Gear

Two questions -

Firstly, what time of year will you be going (and therefore what sort of night time temperatures would you expect)?

Secondly, what is your budget?

Even withough answering these questions, my first suggestion would be to contact PHD (Peter Hutchinson Designs) and discuss your needs with them.  They are able to taylor a bag specifically to your exact needs, well worth considering if you are willing to pay for the upper end of the market.

James

Its not a simple as thermal efficiency - as, presuming the fill is exactly the same quality and quantity, the thermal efficiency will be identical.

However...in the field, there are so many other variables at play that just looking at thermal efficiency is a complete waste of time.

Best bet is to

Select a few in your price budget range and ask what people think of them - particularly Metric Kate who is, by reputation, the coldest sleeper in the universe1

GOF wrote (see)

Its not a simple as thermal efficiency - as, presuming the fill is exactly the same quality and quantity, the thermal efficiency will be identical.

That may be true but the design of the bag can make a huge difference as to how well the filling is utilised, you need to take into account the baffles, baffle design, differential cut, differential fill, are you a back or side sleeper, which mat are you thinking of using.

Sara, a lot of manufacturers now use a University (Leeds?) to test their bags  using a standard manikin.  PHD, Cumulus and RAB all do this I think so at least you can compare price , weight and relative efficiency.  Also note that 800 fill power US, is not the same as 800 fill power EU.  I think RAB and Cumulus now use the US system, not sure about PHD.

Steve D

Its not a simple as thermal efficiency - as, presuming the fill is exactly the same quality and quantity, the thermal efficiency will be identical.

I don't think so...

However...in the filed, there are so many other variables at play that just looking at thermal efficiency is a complete waste of time.

I think it's the case that "other variables" will impact thermal efficiency though.

Shell design, for example, has little to do with down fillpower.  Yet we're pretty sure a mummy shell is more thermally efficient than a rectangle, and we're pretty sure that slant-wall baffling is more thermally efficient than stitch-through or even box-wall.  ME's Extreme Range lightly elasticated shells cut out free space immediately around the body to help the down loft more.  It costs more and compared to a simple box wall weighs more, but lets you reduce the weight of down for the same overall effect thanks to being more thermally efficient.  And so on.

Back to the original question, if all else is equal then free space inside a bag is probably a Bad Thing because it allows convection currents to form and they can moce heat away from your body.  What actually does the insulating in most insulating products is static air.  The down (or whatever) doesn't insulate directly so much as keep the air still so it can do the insulating.  That's also why a foam mat insulates better than an airbed.

So a bag that's closer cut to you should probably be better (if all else is equal, which of course is a fairly big "if"...).  There's a further advantage that you're not toting round extra weight and bulk of a bigger bag.

I'd suggest a "test lie" to make sure you can scratch itches etc. without needing to be a contortionist.  Also, don't forget that down gives no insulation to speak of when compressed (i.e., when you're laying on it), so you should combine your bag with a decent mat to get the best effect.

Pete.

i would think a woman bag would have different down distribution to a man bag with down being more concentrated to where women feel colder than men.

Up to a point, but I would suggest that the general variation in Where People Get Cold probably renders any vendor's ideas about average men vs. average women pretty meaningless.

What you can usefully say is that your average femaile customer will be shorter than your average male customer, and probably have proportionately wider hips.  So you'd want the hip area a little bigger so as not to compress the insulation there.  That's not to say that a women's bag will necessarily be better for A Random Woman, or indeed that a man/"unisex" design will be better for A Random Man.

But an alternative does give something else to try and see if it fits better.  As do alternatives from other manufacturers (Roos uses a Rab "unisex" bag, having found me ME one a bit constricting, but she'd be too tall for a Rab women's bag, for example). 

I suspect (though haven't looked into it) basic fit is more of an issue than some sexual predetermining of who gets the coldest toes etc. 

Pete.

As you would expect, there has been a significant amount of research on the performance of sleeping bags/clothing insulation.

I am not aware of anyone coming to definitive conclusion as to a major limiting factor.

Manufacturers do wibble on about some of the more esoteric features for their women's bags, but I think the issue of length is fundamental; if you're carrying stuff, you want it to be as weight-efficient as possible.  And if you're carrying a bag that is significantly longer than it needs to be, then you're carrying unnecessary shell, inner & down material to no useful purpose.  Not to mention the lower themal efficiency because you're having to warm unused space in the bag.

So get a bag that's sized to fit you properly.  The 'selective fills' some manufs use for women's bags (often in footbox and hip areas) are less important, IMHO.

Oh, and, as Pete says, get one you can fit into without compressing the down around your body (underneath you excepted, of course...).

> Also, don't forget that down gives no insulation to speak of when compressed (i.e., when you're laying on it)

Oh God, don't say that if Richard Nisley from BPL is around...

check out mont-bell

they've had rave reviews for their spiral hugger models, the compartments are arranged like a piral with stretch stichging that pulls the back closer in eliminating cold air pockets around the body but it stretches as you move. 

http://www.montbell.us/products/list.php?cat_id=5

Montbell sadly not generally available in the UK.

However, the body hugging thing is a patent they share with ME, and it's found on their Extreme range of bags.  I have and like those (a Dewline and a Lightline), the elastication is actually unobtrusive.  Roos found the overall cut a little too restrictive for her though, to underline the usefulness of a test-lie (she has a Rab Q600 and a PHD Minim for the same jobs as my ME pair).

Pete.