Calculating Overstuff/Overfill in Down Bags/Quilts?

Yes, it is based on the amount of down by weight, not loft as the baffle height is still the same. (Although it can/will gain more loft between the baffles.) And it is across the entire item unless stated otherwise.

I sleep colder up top so I had two quilts made with extra down in just the top third of the quilt to make it right for me at the temp rating. Those are examples of selective overfill.

Let's say your quilt is 36" x 72" and there's enough baffle fabric to make 2" loft

volume = 5184 in3

if you used 900 fill down you would need 5.76 oz of down

if you overstuffed 20%, you would use 6.9 oz of down

if your baffles were all the same, split it evenly between them

if a baffle was half as long, I would use a little more than half as much down because it takes extra down to fill out the ends of a baffle

you could put more down in the baffles over your torso because you're warmer there

Yep – what Jerry said.

He didn't mention that the "norm" is to assume a loft height of BaffleHeight + 0.5" when calculating how much down to use, since the quilt/bag puffs out between the baffles.

Also, see my post that shows how much down per cubic inch a few different companies use:
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=68798&skip_to_post=591190#591190

If you don't overfill, then the down will fully loft the bag/quilt only in absolutely optimal conditions. I overfilled my quilts by 35-40% and I'm stickin with it for future projects. Most people overfill by 10-20%. Nunatak underfills (i.e. products don't even fully loft in optimal conditions), which is why many people order extra down when they order Nunatak products. I have a Nunatak Skaha off-the-shelf and one with substantial extra down, and they are about 20 degrees different in warmth. EDIT – see my note two comments down this thread.

AmyL

"Nunatak underfills (i.e. products don't even fully loft in optimal conditions)"

Not in my experience. I have three of them right now that still loft higher than their baffle height without overfill. Which model(s) did you have problems with Amy?

Raymond –

Thanks for asking and for challenging my assertion.

My off-the-shelf Skaha (and my husband's off-the-shelf Skaha) does not come close to fully filling out the channels, even though it has been only lightly used, never been dirty or wet, and never been tightly stuffed. Compared to the overfilled Skaha which I also own it is like night and day. I have opened the side seams of my under-filled Skaha and added an oz of down to the lowest three chambers, but I don't have the patience to open all the seams in the upper torso and sleeves to add down to each chamber. That said, the Skaha I bought from Tom that was ordered with significant overstuff is great. I think it's a shame he sells the Skaha without adequate down — the design and sewing is fantastic, and the baffles allow it to be super warm when fully stuffed. Adding down after the fact is time consuming and requires the customer to have a sewing machine and the skills to handle down.

My 2002 BackCountry Blanket was ordered with overfill, and even new it didn't really "puff" between baffle lines.

I can't speak to any other products, and I stand corrected that I really have no idea how fully those stuffed other Nunatak products may be. I mistakenly extrapolated based on my experience with the Skaha and BCB. Maybe the Skaha is intended to be an under-stuffed sweater, and the BCB is intended to be a casually used cabin blanket – that would explain why both are under-stuffed.

As per Nunatak's website, a BackCountry Blanket size Large is 83" long and 61-53-46" wide with 2.5" baffles.

To calculate cubic inches:
83" long *
53.33" wide (which is the average of the three widths) *
3" height (baffle height plus 0.5") =
========
13,280 cubic inches.

To fully loft a Large BackCountry Blanket in optimal conditions would take down rated to 1100 cubic inches per oz.

Because the garments and all of the other sleep systems are shaped, I can't easily figure out how much down per cubic inch is used for those products.

I believe Tom uses 850 or 875 down. In any case, I'm pretty sure he's not using >900 down.

Different brands or models use different amounts of down

If a brand uses less down, then it's more important to over-stuff

And if you talk about over-stuff of an existing product, it would be relative to their normal amount. So if they use 10 oz of down, for example, 20% over-stuff would use 12 oz of down.

I sent my WM Megalite to them last summer for a 1 oz. overfill. With the extra 800 cu. in. of down I can say its loft has improved to where I can no longer redistribute down from top to bottom, as it was originally designed to do.

That's OK by me. I have now tested it to 24 F. (with light polyester longies and a stocking cap) and found it warmer than my previous 24 F. experience. In fact it was quite comfortable. So I'm guessing it's good to at least 20 F. That's about a 10 F. improvement – and I'm a sleeper who always needs more insulation than most.