In using a down comforter, one usually places it inside a duvet cover
in order to provide protection for the comforter. With regards to
actual warmth provided to someone using the comforter, presumably this
extra layer also has some benefit. If, however, instead of placing the
comforter inside the duvet cover, one simply laid the cover on top of
the comforter, would the person under the comforter be kept as warm as
he is when the comforter is inside the duvet cover? Is there actual
benefit, insulation-wise, to having the comforter _inside_the duvet
cover? I would prefer to ignore the fact that someone might be more
likely to toss the cover off while sleeping if it is simply laid on
top, and focus on the thermodynamics of how comforters and their
covers keep one warm.

Lisava - -

Hmm.  Sounds like the kind of domestic argument we might see in my
household.

Heat flows away from a warm body via conduction, convection and
radiation.  Many health professionals see conduction and convection
generally to be similar in transferring thermal energy, unless a body
is in water or touching a metal – both more efficient conductors than
clothing fibers.
Industrial Safety & Hygiene News
"Monitor Your Heat Stress"
http://www.ishn.com/CDA/ArticleInformation/features/BNP__Features__Item/0,2162,2564,00.html

Insulation depends on dead air space to trap thermal heat.  That is
how the down comforter works and why a damp comforter with compressed
feathers loses its ability to keep a body warm.  One inch (2.54 cm) of
dead air alone carries an R-value of 5.5:
Los Angeles Department of  Water and Power
"Insulation"
http://www.ladwp.com/energyadvisor/PA_45.html

The duvet cover and the shell of the comforter are both likely to be
made of cotton – no differences in conduction directly from the body.

Is there different convection, depending on whether both layers of the
duvet cover are ON TOP of the comforter – or surrounding the
comforter?  In the first case, the duvet cover is likely to be
compressed, but still provides two thin thermal barriers: between
comforter and the bottom of the duvet cover + between the two layers
of the cotton duvet cover.

What’s the difference in insulation between the two?  Probably none. 
Any differences will rely on other arguments, such as that the “dead
air” seal of a loose duvet cover is worse (or better) than when the
cover is properly installed.

Note that there is a third kind of heat produced by the body, radiant
heat, which accounts for 19% of the energy given off by a human. 
Radiant heat can be reflected by silvered surfaces but still relies on
a layer of air to prevent transfer of energy through the “foil”
directly.

Position of the duvet cover will make no difference in retention of
radiant heat.  However, the COLOR of the cover will make a large
difference – with silver being highly reflective and even a white
being much more reflective than dark colors (which will absorb and
distribute the radiant heat):
NASA
"Absorption and Radiation"
http://quest.arc.nasa.gov/space/teachers/suited/9d4absor.html

Google search strategy:
insulation + "air space" + theory
convection + radiant + heat
color + "radiant heat" + reflection

If any part of this answer is unclear, please let us know before
rating the answer.

Best regards,

Omnivorous-GA

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Request for Answer Clarification by lisava91-ga on 06 Mar 2003 01:17 PST

To what degree does the dead air seal that you mention impact how warm
one is under the comforter? What is that seal, exactly? Is it possible
that one would be measurably (if not noticably) warmer just because
that seal was better?

---

Clarification of Answer by omnivorous-ga on 06 Mar 2003 03:36 PST

Lisava --

As all of the articles indicate, it's the dead air -- which combines
to use the low conductive properties of air to pass along thermal
energy AND the absence of convective movement of air -- which provides
insulation value and keeps a body warm.

Admittedly the comforter, with a thickness of 3"-4" (7.6-10 cm.)
itself is providing the bulk of the insulation value.  But the cotton
outer shell adds to the trapping of dead air.  Several factors could
make the duvet cover a better insulator:
*  not ironing the cover (allowing more crenulations to trap air)
*  keeping the duvet cover from being compressed -- perhaps one
advantage to having it around an irregularly-shaped comforter, rather
than just sitting atop the comforter
*  lighter colors, providing better reflection of radiant heat
*  thickness and consistency of the material used in the duvet cover
(silk would be a better insulator than cotton)

Buttoning or sealing the duvet cover will provide a small additional
amount of dead airspace, but the total volume involved of duvet +
comforter it's in the range of 1% -- not enough value to be felt.

Best regards,

Omnivorous-GA

My experience with duvets is that they insulate by trapping air among
the down feathers.  The degree of insulation, and therefore their
warmth, depends on the "loft" - that is, the height of the down pile. 
Therefore, I would expect that placing the cover on top would double
the compression of the down layer and result in a DECREASE in warmth. 
To improve warmth, just shake up your duvet to fill the down with air.
 It should be like covering yourself with a comfy warm cloud, rather
than putting on a layer of blankets.

And... an old (and bad) joke:

How do you get down off an elephant?


You don't - you get it off a duck!