I backpack in MA, CT, and NY mostly in June, July, Aug, and Sept. where the normal lows are between 50 and 65 F.
I *occasionally* might backpack in April and October with normal lows in the low 30's.
If I plan on getting two good bags, should I get a 40 and a 15 or a 35 and a 10 or some other combination?
I currently have a cheaper synthetic Mountain Hardwear Flip 25/40 which is a little too hot for summer and too cold for low 30's, even with a liner. This is why I am stearing away from one 30 degree bag. I expect a little discomfort when backpacking, but am eager to minimize it!
I'm leaning toward a good down Montbell for the summer bag and WM for the spring / fall one.
I guess I would switch to the warmer bag when it got down into the lower 40's. I like to be warm not borderline or cool.
What do you think? Oh, Contrail Tarptent and Therma rest Z lite if you're curious.
You might consider a 40-degree bag like the Western Mountaineering Mitylite. It might be multi-purpose enough that you won't need that second bag. I believe, from my own experience, that it may really be a 35-degree bag.
I camp in the same temperature ranges you do, in Ohio and Kentucky.
The Mitylite is a hoodless bag, and actually has 2 zippers: one around the foot, and a long one from ankle to shoulder. Unzip them both, and the bag lays open like a quilt at home. Zip only the foot zipper, and you have a quilt with a footbox. Zip up the long zipper, and leave the foot open for ventilation. Zip them both as the temperature falls.
In the warmer temperatures, I sleep in shorts and a T-shirt. Depending on how warm it is, I may leave the foot zipper closed. However, I always leave the side zipper open and use it like a quilt.
As the temperatures fall toward 40, I wear midweight long johns and top to sleep in, zip both zippers, and cinch the bag around my shoulders. I wear a stocking cap or balaclava, depending on temperature, to keep my head warm (no hood, remember?)
To go lower than 40, I take along down booties, light down jacket and pants (think Patagonia hooded down sweater and Montbell UL down inner pants), and maybe down mittens. You could use similar synthetic garments (Patagonia Micropuff hooded jacket and pants, for example.) This gives me clothing to sit around camp in the evening. I then simply wear some or all of it to bed, over midweight long johns and top. The hood on the jacket works as well as any sleeping bag hood (and lets you roll from side to side more easily.) I've used this combination of clothing and bag into the mid-20's and stayed nice and warm.
Lately, I've also been experimenting with a Thermarest Ventra quilt, though I haven't had it down to freezing yet. I like it, and took it down close to 40 with lightweight long johns and a stocking cap a couple of weeks ago. So far, I like it really well.
One last thing: if you're going to push a bag into colder weather, you might want to beef up that pad. It has an R2.2 rating; the Prolite Plus pads have an R3.8 rating. It won't matter how warm the bag is, if the cold seeps up from the ground. (But you probably already know that, and add padding in colder weather.)
Loc: Portland, OR
The temp ratings for sleeping bags are notoriously unreliable. Some companies rate bags very conservatively. Others rate them very unrealistically. Some people sleep warm. Others sleep cold.
My only point is that your question revolves around sleeping bag ratings in very small 5 degree increments. But the ratings are so inexact that they wholly negate the small differences you are inquiring about.
The answers you get will need to navigate this ratings problem.
I agree - ratings vary significantly from brand to brand. (For example, the Ventra and Mitylite I mentioned are both rated as 40-degree bags by their manufacturers. However, based on my experience with both, I'd put the Ventra at 45, and the Mitylite at 35 - not quite comparable.
However, within a single manufacturer's line of bags, you can at least rely on the ratings to give you an idea of how warm one bag is versus another. (For example, my WM Mitylite is about 10 degrees warmer than my Megalite - and both are about 5 degrees warmer than their 40 and 30 degree ratings, respectively.)
You're right, either "split" will probably work just about as well as the other.
I'm pretty much sold on the roomier WM 15 degree bag for those cool fall nights (and cool early spring ones), because the WM 40 degree Mitylite bag is heavier, and, I think, shorter (I'm 6' and move a lot when I sleep) than Montbell UL SS Stretch 5 long.
I guess what I'd like to know next is whether the Montbell bags at the cooler end of the range are bags that most people would rate accurately. Is the Montbell U.L. 40 degree bag one that seems to experienced folks closer to 35 or 45 (granted, from their point of view)?
Doesn't look to me like either split fits the temps you hike in. But I would second something from Western Mountaineering (30-35 degree), they are well known for conservative temp ratings, and made in the USA. And gorgeous. Then buy an ultralight quilt for hiking in the 50s.
I camp at similar times and find my western mountaineering bags rated at 35 and 20 cover all the bases. I have a highlite rated at 35 and a alpinelite rated at 20 degrees. I have taken the highlite down to 30 and the alpinelite down to 15 without problems. I would recommend a sleeping pad or combination of sleeping pads with a higher N rating if you plan on camping on frozen ground. These bags cover me for everything except my winter camping. Personally I find it hard to beat the western mountaineering bags. Oh and I also use my bags with a tarptent, specifically the original squall without problems.