Adding an over bag or quilt to your system works, but is typically not as lightweight as using a bag/quilt rated for the temps you anticipate. Here is an equation another poster added on these forms awhile back that is pretty accurate in my experience to determine what a second bag/quilt would add to your system’s temp rating:

x -(70 – y)/2 = z

x = first bag (higher rated/lower degree)
y = second bag (lower rated/higher degree)
z = rating of doubled bags

Using that equation you would want to add a 30 degree over bag/quilt on top of your existing system with some extra clothing to hit zero.

Vbl and bivy to start. Add overbag if needed. Also make sure you have your pad system figured out. I have seen many who complain about being cold and using insufficient pad such as neoair on snow.

As Jerr mentioned, you must have enough room inside your bag to accomodate insulated pants and jacket. Otherwise you will be compressing the loft in your bag and losing insulation, defating the purpose of insulated clothing.

Try in your back yard to see if it does work.

My WM Megalite is large enough for me to wear my insulated pants and jacket even though I had the bag overfilled by WM. The WM Summerlite, on the other hand, would be much too narrow for these insulated clothes.

Mike, thanks for the calculation and for everyone elses comments. I don't have any problems with wearing extra layers in the MB but you all have made a good point about seeing if loft is affected in the overbag.

I tried to see how low I could go a few years ago. Took my 20f rei sub kilo to -2 and had one of my best nights ever. Go figure! Wore plenty of extra clothes and had my neoair sandwiched between 2 ccf pads. Wish I had down booties.

How warm is bag for you? With a tent and baselayers and a hat, is it comfy for you at 25F? Or only down to 30F? Or even 20F?

The point i am making is, how much colder will it be than the lowest temp you are comfortable at without those measures?

After that, I would first use high loft clothing, if it fits inside, since it's gear you will want any way, and it's nice not to have to get up to pee or in the morning and have to get dressed first.

My second one would be an wide overquilt, because you could use it for other things in the summer.

Hi Ben,

One significant advantage in using a synthetic overbag or quilt over a down bag is that the condensation point should move from inside the down bag to inside the synthetic overbag/quilt. Your insensible perspiration will condense somewhere in your insulation; better for this to occur in an outer synthetic layer than inside your down insulation.

I like this system as well because we can have such wildly varying weather in winter in the southeast. It's not unusual over the course of 3-4 days for weather in the high country to swing from low 30Fs and rainy, to clear with temps in the single digits the next. Multiple layers for my sleep system allow me to comfortably accomodate this, much as variable layering systems allow for greater comfort during the variable conditions we all experience during a day on the trail. You'll pay a weight penalty for this versatility, but if you're going to be out for more than 1-2 nights, that moisture in your sleeping system has to be managed, or your down will become damp enough that it's insulative capacity is significantly diminished.