Don’t Mess with Mr. Hyde: Modern Hygrothermal Performance Assessment

Recent changes in energy efficiency
requirements make
white roofing more and more
popular, as it can reduce the
cooling demand in warm and
hot climate zones. Also, recent
research has shown that the downside of
the application of a cool membrane makes
a huge difference regarding moisture management.
Using the hygrothermal performance
assessment tool WUFI, and based
on the latest research findings, the author
will show the consequences of white membranes
applied to a residential wood deck
construction.
According to the U.S. Department
of Energy, 40% of the entire U.S energy
demand is related to heating and cooling
of the millions of buildings in this country,
so it is probably a good idea to focus on
increasing the energy efficiency of our buildings
and building enclosures.
But who does not know the story of
Dr. Jekyll and Mr. Hyde—a man with two
opposing sides? The accompanying movie
poster of the duo (Figure 1) has been drawn
by my son Christian. Building science today
has some similarities, called heat and moisture.
Dr. Jekyll, who is the nice guy, saves
energy on all of our personal heating and
cooling bills, and we love him. But most
people don’t tell you that Dr. Jekyll comes
in a package with Mr. Hyde, who is the bad
guy in building science called moisture. The
facts are pretty simple: Increased energy
efficiency can include a higher risk of moisture
failures. Why? Well, in order to dry
moisture out of a building enclosure, you
need to spend the evaporation energy of
water. But remember, since we want to save
energy, modern building enclosures often
no longer provide for energy release. So the
risk of moisture accumulation and failure is
increasing. This is based on simple physics,
which doesn’t play favorites.
So how to deal with this dilemma? Well,
that can be complicated. Building scientists
weren’t lazy the last three decades, and
as the Europeans were forced to work on
this problem back in the ’80s, when their
energy efficiency codes were getting stricter
(and, as a consequence, they faced the Dr.
Jekyll/Mr. Hyde issue earlier than building
designers in this country), we have the
solution today called Modern Hygrothermal
Performance Assessment. A man named Dr.
Hartwig Künzel made the important step to
simplify the very complex physics of coupled
heat and moisture transfer and developed
WUFI® (Wärme Und Feuchte Instationär) at
Fraunhofer IBP, Germany. WUFI, a numerical
simulation tool for architects and engineers,
is based on Künzel’s work in 1995
and was significantly improved in 2006
and in 2008. WUFI is a tool to evaluate the
potential for moisture accumulation in a
building wall or roof and has been validated
with field measurements all over the world.
So, let’s go into some details now and
see how WUFI Pro can help to keep Mr.
Hyde under control. Most people know
about cool roofs and their benefits in terms
of cooling energy savings by reducing heat
gain. These can be significant savings. In
some states, cool roofs are mandatory for
O c t o b e r 2 0 1 7 R C I I n t e r f a c e • 4 1
Figure 1 – Dr. Jekyll and Mr. Hyde, by Christian Kehrer.
new construction and major roof replacement—
e.g., California’s Title 24.
However, cool roofs can have moisture
problems, particularly if they are not fully
adhered. Mechanically attached membrane
systems can be lifted by wind forces, creating
billowing. Billowing caused through
wind uplift can draw indoor air through
the joints and penetrations in the roof
deck and deposit moisture underneath the
roofing membrane. During cool exterior
temperatures, this can result in condensation
under the membrane. However, fully
adhered roofs can also have condensation
problems due to vapor diffusion. Further
research shows that four main parameters
affect this condensation potential: airtightness,
indoor air moisture content, project
location, and membrane color.
Figure 2 depicts a typical residential flat
roof construction. This includes a membrane
system over plywood with batt insulation
below the roof deck. Physics proves that
the color of the membrane makes a huge
difference with this construction.
Figure 3 shows the results for a WUFI®
Pro simulation for both white and black
surfaces under these conditions and the
moisture content in the plywood roof deck
using San Francisco, California, as an
example. Hutchinson, in 2009, found ice
formation underneath partially attached
cool roof membranes in cold climate zones
during the winter, while an ex-colleague and
RCI.indd 1 9/9/2017 7:28:56 AM
Figure 2 – Typical West Coast flat roof, wooden 2×8 construction with R-19 fiberglass
insulation.
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4 2 • R C I I n t e r f a c e O c t o b e r 2 0 1 7
I (Kehrer and Pallin, 2013) have also shown
that the selection of a cool membrane has a
major effect on moisture performance. The
construction with the white surface shows
continuing moisture accumulation year by
year, exceeding the threshold moisture content
of 20 mass percent (M.-%) after less
than two years, according to the ASHRAE
Handbook of Fundamentals. This can result
in severely deteriorated wood decks after a
few more years.
Figure 4 shows an even more drastic
analysis of the problem. According to
ASHRAE Standard 160-2016, the Mold
Growth Index, a model used to determine
mold growth in building enclosures, should
stay below 3.0. The construction with the
white surface shows that this threshold is
exceeded after about a year, while the black
surface construction stays well below 3.0.
Further details can be found in Pallin et al.,
2013, “Hygrothermal Performance of West
Coast Wood Deck Roofing Systems.”
CONCLUSION
White roofing is a good option to reduce
the demand of cooling energy for roofs, as
the roof will stay cooler on average because
of the solar reflection. The downside is
that the lower roof membrane temperature
increases the potential for condensation
and provides less drying potential
to mitigate moisture accumulation. When
O c t o b e r 2 0 1 7 R C I I n t e r f a c e • 4 3
Figure 3 – Moisture content in the plywood for a roof with a white and
a black surface.
Figure 4 – Mold Growth Index according to the evaluation criterion
of ASHRAE Standard 160-2016.
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using these roof membranes, particularly
in residential construction, we need to be
mindful of details that can overshadow the
energy savings. These include air barriers,
membrane attachment methods, reducing
interior air moisture content, and surface
temperatures. If we do not consider these
effects, Mr. Hyde may come calling.
REFERENCES
T. Hutchinson. “Potential Pitfalls of Cool
Roofing.” Ecobuilding Pulse. May
2009.
M. Kehrer and T. Schmidt.
“Temperaturverhältnisse an
Außenoberflächen unter Strahlungseinflüssen.”
International
Building Performance Simulation
Association (IBPSA) conference.
2006: Munich.
H.M. Künzel. 1995. “Simultaneous Heat
and Moisture Transport in Building
Components.” IRB. Stuttgart.
H.M. Künzel and M. Kehrer. (2008).
“Einfluss der Hinterlüftung auf
das Feuchteverhalten von Holzbauwänden.”
Umweltbewusstes Bauen.
Hrsg. A. Maas, Fraunhofer IRB
Verlag 2008, pp. 611-620.
S. Pallin, M. Kehrer, and A. Desjarlais.
(2013). “Hygrothermal Performance
of West Coast Wood Deck Roofing
Systems.” ORNL Report ORNL/
TM-2013/551.
Title 24, The 2016 California Building
Standards Code (California Code
of Regulations, Title 24). California
Building Standard Commission,
2016.
4 4 • R C I I n t e r f a c e O c t o b e r 2 0 1 7
Manfred Kehrer,
a senior associate
at Wiss, Janney,
Elstner Associates,
has been active in
the field of building
science for over 25
years. After more
than two decades
at Fraunhofer IBP,
Germany, where
he was leading
the WUFI® software
development
team, he worked for the Oak Ridge National
Laboratory for five years as a senior
researcher, followed by a one-year period as
president of the startup consulting company
justSmartSolutions LLC. Kehrer has been
the Official WUFI® Collaboration Partner for
USA/Canada since 2011.
Manfred Kehrer
Numerous states and municipalities are banning the use of coal tar sealants commonly used in the paving of driveways,
parking lots, and roadways. Research indicates that coal tar sealants are a major source of polycyclic aromatic
hydrocarbons, or PAHs. At high levels of exposure, these chemicals are known to be toxic. In 2015, the U.S. Geological
Survey (USGS) did two studies that determined that runoff from pavement seal-coated with coal tar was toxic to aquatic
life, had detrimental effects on DNA, and “are probable human carcinogens.”
An estimated 85 million gallons
of the black, shiny, coaltar-
based sealant are applied to
pavement each year, primarily
east of the Continental Divide
in the U.S. and parts of Canada,
according to the USGS.
Anne LeHuray, of the
Pavement Coatings Technology
Council, claimed that coal-tarbased
products last longer,
protect better, and are proven
products. Coal tar can also be
applied in cold weather. The
most common alternative to
coal-tar-based sealants is an
asphalt-based sealant, which
LeHuray said does not protect
as well as coal tar.
— Durability + Design
and usgs.gov
Photo by Barbara Mahler, USGS
Coal Tar Sealants Facing Bans