ASHRAE Journal - July 2019 - 52

COLUMN BUILDING SCIENCES

52

ASHRAE JOURNAL

ashrae.org

PHOTO 1 Operable Windows in a Commercial Office
Building. Somewhere in a European country that is
known for pasta.

distributed mechanical systems
that are limited to serving a floor or
a unit or a "compartment." You can
move chilled water and hot water
across "real" compartments and
across floors but not air in ducts
and shafts. The "ducts" and "shafts"
are "holes" in the compartments. To
deal with this typically means more
fans and apparently that drives
some engineers insane.
The architect argument against
compartmentalization and
distributed mechanical systems is
that they don't like the "holes" and
"penetrations" in their nice façades.
Come folks, make them look good.
You want operable windows,
you are going to have to live with
more holes. If you want that LEED
"compartmentalization" credit, you
are going to have to live with more
holes. Running exhaust ventilation
ducts up to the roof is not going
to fly in a truly high performance
building.
Achieving compartmentalization
is not easy. You have to think threedimensionally and treat your
interior walls like exterior walls
(Figure 4). Back in the day some of

J U LY 2 0 19

PHOTO 2 Old School. Magnificent depression era
building with operable windows. Not bad for a
bunch of geezers, eh?

PHOTO ISTOCK/SIMEON)

can be controlled (Figure 3). Today,
we call this compartmentalization.
The most elegant argument for
compartmentalization of tall
buildings came from Handegord
(2001).
Basically, you turn a 10-story
building into 10 one-story buildings
that are stacked on top of one
another. So why do it? Well, now the
air pressure differences are much
lower. Rather than a 10-story stack
effect acting across your building,
you have ten one-story stack effects.
Neat, eh? With one story stack
effects, you can have operable
windows in tall buildings. Check
out Photo 1. Clearly, not a North
American building. If you proposed
this to your mechanical engineer,
their head would explode. You
might want to mention this to them
for the entertainment value alone.
The irony is that we pioneered
this in North America in the
1930s. North America is where tall
buildings were first built, and we
built them with operable windows
(Photo 2). We were able to do this
because we compartmentalized,
and we didn't move air between
floors for conditioning and
ventilation. Ducts and shafts
running between floors defeat
compartmentalization. Elevator
lobbies needed to be in vestibules
and they were.
I don't know any architect
that would argue against having
operable windows in a tall building.
I don't know any occupant or
tenant or employee that would
argue against having operable
windows. Only engineers do.
But they didn't in the past. The
reason engineers argue against
operable windows is that you need

FIGURE 4 More Compartmentalization. You have to
think three-dimensionally and treat your interior
walls like exterior walls. A minimum resistance or
air permeance of 2.00 L/(s·m2) at 75 Pa of unit air
tightness is necessary to control stack effect air
pressures and to limit airflow from adjacent units
and cross contamination. Individual units are air
sealed and tested by pressurization or depressurization for compliance.

us guessed at what unit airtightness
should be. I proposed a minimum
resistance or air permeance of 2.00
L/(s·m2) at 75 Pa (Lstiburek, 2005).
This works out to 0.4 ft3/(min·ft2) at
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ASHRAE Journal - July 2019

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