Designing with Down

Designing with Down

The comments in this article relates to Nunatak's most popular single person quilt: the Arc UL.

Down is comprised of thousands of individual clusters. The expanded clusters cling loosely together and form a mat of insulation. The trapped air inside this mat is what keeps us warm.

What can go wrong? Well, besides moisture infiltration, mishandling and age - there are also a few other things that need to be in place for a quilt to be effective. Some of these are related to design.

The down.

Without fine raw materials most design aspects of a quilt becomes merely bandaids. 

A well known definition of down quality is ‘fill power’. The down producer determines the fill power of his batches, then the quilt maker decides which level to purchase and use. Nunatak quilts have a fill power of 900.

This number do not determine directly how warm the quilt is, but rather the weight to warmth ratio. A quilt with 750fp down can be as warm as one with 900fp down, but will be heavier and compact less during transport.

Down is collected mainly from ducks and geese. Discussions usually tries to defend duck down as a viable alternative to goose, not the other way around. Nunatak has never used duck down, and for a company providing fine down products we’ve yet to find a compelling reason to change this.

Although not related to warmth and weight, it bears mentioning that all Nunatak's down is responsibly sourced and ethically harvested via the stamp of the Responsible Down Standard (RDS)

Box Baffles 

Nunatak quilts are 'box baffled', not 'sewn through'. The illustration to the right shows clearly how the mat of insulation is continuous in a baffled design and interrupted and weakened when the shell and liner material is sewn through. The height of the vertical dividing baffle defines the size of the down filled cavity, which in turn sets the temperature rating of the quilt: smaller space - less warm, and vice versa.

The dreaded shifting of the down volume and the resulting bare spots.

Even experienced users certainly struggle with this. It's an issue as inherent to backcountry down as its dislike of moisture, but much less talked about. So what's going on?

A restless, overtired, slightly uncomfortable backpacker tosses and turns throughout the night, causing the down clusters to separate and tumble away from the high spots of shoulders and hips (if side sleeping) to lower areas along the sides. As heat rises inside the quilt it escapes at these bald areas and the already struggling camper now also gets cold.

While attempts at completely eliminating the issue may result in a quilt that’s unsuited for backcountry use, we do have a number of ways to at least limit this downward down migration. I list them here in descending order of importance.

Differential cut

All our down quilts are cut differentially, meaning the inner layer of fabric has a smaller circumference than the outer. This greatly assists in keeping the down in place and also resists compression of the insulation.

Proper fill amounts

During manufacturing down is added to the cavity created by the vertical baffle piece between the shell and liner fabrics. Hitting the precise amount needed is a balancing act. Too little might get close to providing the desired temperature rating, and will display a lighter total weight value in the specs chart, but will not hold the down in place. On the other hand too much down adds weight and expense without much return.

We feel the proper amount is a slight overfill that will expand the fabric between the baffle lines as shown on the right.

Baffle orientation

By mapping out the effects various sleep positions have on the down volume, we designed a mix of transverse and longitudinally oriented baffles.

Starting at the head end is a single top baffle running perpendicular to the body, This proved the most comfortable and ‘snugly’. A slight overfill here made sense to create a firm barrier against cold intrusions.

Next comes a section of baffles running head to toe to combat the sideways down migration of side sleeping. These baffles are also tapered gently with the narrow part downhill in the leg region.

The lower area of the quilt goes back to transverse baffles to prevent the down from shifting away from the feet, and allowing an overfilled box baffle in the important area near the toes.

Baffle wall material

For the inner, hidden box-baffle walls separating the shells we use a high friction mesh similar to the no-see-um netting found in tents. The down structures adjacent to the mesh clings to it tenaciously, creating the first step in the interlocking down mat stretching across the box-baffle to the mesh at the other side.  

Width and taper of the box baffle cavity

Finally we reduce the width between the stitch lines holding the mesh in place, to a practical minimum. The extra mesh walls typically adds under 1% in total weight, but our tests show that these smaller spaces compounds the effects of the over-filling mentioned earlier. 

The lengthwise box baffles in the torso region are also tapered from wide at the top to slightly narrower at the lower end. This further discourages the down form packing into the bottom of the slope.

Area specific down fill

We deliberately under and over-fill certain chambers in order to fully maximize the thermal value of a given down volume. See graphic to the right.

For instance the outer box baffles in the torso region typically ends up under the user on cold nights and therefore receives less fill. 

It is not unusual for the feet to become cold before anything else, so we overfill slightly the round footbox and the transverse baffle directly up from here.


    • Avatar
      Willem knopper
      Aug 18, 2019

      does the arc ul have a differential cut