ASHRAE Journal - February 2014 - 44

Column BUILDING SCIENCES

and this product is added to the next product of the
"effective area" of a hole multiplied by its distance to the
neutral pressure plane for all of the holes above the neutral pressure plane. Yes, you all know where this is going;
this sum must equal the sum of the same exercise of all of
the holes below the neutral pressure plane.
Bottom line: sealing big holes in the attic moves the
neutral pressure plane down, and sealing big holes in
the basement moves the neutral pressure plane up. This
has very interesting consequences. Let's say you want to
keep radon gas out of your basement.§ Sealing big holes
in your attic helps as it lowers the neutral pressure plane
and reduces the pressures across the holes at the bottom
of your building.
Let's say you were running an energy conservation
program in a country where it is cold and they play
hockey outside and you wanted to insulate attics. But
when you first just added a bunch of attic insulation,
you rotted out a whole bunch of attics because you made
the attics colder and negated the effect of attic ventilation on controlling attic moisture problems because
attic ventilation needs heat loss to work. So you wanted
to fix this and you did fix this by sealing the big holes
in the ceiling-the "attic bypasses"-to prevent interior
moisture-laden air from getting into the attic and causing those nasty attic moisture problems. You fixed it by
mandating air sealing your attic ceiling as part of your
insulation strategy. Guess what happened to the location of the neutral pressure plane? Yup. You lowered it
and exposed more of your exterior walls to exfiltration.
You just moved the attic moisture problem to your walls.
Bummer. It got worse.
Most homes in cold climates were heated with fuel
burning furnaces: gas and oil heat. These appliances had
chimneys. As far as building enclosures are concerned,
chimneys are exhaust fans-big ones at that. Exhaust
fans and chimneys tend to raise the neutral pressure
plane. Most houses in cold climates with big oil and gas
furnaces had neutral pressure planes located above attic
ceilings (Figure 6). This had enormous implications for
building durability.
With neutral pressure planes located above ceiling
lines the entire building enclosure sees infiltration.

Neutral Pressure Plane:
Indoors to outdoors
Neutral Pressure Plane:
Chimney to outdoors
Neutral Pressure Plane:
Fan Duct to outdoors
Pressure in Fan Duct
Pressure Difference
Fan Duct to Indoors
Pressure in
operating
Chimney

Pressure Indoors
Pressure
outdoors
Pressure Difference
Indoors to Chimney

Absolute Pressure
FIGURE 6 House With Operating Chimney. The operating chimney depressurizes

the house by raising the neutral pressure plane well above the ceiling (adapted
from Handegord2). It was hard to rot a house in a cold climate that had a neutral
pressure plane located above the ceiling. It was hard, but not impossible. You
could do it with rain leakage.

Infiltrating air does not bring moisture with it in a cold
climate during the heating season. Exfiltration, not
infiltration, hurts you in a cold climate. And, exfiltration
hurts you only if the exfiltrating air has lots of moisture
in it. It was hard to rot a house in a cold climate that had
a neutral pressure plane located above the ceiling.# The
old big chimneys did two things: they raised the neutral
pressure plane above ceilings, and they also provided a
large air change that served to dilute interior moisture
levels.
Of course, the "bad thing" about these old oil and gas
furnaces with big chimneys was that they were very
energy inefficient. We began to replace them with sealed
combustion appliances. We also discovered heat pumps
and electric heating. No chimneys. Neutral pressure
planes dropped like rocks, and dilution air change that
came from operating chimneys disappeared. So interior
moisture levels went up precisely at the same time large
portions of building enclosures became exposed to exfiltrating air.
Getting rid of active chimneys and air sealing
attic ceilings dropped neutral pressure planes and
caused moisture havoc in cold climate exterior walls

§For many years I have thought that the ePA placed a misguided emphasis on radon mitigation, but I kept silent-oK, not always-because it had a fabulous collateral outcome of handling soil gas and
pesticide and ground contamination issues, which in my opinion were more significant. If you can measure stuff, you can scare the peasants. radon was easy to measure and the radiation thing was so
sci-fi and easy to relate to every pseudo disease.
#It was hard, but not impossible. You could do it with rain leakage.
44

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