ASHRAE Journal - July 2019 - 51

COLUMN BUILDING SCIENCES

FIGURE 1 Stack Effect in a Tall Building. Interior
airflows in tall buildings compromise smoke control,
fire safety, indoor air quality, comfort and energy use.

FIGURE 2 More Stack Effect in a Tall Building. The air in lower units ends up in the upper units. I guess that is
why folks in the upper units pay more for the privilege.

Stack Effect Flow Out
(Exﬁltration)

Poutside Drops with
Height Faster than
Pinside

That by itself "wins" the value
proposition and water control is
why most owners buy "air barriers"
and water control is why most
architects specify "air barriers."
Assemblies leaking rain just don't ﬂy
and water leakage liability is a big
deal. But in terms of overall building
performance the "real action" is on
the inside. That's where the energy
penalties lie, that's where the indoor
air quality penalties lie and that's
where the smoke and ﬁre control
penalties lie.
Most folks in the air barrier
business think that the "air barrier"
is the primary air enclosure
boundary that separates indoor
(conditioned) air and outdoor
(unconditioned) air. Most folks
forget that in high-rise buildings
and in multi-unit apartment
construction the air barrier system
is also supposed to separate the
conditioned air between ﬂoors and
from any given unit and adjacent
units. Even more importantly, the
air barrier is also the ﬁre barrier
and smoke barrier in these interunit separations. This inter-unit
separation must meet speciﬁc ﬁre
resistance ratings. There is a huge
†

business opportunity here that is
currently untapped-more surfaces
that need to be air barriers, hint,
hint.
The key to this argument is
understanding the stack effect. We
were here before ("How Buildings
Stack Up," ASHRAE Journal, February
2014). 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 building can be considered
a giant chimney where we live
and work inside of. The taller the
building, the greater the stack effect.
The colder the temperature, the
greater the stack effect. In heated
buildings, the air tends to ﬂow out
of the top of the building while
inducing air to ﬂow in at the bottom.
A real smart old guy wrote about this
in 1926.†
Stack effect driven airﬂows in
tall buildings compromise smoke
control and ﬁre safety, adversely
affect indoor air quality and comfort
as well as increase operating costs
for space conditioning energy. The

No, I never met Professor Emswiler. Not even I am that old. Check out the bibliography
at the end of this column.

Pinside Drops with
Height Slower than
Poutside
Neutral Pressure Plane

FIGURE 3 Compartmentalization. Basically, you turn
a 10-story building into 10 one-story buildings that
are stacked on top of one another. By isolating the
units from each other and from corridors, shafts,
elevators and stairwells, stack-effect driven interior
airflows can be controlled.

Reduced Individual Unit Stack Effect

air in lower ﬂoors ends up in the
upper ﬂoors (Figures 1 and 2). I guess
that is why folks in the upper ﬂoors
pay more for the privilege.
By isolating the ﬂoors and units
from each other and from corridors,
shafts, elevators and stairwells,
stack-effect-driven interior airﬂows
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of
Building Science Corporation in Westford, Mass. Visit
www.buildingscience.com.

J U LY 2 0 19

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ASHRAE Journal - July 2019

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