ASHRAE Journal - February 2014 - 42

Column BUILDING SCIENCES
Joseph W. Lstiburek

How Buildings Stack Up
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

We live at the bottom of an ocean of air. Each of us is carrying around 14.7 pounds
per square inch when at the beach in Miami.* We are powerful creatures indeed.
Imagine carrying around 101,000 Pascals on that beach. Or 1,010 millibars. As elevation increases, the weight of the air we are carrying around decreases. This decrease
in weight with elevation is called the lapse rate (Figure 1).
the building, the greater the stack
effect. The colder the temperature,
the greater the stack effect. So, in
heated buildings, the air tends to
flow out of the top of the building
while inducing air to flow in at the
bottom.
Now let's look at Figure 3. The
hole is at the top of the building. All other assumptions are the
same. Once again, at the hole, the
pressure inside equalizes with
the pressure outside. As we go
down in height, the pressure difference between the inside and
outside gets bigger-but it is in the
opposite direction. Neat, eh? The
interior pressure line (the interior
lapse rate "line") moves laterally-
horizontally shifts to the left from
Figure 2 to Figure 3. The "solid" line
does not move. The "dotted" line
moves. The slope of the "dotted"
line does not change, it just shifts
left.
What happens when we have two
holes the same size? One at the top
and one at the bottom? Check out
Figure 4. The "dotted" line shifts

100

160

Temperature (K)
180 200 220 240

260

280

Thermosphere (Heated by
Oxygen Absorption of Solar UV)
Mesopause

Height (km)

The lapse rate thing gets interesting with buildings. In a heated
building, the lapse rate inside is
less than the lapse rate outside.
This is due to the reduced density
of heated air compared to unheated
air. Check out Figure 2. The assumptions in this figure are important.
There is only one hole in the building enclosure, and it is at the bottom. There are also no interior
floors or partitions. So we have an
airtight building (except for the one
hole at the bottom) with no interior
flow resistance. And, the building
is heated. At the hole, the pressure
inside equalizes with the pressure
outside. As we go up with height,
the pressure difference between
the inside and outside gets bigger.
This difference in pressure is called
the stack effect.
The stack effect gets its name from
the same phenomenon that causes
hot combustion gases to rise in a
chimney or chimney stack. A heated
house or heated building can be
considered a giant chimney that
we live and work inside. The taller

Mesosphere
Stratopause

Stratosphere
(Heated by Ozone
Absorption of Solar UV)
20
Troposphere
Tropopause
(Heated by Surface
Absorption Solar Visible)
0
10-6 10-4
10-2
100
102
Pressure (hPa)

104

FIGURE 1 Lapse Rate. This is a chart of the U.S.

Standard Atmosphere (1976). The red line is the
temperature lapse rate. The blue line is the pressure lapse rate. The pressure lapse rate is around
12 Pa per meter or 3.6 Pa per foot.

right partially. It crosses the "solid"
line smack dab in the center of the
building elevation wise. Where the
interior lapse rate line crosses the
exterior lapse rate line, the pressure inside equals the pressure
outside. No pressure difference
exists. We call that the neutral

*Why Miami? Ah, I am thinking warm thoughts. Any sea level location would have worked. Unfortunately, the International Civil Aviation Organization (ICAO) Standard Atmosphere assumes a temperature of
15°C (59°F). Not exactly bathing suit weather except for Canadians. It also assumes no water vapor. So it is a "dry cold."
42

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