ASHRAE Journal - February 2014 - 43

Column BUILDING SCIENCES

Total Stack Effect

Pressure Difference
Across Exterior Walls

Pressure Outdoors

Opening to Outdoors

Pressure Indoors

Pressure Difference
Across Exterior Walls

Pressure Outdoors
Opening to Outdoors
Total Stack Effect
Absolute Pressure

Absolute Pressure

FIGURE 2 (LEFT) Stack Effect. Building with no internal separations with opening at the bottom (adapted from Handegord2). FIGURE 3 (RIGHT) More Stack Effect. Building with no

internal separations with opening at the top (adapted from Handegord2).

Pressure Difference Acting
Outward Across Ceiling

Outward
Pressure
Indoors
Openings at Top
And Bottom

Pressure Difference
Acting Outward
Across Exterior Walls

Neutral Pressure Plane
Pressure
Difference Across
Exterior Walls
Inward

Pressure
Outdoors

Pressure Difference
Acting Inward
Across Exterior Walls
Pressure Difference
Acting Inward at Grade

Absolute Pressure

Neutral Pressure Plane
Indoors to Outdoors

Absolute Pressure

FIGURE 4 (LEFT) And Still More Stack Effect. Building with no internal separations with openings at the top and bottom (adapted from Handegord2). Beginning to look familiar,
eh? Notice the "neutral pressure plane." At the neutral pressure plane no difference in pressure exists between the inside and the outside. FIGURE 5 (RIGHT) House. Basic twostory house with vented attic (adapted from Handegord2). The assumption here is a uniform distribution of leakage areas. All surfaces are uniformly leaky. All holes above the
neutral pressure plane have air "exfiltrating." All holes below the neutral pressure plane have air "infiltrating."

pressure plane or neutral pressure zone. A real smart
old guy wrote about this in ASHVE Journal in 1926.†
Now let's modify the assumptions a bit. Let's assume
a uniform distribution of leakage areas. All surfaces are
uniformly leaky-same size holes-same distribution.
Also, and this is the tricky assumption, let's assume "no
flow."‡ Now let's put a vented attic on top of the box. We
get Figure 5. All holes above the neutral pressure plane
would have air "exfiltrating" if there was flow. All holes
below the neutral pressure plane would have air "infiltrating" if there was "flow."
†No, I never met Professor Emswiler. Not even I am that old. Check out the reference at the end of
this column. University of Michigan lad. Go Wolverines. Professor Emswiler also wrote a classic text
on thermodynamics amazingly enough called Thermodynamics.1
‡Don't bust my chops on this, OK. Actual flow will result in modified pressures; there is friction and a
bunch of other stuff to consider. These types of diagrams represent the maximum theoretical pressures.
If you want to go into the weeds, get the original paper or even better read Handegord.2,3 ASHRAE
Handbook-Fundamentals is good too. Or go take Collin Olson of the Energy Conservatory to lunch.

How does the neutral pressure plane know where it is
supposed to be? Ah, Grasshopper, it always knows. In the
absence of wind and exhaust fans, supply fans and ducted
HVAC systems and interior partitions-weasel words for
sure-roughly half the holes are above the neutral pressure plane and roughly half the holes are below the neutral pressure plane. We are talking surface area wise, sort
of. There are cracks, and there are straight through to the
outside holes, and there are tortuous paths. It gets worse.
Holes farther away from the neutral pressure plane are
more "important" than holes closer to the neutral pressure plane. It is sort of like a "moment" calculation in
structural engineering. The "effective area" of a hole is
multiplied by its distance to the neutral pressure plane,
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Somerville,
Mass. Visit www.buildingscience.com.
FEBRUARY 2014

ashrae.org

ashraE JourNal

ASHRAE Journal - February 2014

Table of Contents for the Digital Edition of ASHRAE Journal - February 2014