Loft and required insulation?

3.5″ will get you to around 0F.  Starting from room temperature (70F), 10F/0.5″ loft works to around 0F and then things get less linear.  See what manufacturers list.  If you can, you might also refer to the bag’s original spec’s to guestimate its new rating.

I can’t make sense of your second question, re. overbag.

Well the original spec was for -30C/-40C with the use of the down suit but that was a long time ago and with the overfill there is no longer any room inside to wear insulated clothing.
It’s closest approximation to a new bag would be a WM Kodiak I guess. If the Kodiak was zipperless.
Second part of the question was a convoluted way of asking how may more inches of insulation would be needed in an overbag to get to -40 seeing as the tables in “Man in a cold environment” are so optimistic.
I was guessing that I needed between 30 and 50mm extra and seeing if I could do that more cost effectively than a $1k- overbag from PHD as my Nunatak overquilt was ordered to go over a much warmer bag and is only 20mm thick [ 100GSM] but I have a S2S down blanket similar to the WM Everlite with an average loft of 25mm/ 1″ I could use in conjunction even if this combination wasn’t as light as a dedicated overbag.
https://www.westernmountaineering.com/sleeping-bags/extremelite-series/everlite/
This is related to my previous post asking about the effect of layering over a sewn though sleeping bag such as the S2S blanket or the WM Caribou

Down retains its insulating properties even when compressed well under half its fully lofted volume.  I think Richard Nisley said this actually holds up to 8x.  I think the down suit would still perform as expected even with the overfill.

Rene that may well be true, but I outgrew and sold the down one-piece suit decades ago. I did try and wriggle in wearing my old down parka and I simply couldn’t do it easily enough to feel comfortable. This is the massively overstuffed Everest parka I bought and never used in the Himalaya and it weighs ~1300g; 650 of which is 650FP down, I just feel too confined to use it like that now. It wasn’t a big problem before the accidental overstuff.
I have thought briefly about loops and buttons but I am a side sleeper and the parka would not always be on top due to me being a wriggler when I sleep

If so is 87mm/3.5″ of insulation really adequate at -40 as suggested by the Armys insulation tables or is this an approximation of the “survival” rating.

I have never seen a table of 3.5″ good to -40 (I’m assuming Celsius). The tables I’ve seen (as Rene alluded to) is 3.5″ good to 0 degrees F. These tables we have used for decades are based on an old 1969 Natick paper, The Comfort and Function of Clothing (Tech Report 69-74-CE) back when 550 fill power was common. The zero temperature was for comfort at .90 MET (sleeping).

But because the baffles are not that high the free loft at the stitch line is only 155mm.
Does this approximate to an insulation thickness of the average?

Good question and probably someone like Nisley can answer. But 155mm at the seam vs. 175mm at the torso is a 20mm difference, which is nearly .75″ to this non-metric guy. That’s a lot and I would consider it a gap in insulation. In temperatures around 0F, that would be a big problem, I would think.

One of the problems in dealing with down is manufacturer’s don’t have a standardized methodology of filling spaces between the fabric caps, thus we need lab test such as EN-ratings done on specific pieces of gear.

As far as compression and insulation goes, I did a lot of research when I remodeled by house with some serious energy upgrades. I found that R-19 fiberglass batting is 6.25 inches thick. But the wall cavities for this insulation is made from 2″ X 6″ studs, which is really only 5.5 inches thick, meaning the insulation has to be compressed .75 inch. However, the insulation per inch increases while the overall insulation value decreases. In this example, the R-19 value is actually R-18 when compressed 3/4 of an inch. Not much of a decrease.

Man in a cold environment is well outdated but when Gerry wrote his series of articles and the DIY book I think it might have been the best available information at the time and I have always thought them optimistic for a good sleep.
Also possibly why so many combat soldiers froze to death during WW2 and Korea.
I just weighed some possible outer layer combinations to see how much extra weight the extra insulation would add in cheap layers that I already own.
Nunatak overquilt ~790g Rated 14C
Cheap 50GSM overbag also ~800g Rated 14C
S2S down blanket 660g Rated 14C
A dedicated overbag from PHD to give a 20 degree boost weighs 1100g and would cost $1350- to my door, a heck of a lot of money to save ~350grams if using the combination of two of the above layers would do the job. I’m not ever buying another Arctic bag but for comparison a WM Puma bought here would cost $1855-AUD.
A cost benefit analysis is always needed on purchases and using this forum as a sounding board helps my thought processes. I keep hoping S2S will bring out an UL version of their Traveller series. I only have the down blanket because it was on sale cheap.

We can’t cut corners on sleeping bags in cold weather! Good bags are expensive.

I clicked your link to WM and now see my rather old Ultralite now lists for $525. Ouch. I sleep cold and am not comfortable at its rated 20F (even with an appropriately insulated pad).

I also have a 32F Nunatak Arc Specialist that is 12 years old. A comparable Nunatak is now over $400. That’s a lot of money for a couple pieces of sleeping gear.

But I have learned over the years you need to buy quality when it comes to sleeping gear.

Add $55- USD for postage then double it for costs in Australian dollars and then add the 10% G&S tax and you might understand why I’m pinching pennies in my research for a trip that most likely will remain a dream
For car camping I have systems that easily reach -40C but I could not carry one very far.
Some thinking to do with making some adjustments for age related loss of sleeping metabolism and based on 70% of resting

EJ, maybe I’m confused, but the current exchange rate is $1.44 AU for each US $.  I’ve never had an AU or other overseas transaction where the current exchange rate was not figured into my costs in $US.  Don’t know about the G&S tax that you mention, but the value-added tax in Europe has not been added either for items shipped to me in the US.  As you might guess, all these transactions were done by credit card direct or through PayPal.

Extrem Textil is hammering me a bit for shipping on some recent transactions, they say due to the pandemic; but for what I want, it is worth it.  They have never charged me the value added tax that all in the European Union must pay.

I did order a tent from AU some years ago where the US-AU exchange rate was around the opposite, and cost me considerably more in US$ charged to the card than the AU $ price.  But it was worth it then, also.

Hi Sam, you can’t buy WM direct from the USA anymore. WM have an Australian agent and their intermediate mark-up is high and I was taking that into account in making my comparison. While the exchange rate might be $1.44 / $1 the banks charge a huge exchange commission here as does PayPal if you use it
GST is added onto imports if not paid by the shipper although some small packages do make it through untaxed anything insured for a declared value of more than a $k gets hammered and you get taxed on the value plus shipping.
My guesstimates might be a little off but I was trying to show why I wasn’t planning on buying a new Arctic bag and making do with what I have in my stash. A trip back to the Northern hemisphere isn’t likely unless we win the lottery, things are no better here than in the USA at the moment

If it’s for one trip and your outfit is a little cold, maybe get a down or synthetic cumforter for the outside, or wear more inside your existing bag?

Edward,

Out of curiosity, where are you car camping at -40?

Wikipedia sez the lowest temperature ever recorded in Australia is -23C.

And, a point of trivia: -40F = -40C.

Rene I was thinking about my proposed but cancelled road trip to Alaska and Canada.
Snow camping here is in the hills and usually ranges between 5C and -5C with lows around -12C
However I have family in the USA, in Upper NYS

Not far from Mt. Washington :)

Just a little feedback on the combination used on my short trip.

Using the S2S down blanket plus the cheap XL sleeping bag I needed to put on a light fleece layer at 5C, although conversation with S2S said it should be good to freezing that guesstimate would have been for somebody younger and fitter than I am, my inner furnace is not firing well lately.

Yes buying a dedicated deep winter bag is more efficient but the layering  works, if only S2S would bring out an UL down Traveller series using lighter/tighter fabric and 900FP down it really would be a viable option for some people.

I don’t think that a simple stitched through overbag would work anywhere near as well as one with baffles.

Edward J,

Thanks for more info about purchasing from Australia these days.  Sounds daunting.  I have been purchasing from Extrem Textil in Germany lately, with few problems, the only one being they charge more for shipping to the USA due to the pandemic.  So will now treasure my modified Aussie One Planet tent even more.

You lost me with your reference to “WM.”  If not Western Mountaineering, a USA company, then what might it be.

Western Mountaineering, I was being lazy

Which One Planet tent?

It’s importing to here gives problems and more due to our klunky way of collecting tax, exports don’t get taxed usually

Here’s another thought.

I used to climb and I have bivvied a few times and been comfortable or cold depending on how much gear I took with me, usually cold tho.

Also in my ski touring I have been caught out a couple of times due to gear failure, a couple of shivering nights and a couple of OK ones that were fun in retrospect.

Decades ago when Goretex first came out I made a half sack

I found it added a good few degrees to my survival rating being totally windproof, I assume 5C the same as the bivvy sack which replaced this.
475grams which was UL in its day.
I basically copied the Chouinard design which was insulated with a very lite layer of Thermolite and wasn’t considered by my mates worth the extra weight.
I’m tempted to replicate this old one using LW WPB and add insulation to give a 10C boost to whatever clothing I am wearing, just enough to stave off hypothermia

Edward J,
Ah, the light dawns – you must live in Australia or thereabouts.
The One Planet is a Goondie, modified:  https://backpackinglight.com/forums/topic/81307

Southern Australia, about 50 miles south of the capital Melbourne and a 700 miles South of Sydney where Roger lives and I have to drive 200 miles to see snow and ski