ASHRAE Journal - January 2023 - 29

COLUMN BUILDING SCIENCES
PHOTO 3 Ductwork. Buildings now have ducted
and fan-driven heating, cooling and ventilation
systems and are significantly more airtight, so
moving large quantities of air within them can
lead to unacceptably high negative and positive air
pressure differences.
and relative humidity adjacent interior
surfaces is higher, which can lead
to moisture problems. The bad gets
worse if we do not bring in suffi cient
exterior air. We can create indoor
environmental issues.
The opposite is also bad. If we
bring in too much exterior air, we
can create indoor environmental
issues such as excessive interior
surface wetting in hot-humid and
mixed-humid climates or excessive
interior dryness in cold climates
leading to discomfort and material
damage to hygroscopic interior trim
and fi nishing materials.
It gets even more complicated
with ducted and fan-driven heating,
cooling and ventilation systems
(Photo 3). Buildings are signifi cantly
more airtight, so moving large quantities
of air within them can lead
to unacceptably high negative and
positive air pressure differences.
Exhaust systems such a kitchen
range hood can also lead to signifi -
cant negative pressures (Photo 4) and
backdraft combustion systems such
*I never get a call going- " Joe, things are
going great...let's have a beer.. "
†I consider Rudolph Clausius the big
dog of thermodynamics-entropy, baby,
entropy.
as fi replaces (Photo 5). Because of the
huge improvements in effi ciency,
typical air-conditioning systems
no longer run long enough to suffi
ciently dehumidify the interior of
buildings. On top of that, modern
high effi ciency air-conditioning systems
provide less dehumidifi cation
than old ineffi cient units.
Over time the moisture sensitivity
of assemblies has gone up as
well as the " dwell time " for water in
these building assemblies. Building
assemblies are staying wetter longer,
and the modern materials are
unable to tolerate the moisture
stress. This is coupled with heating,
cooling, and ventilation systems that
have to address air change, air pressure,
and interior humidity control.
Things are much more complicated
than they used to be.
The New Reality
Because there has been a big
change in energy fl ow, materials and
conditioning, we have to change the
way we build. When we insulated old
buildings, the paint peeled off them
and attics rotted. When we caulked
and sealed old buildings, we reduced
air change, increased pressure differences,
messed up combustion air
and chimney performance, spilled
and back-drafted furnaces and water
heaters, collected radon and other
soil gasses, increased mold, increased
part-load humidity, increased mold
again. We should have learned.
Actually, we did-but we seemed to
have forgotten today.*
Building assemblies need to be
protected from wetting via rainwater,
groundwater, air transport and
from vapor diffusion. The typical
strategies use water control layers,
air control layers, vapor control
PHOTO 4 (TOP) Kitchen Range Hoods. Exhaust
systems such kitchen range hoods can lead to
significant negative pressures.
PHOTO 5 (BOTTOM) Back-Drafting Fireplace.
Negative pressures due to air-exhausting systems
backdraft combustion systems such as fireplaces.
layers, thermal control layers ( " insulation " ),
air pressure control and
control of interior moisture levels
through ventilation, dehumidifi cation
and source control. Climate
location, season and interior loads
determine the strategies.
The reduction in energy fl ow is
going to get even greater. The change
in materials is going to get even
greater. Wait for the fun that is coming
when we add carbon and carbon
sequestration and embodied energy
to the mix. This is complicated
enough with new buildings where
we start with a clean sheet of paper.
Wait until we do all of this in existing
buildings. Retrofi ts are going to
be way more fun. There is no such
thing as a free thermodynamic
lunch. I bet the ghost of Rudolph
Clausius†
has a big grin.
References
1. Hutcheon, N.B. 1953. " Fundamental
considerations in the design of exterior
walls for buildings. " Engineering Journal
36(1):687 - 698.
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