wrote:

Some people would rather not tolerate relative humidity below a certain
level, even if they "feel" warm enough, so "comfort zone" does not
pertain to temperature alone...

It does in a narrow technical sense, based on surveys of thousands of people
("Does it feel warm enough now?"), but as you say, some people have quirks.

We agree that constantly replenishing water in the air generally requires
greater energy than the slight increase in temperature that would make it
"feel" just as warm, unless the humidity lost to air leaving the house can
be made small.

Extremely small, like a submarine :-) Most US houses leak about 10 times
too much air to save any energy with winter humidification.

Sealed combustion eliminates one of these losses. It is my observation that
sealed combustion can eliminate the need for supplemental
humidification altogether.

That seems unlikely to me, in a typical US house with 0.7 ACH. Then again,
is there any such need?

It also decreases the potential for air infiltration in certain areas which
would make those areas "feel" cold, inciting the occupants to increase
the thermostat setpoint.

And if exterior walls are warmer, with less inward infiltration, they lose
more heat to the outdoors. And uniform infiltration through wind barriers
could make them act like Scandinavian "breathing walls" with no heat loss
to the outdoors.

That's food for thought, but I remain skeptical that infiltration
induced by non-sealed furnace combustion will significantly decrease
the overall exterior temperature of conventionally built exterior walls

It might move an extra 20 cfm in through the walls, cooling the insides
of the walls a bit and adding fresh air to a house (at a time when it
probably isn't needed :-)

or make them perform like these Scandanavian walls of which you speak.

They are carefully designed for a slow uniform inward airflow. Their
only "insulation" is a 1/4" porous felt with an infinite R-value :-)
It's unlikely a US wall would work that way, even with a wind barrier.
Tyvek and Typar are less porous than that breathing wall felt.

If infiltration creates cold spots on the interior surface, they'll
draw heat from the room.

And lose less heat to the outdoors. A wall with a 60 F interior surface
because of air infiltration will lose less heat to the outdoors than
the same wall with a 65 F interior surface with no air infiltration,
even with room air at the same (eg 70 F) temperature, no?

It seems to me that even if unidirectional infiltration (i.e. enter
through wall, exit through furnace exhaust stack) were to reduce the
wall exterior surface temperature to outside temp so as to eliminate
heat loss from that surface, it wouldn't eliminate heat loss to the
outdoors altogether. The heat-losing surface has simply relocated to
somewhere within the wall or the living space.

I could see how the R value for a well designed breathing wall would be
unexpectedly high compared to a conventionally framed and fiberglassed
wall, if one's expectations are based on looks alone.

I don't see how this has to do with looks. If a uniform sheet of outdoor
air is slowly flowing towards you from a wall (at a velocity less than
a perceptible draft), you can't lose any heat to that wall by convection,
because the warm air won't travel upwind. You can still lose heat by
radiation to the wall, and it still takes power to heat the air that
flows in through the wall, but the heat loss by convection through
the wall is zero, even with 1/4" insulation. Scandinavians have been
building and measuring walls like this for 30 years.

Nick