Energy and Moisture Performance of Attic Assemblies

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ENERGY AND MOISTURE PERFORMANCE
OF ATTIC ASSEMBLIES
ANDRÉ DESJARLAIS, FASTM, AND
WILLIAM MILLER, PHD
OAK RIDGE NATIONAL LABORATORY
PO Box 2008, Oak Ridge, TN 37831
Phone: 865-574-0022 • Fax: 865-574-9354 • E-mail: desjarlaisa@ornl.gov
Nonpresenting Authors:
SUDHIR RAILKAR, PHD; AND A. CHICH
GAF
ABSTRACT
The effect of natural ventilation in a residential attic cavity has been the topic of many
debates and scholarly reports since the 1930s. The purpose of ventilating an attic cavity is
to prevent collection of condensate on the structural surfaces and to create a thermal buffer
between the conditioned space and the ambient air. Additionally, the energy and moisture
performance of attic assemblies can be varied by a large number of construction features
that have been recently introduced into the marketplace. Specifically, the introduction of
underlayments with highly variable water vapor permeances, the inclusion of a radiant barrier,
a cool roof, the amount of ventilation area, and above-sheathing ventilation all can
change how an attic performs hygrothermally. The recent use of sealed attics and the movement
of the insulation from the attic floor to the rafters can also impact attic performance.
SPEAKER
ANDRÉ DESJARLAIS, FASTM – OAK RIDGE NATIONAL LABORATORY, OAK RIDGE, TN
ANDRÉ DESJARLAIS is the group leader for the Building Envelopes Research Program
at the Oak Ridge National Laboratory (ORNL). He has been involved in building envelope and
materials research for over 35 years, first as a consultant and, for the last 20 years, at
ORNL. He is active in the building industry, participating in ASHRAE, ASTM, the Cool Roof
Rating Council, SPRI, RCI Foundation, RICOWI, Federal Roofing Committee, and the
Building Environment and Thermal Envelope Council. Areas of expertise include building
envelope and material energy efficiency, moisture control, and durability.
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INTRODUCTION
Natural ventilation became part of a
standard attic design in the late 1930s. In
1939, the Federal Housing Administration
set a natural ventilation area ratio standard
as a means of minimizing condensation on
the underside of the roof sheathing. This
requirement was the result of the research
performed by Frank Rowley, professor at
the University of Minnesota, and published
in ASHRAE Transactions (Rowley, Algren,
and Lund, 1939). Rowley concluded that
natural ventilation openings are required
for the circulation of air in the attic space to
prevent surface condensation on the underlayment
of the roof. Following Rowley’s
work, the National Housing Agency published
“Property Standards and Minimum
Construction Requirements for Dwellings”
for the Federal Housing Administration
(FHA) in 1942. This document contains the
first record of the 1:300 specifications.
(FHA, 1942).
Attics have dramatically changed since
the days of Rowley. Cool roofs now represent
a growing portion of the overall residential
market. Underlayments with water vapor
permeances ranging from near 0 to 100
perms are available in the marketplace.
Radiant barriers are traditionally installed in
attics located in cooling-dominated climates.
Above-sheathing ventilation (ventilation
space created between the roof sheathing
and waterproofing layers) has shown
promise in improving the energy efficiency of
attic assemblies. Airtightness of building
envelopes has increased. Finally, the introduction
of sealed attics (moving the insulation
layer from the attic floor to the roof
sheathing and eliminating attic ventilation)
has been proposed as an energy-efficient and
durable solution related to the entry of moisture-
rich air in hot and humid climates and
the need to recapture energy losses from a
leaky distribution system located in the attic
space. The result of this experiment showed
that sealed attics with an air and thermal
barrier on the sloped roof deck can be built
without associated energy penalty in hot climates,
as compared to traditional ventilated
constructions (Rudd and Lstiburek, 1998).
The purpose of this research project was
to create attic assemblies that included all
of the features that would impact the
hygrothermal performance of an attic
assembly. An existing test facility was modified
to accept a family of different attic
assemblies. These assemblies were instrumented
and monitored for a period of more
than one year. These test data are used to
compare performance between the attic
assemblies and to validate a computer simulation
tool that can extend the comparisons
to other assemblies and climates.
The Facility and the Experiments
A series of experiments was planned to
be performed on the South Carolina Natural
Exposure Test Facility (NET) located in
Charleston, SC. A photo of the facility is
shown in Figure 1. This facility was constructed
in 2004 and was originally used to
test wall systems. The footprint of the NET
is 80 ft. long by 25 ft. wide with the long
dimension facing south. The roof was a traditionally
constructed rafter system with 2
by 6 rafters, 24 inches on center; oriented
strand board sheathing; and a slope of
approximately 4:12.
For these experiments, the attic of the
NET was subdivided into seven separate
attic modules, with divider walls constructed
to create the individual attics. The
divider walls were approximately R-15 and
were air-sealed. The old roof and sheathing
were completely removed and replaced during
the construction. The interior of the NET
is both temperature- and humidity-controlled.
During the course of the experiments,
the interior conditions were maintained
at 75°F and 45% relative humidity.
Descriptions of the test attics follow.
The attics are described in the same order
that they appear in the NET starting on the
west side of the building (left side of Figure
1). A schematic of the attic assemblies is
shown in Figure 2.
Attic 2: Sealed Attic (SLD)
This attic has 5.5 in. of open-cell foam
installed on the roof sheathing. The sheathing
is covered with 15-lb. felt paper (8
perms) and traditional, dark-colored shingles
with a solar reflectance of 10%. This
attic is unventilated and has no insulation
in the attic floor. The attic assemblies were
installed in May 2010.
Attic 3: Nonbreathable Underlayment (NB)
This attic has R38 fibrous-batt insulation
installed on the attic floor. The sheathing
is covered with a nonbreathable sheet
underlayment (0.04 perms) and traditional
dark-colored shingles with a solar
reflectance of 10%. Appropriate lengths of
soffit and ridge venting are added to obtain
a 1:300 ventilation area.
Attic 4: Cool Shingles (CC)
This attic is identical to Attic 3 except the
dark-colored shingles are replaced with cool
shingles that have a solar reflectance of 28%.
Attic 5: Above-Sheathing Ventilated
Roof (ASV)
This attic has R-38 fibrous-batt insulation
installed on the attic floor. 2-by-4 wood
nailers are installed on top of the roof
ENERGY AND MOISTURE PERFORMANCE
OF ATTIC ASSEMBLIES
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Figure 1 — The southern exposure of the NET facility in Charleston, SC.
sheathing, and a second layer of oriented
strand board is nailed on top, creating a continuous
¾-in. airspace between the two layers
of oriented strand board that can communicate
with the soffit and ridge vents. The
sheathing is covered with 15-lb. felt paper (8
perms) and traditional, dark-colored shingles
with a solar reflectance of 10%.
Appropriate lengths of soffit and ridge venting
are added to obtain a 1:300 ventilation
area. A fascia vent (2 times the net-free area
versus standard soffit vents) was installed
on the ASV section as well as the ridge vent,
while a traditional soffit vent was used for
intake in the attic portion of the roof. A ridge
slot cut into the roof exhausted for both ASV
and attic. Hence, this bay had a higher ventilation
rate than any of the other attics.
Attic 6: Radiant Barrier (RB)
This attic has R-38 fibrous-batt insulation
installed on the attic floor. A radiant
barrier that was factory-applied to the
underside of the oriented strand board
sheathing was installed in this attic. The
sheathing is covered with 15-lb. felt paper (8
perms) and traditional dark-colored shingles
with a solar reflectance of 10%. Appropriate
lengths of soffit and ridge venting are added
to obtain a 1:300 ventilation area.
Attic 1: Control Attic (CTRL)
This attic has R-38 fibrous-batt insulation
installed on the attic floor. The sheathing
is covered with permeable membrane
underlayment (16 perms) and traditional
dark-colored shingles with a solar
reflectance of 10%. Appropriate lengths of
soffit and ridge venting are added to obtain
a 1:300 ventilation area.
Attic 7: Increased Ventilation (FF)
This attic has R-38 fibrous-batt insulation
installed on the attic floor. The sheathing
is covered with an impermeable peeland-
stick underlayment (0.1 perms) and
traditional dark-colored shingles with a
solar reflectance of 10%. A fascia vent system
is installed in this attic to increase the
ventilation area to 1:150.
The Instrumentation
During the construction of the NET
facility attics, sensors were attached at pertinent
locations to analyze the in-situ performance
of the individual attic bays. The
sensors were placed in an identical pattern
for each attic cavity to facilitate the comparison
of results.
Thermistor temperature sensors were
combined with relative humidity sensors by
placing the two sensors in a permeable sack
made of spun-bonded polyolefin to protect
the sensors from contact with water.
Thermistor temperature sensors are also
used for the sensing shingle underside temperature,
as well as the temperature across
the fiberglass insulation. Figure 3 depicts
the location of these sensors within the attic
assembly.
Heat flux transducers (HFTs) were
attached to the underside of the roof decks,
as well as on the surface of the gypsum
board on the attic floor, as shown in Figure
3. These precalibrated devices generate an
electrical signal proportional to the total
heat rate applied to the surface of contact
and allow collection of conductive heat flux
data through the inclined roof decks and
the attic floor. The HFTs must be calibrated
in the configuration that they will be
operating. Therefore, the roof deck HFTs
were calibrated in a surface-applied configuration,
and the attic floor HFTs were calibrated
in between gypsum board and fiberglass
insulation using a heat-flow-meter
apparatus.
Figure 2 – Test attic assembly layout on NET facility in Charleston, SC.
Figure 3 – Instrumentation layout for each attic assembly.
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Pressure taps were installed near the
ridge and each soffit to estimate the ventilation
flow rate. Taps were also installed on
the south and north façades of the building
and in the building interior. Weather data
are collected on the roof of the NET facility
and include wind speed and direction, solar
irradiance, rainfall, ambient temperature,
relative humidity, and pressure.
Data from all of the sensors are recorded
continuously several times a minute into
the data acquisition system inside the NET
facility. Weekly, these data are downloaded
to the ORNL campus for processing. Data
collection started in August 2010 and, as of
this writing, is ongoing.
To simulate moisture generation from
occupied home conditions, moisture was
added to these attic spaces by installing a
humidifier in each attic (except in the
sealed attic). The relative humidity level was
determined using an earlier ORNL study of
relative humidity in several attics from
hot/humid climates and was varied on a
monthly schedule.
The Experimental Results
There are numerous comparisons that
can be made with the data gathered in this
research project, and only some of these
comparisons have been concluded to date.
A sampling of the data and some of the initial
conclusions will be discussed in this
section of the paper.
Winter Ceiling Heat Flux Comparison
Heat flux through the attic floor into the
conditioned space has been determined to
be a primary indicator of an attic’s comparative
load on the HVAC system of a building.
Each attic in the facility was outfitted with
a heat flux transducer to measure the
amount of heat entering (or leaving) the
conditioned space. During the summer and
winter months of 2011, these data were
analyzed in BIN averages as well as on the
basis of an overall integrated value for comparison.
Winter ceiling heat flux is shown in
Figure 4 on the basis of a time-step BIN
average.
The sign convention of the ceiling HFTs
is positive for heat flowing from the attic
into the conditioned space. As expected, in
winter, the heat flow is mainly in the negative
direction, indicating an attic space that
is cooler than the conditioned space. The
data suggest that the CC and RB attics
remain cold enough that, on average, the
direction of heat flow never reverses.
Attic 5, outfitted with above-sheathing
ventilation, has the least average diurnal
variation (amplitude of oscillation across
the zero line). It also shows the least
amount of heat lost from the conditioned
space during the early morning.
Open-cell spray foam was applied to the
roof deck and gable ends of Attic 2 to form
a sealed (SLD) attic. Once cured, the foam
fully covered the 2-by-6 rafters, and its Rvalue
was computed at about R-22. Sealed
attics recommend no insulation in the attic
floor; hence, its heat flux values are much
higher and vary more than other attics.
Figure 5 illustrates the large difference
in ceiling flux for the SLD compared with
the CTRL and ASV attics.
The primary reason for sealing the attic
is to capture lost energy from a leaky distribution
system and use it to make the attic
part of the conditioned space. If the attic air
is relatively close to the conditioned temper-
Figure 4 – Average winter comparative heat flux.
Figure 5 – Sealed attic ceiling winter heat flux.
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ature, then the ceiling flux will be negligible.
A concern with the sealed attic is that the
retrofit increases the volume of the conditioned
space, and the heat system must run
longer to satisfy the increased load.
The collected winter data were reduced
into the total integrated flux, and results
are provided in Table 1. This integration
was performed by a simple application of
the trapezoidal rule. See Equation 1.
Where Yn and Yn+1 correspond to the current
and future time step, ΔXn is the time
step (one hour in this case), and the subscript
“m” denotes the final point in the
series, which varies depending on the summer
or winter data sets. Keep in mind that
this integration represents a summation of
the area under the curve of the heat flux,
which also takes into consideration negative
values; therefore, the total integrated
heat flux should be considered an overall
value.
Overall, the ASV attic reduces the total
heat flux through the attic floor by 53% as
compared to the control (CTRL) attic. CC
and RB show comparable reduction of
14.8% and 11.2%, respectively, which
agrees with the graphical results of BINaveraging
the data in Figure 4. Another
important observation is that 1:150
accounts for a 7% reduction in ceiling heat
flow over a 1:300 attic.
Summer Ceiling Heat Flux Comparison
Observing heat flux through the attic
floor is especially important during the
summer months, because the temperature
of the attic can reach upwards of 50°C
(122°F), and that temperature difference
drives the heat from the attic into the conditioned
space at a generally larger magnitude
than in the winter.
The BIN average
heat flux through the
attic floor is shown
in Figure 6. All attics,
with the exception of
the ASV case, have a
window of time from
3:00 AM to 9:00 AM
where the heat flux
is negative. For this
period, the temperature
in the attic cavities
is, on average,
less than the conditioned
space.
Attics ASV, CC,
and RB demonstrate
relatively the same level of ceiling flux, with
an average peak flux value of 0.86 W/m2.
Adding the low-permeance membrane
appears to increase the heat flux by 0.35
W/m2. Increasing the ventilation area from
the NB at 1:300 to the FF at 1:150 increases
the heat flux from the NB peak of 1.16
W/m2 to the FF peak of 1.36 W/m2, which is
an increase of 0.2 W/m2.
In the summer season, the radiant barrier
and cool color shingles show the greatest
benefit in minimizing the heat flow
through the attic floor. As compared to the
CTRL, the RB and the CC show a similar
43% reduction in total integrated ceiling
heat flow. This pattern is logical because
the increased reflectivity of the cool color
shingles and the long-wave radiation shielding
of the radiant barrier offer protection
against incident solar radiation, the dominant
mode of summer heat transfer in an
attic cavity. See Table 2.
Addition of above-sheathing ventilation
results in a 29% reduction against the standard,
followed closely by the use of a lowpermeability
membrane at 24%. Similar to
Equation 1
Total Integrated Flux =Σ .5 * ΔXn * (Yn+Yn+1)
W
[hr * m2 ] n=0
m
Table 1 – Total integrated ceiling heat flux (winter 2011).
WINTER
Attic Feature Total Integrated Reduction
Ceiling Heat Flux From CTRL (%)
kJ
m2
05 ASV -2700 53%
04 CC -4896 15%
03 NB -5100 11%
06 RB -5287 8%
07 FF -5325 7%
01 CTRL -5746 Base
02 SLD -9735 -69%
[ ] Figure 6 – Summer ceiling heat flux.
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the winter data, the 1:150 FF (as compared
to the CTRL) attic results in a 10% reduction
in total integrated ceiling flux.
The SLD attic is of keen interest
because its integrated ceiling flux is 356%
more than that of the CRTL attic. In other
words, the sealed attic increases living
space cooling load by 356% as compared to
a conventional 1:300 attic.
Winter Moisture Control in Attics
A concern about attic construction is
the accumulation of moisture in the moisture-
sensitive components during that period
of time when moisture would be driven
from the conditioned space and inserted
into the attic. For the NET facility, this period
would be from December through March.
We investigated the temperatures of the
OSB underlayment in each attic to determine
if there was potential for surface condensation
based on surface temperature
depression below the dew point temperature
of the air. Attic air dew point temperature
was calculated using a standard
ASHRAE routine and compared to the field
data on an hour-by-hour basis. Condensate
occurs when the dry-bulb temperature of a
surface is depressed below the dew point
temperature of the surrounding air, which,
in this case, is the air inside the attic cavity.
The method chosen to calculate the dew
point temperature (Tdp) for this report is a
simple estimation based on a psychometric
state point determined by a known air temperature
and relative humidity. Note that
Equation 3 is specifically for temperature
units of Celsius. Coefficient “a” in Equation
3 is 17.27, and coefficient “b” is 237.7
(Simmons, 2008). See Equation 2.
The total number of times that the surface
temperatures of the wood joist or the
OSB sheathing
dropped below the
dew point temperature
of the attic was
logged and compared
to the total time of
data collection as a percentage, Table 3.
According to Table 3, the ASV attic has
the best surface condensation moisture
control, with the possibility of condensation
at only 1% of the recorded time. The
increased ventilation FF attic does provide
better dew point control than the NB attic,
with a 1.7% decrease for the sheathing and
a 2.0% decrease for the joist. As expected,
the breathable underlayment reduces condensate
potential by 33% in the joist. Note
that the SLD attic was not included in this
table because the sheathing has no direct
contact with the attic air.
Modeling the Attics
To be able to extend the experimental
data gathered during this series of tests, a
thermal model of the attic was used.
AtticSim (Wilkes and Rucker, 1983; Wilkes,
1991a; Wilkes, 1991b) is a computer tool for
predicting only the thermal performance of
attics. It mathematically describes the conduction
through the gables, eaves, roof
deck, and ceiling; the convection at the
exterior and interior surfaces; the radiosity
heat exchange between surfaces within the
attic enclosure; the heat transfer to the ventilation
air stream; and the latent heat
effects due to sorption and desorption of
moisture at the wood surfaces. Solar
reflectance, thermal emittance, and water
vapor permeance of the sundry surfaces are
input. However, AtticSim is a heat transfer
model, and it does not model moisture
transport in roof systems.
The model can account for different
insulation R-values and/or radiant barriers
attached to the various attic surfaces. It also
has an algorithm for predicting the effect of
air-conditioning ducts placed in the attic.
The code reads the roof pitch, length, and
width and the ridge orientation (azimuth
angle with respect to north) and calculates
the solar irradiance incident on the roof.
Conduction heat transfer through the two
roof decks, two gables, and vertical eaves are
modeled using the thermal response factor
technique, which requires the thermal conductivity,
specific heat, density, and thickness
of each attic section for calculating
conduction transfer functions. Heat balances
at the interior surfaces (facing the
attic space) include the conduction, the radiation
exchange with other surfaces, the convection,
and the latent load contributions.
Heat balances at the exterior surfaces balance
the heat conducted through the attic
surface to the heat convected to the air, the
heat radiated to the surroundings, and the
Table 2 – Total integrated heat flux (summer 2011).
SUMMER
Attic Feature Total Integrated Reduction
Ceiling Heat Flux From CTRL (%)
kJ
m2
06 RB 4582 44%
04 CC 4666 43%
05 ASV 5779 29%
03 NB 6201 24%
07 FF 7288 10%
01 CTRL 8122 Base
02 SLD 37012 -356%
[ ] Equation 2
Tdp =
a – Ln(RH) – aTdb
b+Tdb
Ln(RH) + aTdb
b+Tdb
b( )
Table 3 – Surface condensation potential.
WINTER
Attic Feature Hours % Time for Hours % Time for
Tsheath<Tdp Condensation Tjoist<Tdp Condensation
(2,015 total) on Sheathing (2,015 total) on Joist
05 ASV 20 1.0% 10 0.5%
07 FF 75 3.7% 32 1.6%
06 RB 83 4.1% 26 1.3%
01 CTRL 102 5.1% 48 2.4%
04 CC 103 5.1% 62 3.1%
03 NB 110 5.5% 72 3.6%
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heat stored by the surface. Iterative solution
of the simultaneous equations describing
the heat balances yields the interior and
exterior surface temperatures and the attic
air temperature at one-hour time steps. The
heat flows at the attic’s ceiling, roof sections,
gables, and eaves are calculated using the
conduction transfer function equations. The
tool was validated against field experiments
and is capable of predicting the ceiling heat
flows integrated over time to within 10% of
the field measurement. AtticSim has been
developed into ASTM Standard C1340,
Standard Practice for Estimation of Heat Gain
or Loss Through Ceilings Under Attics
Containing Radiant Barriers by Use of a
Computer Program (ASTM, 2004) and is
available for general use.
Validating the AtticSim Model
AtticSim was used to estimate the heat
flux into the roof sheathing and through the
ceiling for the CTRL test attic. Comparisons
of these modeled heat fluxes for a week in
the summer, with their corresponding measured
heat fluxes, are shown in Figures 7
and 8. In Figure 7, the heat flux from the
model (bold line) agrees very well with the
measured data. The data in Figure 8 does
not show as good a comparison but does
capture all of the trends created by the
varying meteorological conditions imposed
on the attic. The AtticSim model overpredicts
heat flux in conventional attics. The
small heat fluxes going through the ceiling
make this a critical comparison. The greatest
uncertainty in this analysis is the Rvalue
of the fibrous insulation, which is typically
within +/- 5% of the label value that
was used for the analysis.
Using the AtticSim Model to Study
Ventilation
Using a standard year of typical meteorological
year (TMY2) data (ASHRAE, 2009),
a yearlong performance of the NET facility
was modeled, with a particular focus on the
variation of ventilation. The effect of varying
the ventilation area is observed by taking
three of the attics (CTRL, RB, and ASV) and
performing a parametric study on varying
ventilation rates. Equal part soffit and ridge
ventilation areas were varied in each attic at
footprint-to-ventilation area ratios of 1:25,
1:50, 1:150, 1:300, and 1:500 to show an
overall effect of modifying the ventilation
area heat transfer buffering to the conditioned
space. (Note that we cannot control
moisture in AtticSim.) Increasing the ventilation
area ratios increases the actual orifice
area through which wind-driven flow
passes. AtticSim calculates the air-changeper-
hour (ACH) values at each hour, based
on an empirical routine developed for soffit
and ridge vent systems (Burch and Treado,
1979). This ACH computation has been
Figure 7 – Comparison of measured and modeled heat flux
through the south roof deck.
Figure 8 – Comparison of measured and modeled heat flux
through ceiling.
Figure 9 – Ventilation ratio effect during average summer day.
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benchmarked to the NET facility via tracer
gas analysis. Average diurnal ACH values in
the CTRL attic are presented for the summer
season in Figure 9 to show the effect of
increasing ventilation ratio on the flow rate
during the course of a typical summer day.
The effect of increased ventilation ratio
is an increase in flow rate over the entire
diurnal cycle, but also in the amplitude of
the maximum ventilation at the peak, from
30 ACH at 1:500 to 594 ACH at 1:25.
Maximum ventilation takes place at approximately
5:00 PM, with minimum ventilation
occurring at 4:00 AM.
Increasing the ventilation area for the
attic cavity would logically increase the airflow
rate up to a certain point where the
model would approach the situation of a
system of flat plates suspended in an open
airstream. Therefore, there is an obvious
limit to how much airflow one can achieve
by increasing the ventilation area. This limit
is shown in Figure 10, where the effects of
increasing the ventilation ratio are shown
on the maximum and minimum ACH values
during the summer season.
As seen in Figure 10, as the scale of data
approaches 1:50, the ACH prediction increases
exponentially. The reality, however,
is that an attic will never be constructed
with a ventilation ratio close to 1:50. At this
point, the attic is no longer operating as an
effective barrier against inclement weather.
The range in ventilation ratio that most
effectively increases the attic’s ACH (without
approaching the limit of reasonable ventilation
area) is roughly 1:300 to 1:100.
Even more important than the effective
increase of the ventilation rate is the prevention
of condensation on the surfaces of
the attic. Condensation potential on the
OSB sheathing was calculated by simulating
the dew point temperature of the attic
air and comparing the predicted sheathing
temperature against it. The percentage of
total time for condensate potential is shown
in Table 4, such that the results will emulate
the results of the field data analysis
previously shown.
The simulation results show that an
increase in ventilation area consistently
decreases the potential for condensation on
the OSB sheathing. However, the magnitude
of the effect is relatively small in comparison
to the difference seen by changing
the attic type. For example, the result of
increasing the ventilation area in a CTRL
attic from the standard 1:300 to 1:50 is a
decrease of only 0.2%. If the CTRL attic with
a ventilation area of 1:300 is retrofitted with
an ASV system, however, the percentage
potential for sheathing condensation is
2.9%, a decrease of almost 10%.
The addition of the radiant barrier to the
attic sheathing has the effect of keeping the
surface cooler during the night during the
winter months. This is explained by the
simple fact that the radiant barrier halts the
long-wave radiation down through the roof
deck during the daylight hours and then
performs the same way with the heat from
the conditioned space during the night and
early morning hours, which would depress
the temperature of the OSB. Since the early
morning hours are the most likely time for
condensation potential, the cooler surface
of the OSB would lead to a higher likelihood
that condensation would occur.
These results are higher than the measured
results presented in Table 3. Several
reasons for the differences include the fact
that the measured data were acquired during
a very mild winter and that the leakage
rate from the conditioned space to the attic
may not have been comparable between the
testing and simulation.
Simulations were run to compare the
effect of ventilation on the ceiling heat flux,
which is a best indicator of the influence the
attic cavity has on the HVAC system of a residence.
The data presented for these simulations
are shown on time-of-day BIN averages
to mirror the presentation style of the field
data. Summer simulation shows that the
average peak ceiling heat flux in the CTRL
attic occurs at approximately 5:00 PM, at
which point the maximum stratification of
ventilation effect is observed in Figure 11.
The results of the ASV and RB simulations
are similar. As observed in experimental
data, the model predicts that using a 1:150
attic lowers the heat in the living space by
7% as compared to a 1:300 attic.
CONCLUSIONS
A series of field experiments was performed
on seven attics exposed to meteorological
conditions in Charleston, SC. These
data were used to validate AtticSim, a tran-
Figure 10 – Effect of ventilation ratio on maximum and minimum summer ACH
values.
Table 4 – TMY2 results of sheathing condensation potential.
Percent of total time in which Tsheathing<Tdp
Attic Type 1:25 1:50 1:150 1:300 1:500
RB 15.4% 15.9% 16.4% 17.0% 17.2%
CTRL 11.4% 11.8% 12.5% 12.7% 12.6%
ASV 2.8% 2.8% 2.8% 2.9% 2.7%
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sient hourly attic simulation model.
Increasing ventilation to 1:150 from
1:300 reduces heat load by 10% in summer
and reduces heating load by 7% in winter.
Increased ventilation to 1:150 also reduces
the condensation potential by 33% as compared
to the 1:300 attic.
Sealed attics put the highest heat load
on the living space compared to any other
attic strategy. A sealed attic increases heat
by 356% in the summer over the 1:300 attic
and takes away 69% more heat in the winter
as compared to the 1:300 attic. Hence,
both in summer and winter, there is a very
large energy penalty.
Field data on the ceiling flux heat transfer
showed that the ASV attic had the least
average diurnal variation over the winter
2011 season, with the least amount of heat
lost from the conditioned space during the
early morning (1:00 – 6:00 AM). Integration
of the total heat flux over the winter season
shows that the ASV attic reduces the total
heat flux through the attic floor by 53% as
compared to the CTRL attic.
Integration of the summer data showed
that, when compared to the CTRL attic, the
RB demonstrates a 44% reduction, and the
CC shows a similar 43% reduction in total
integrated ceiling heat flow. Addition of
above-sheathing ventilation results in a
29% reduction against the standard.
As expected, cool-colored shingles
reduce the heat load by 43% in summer.
However, the shingles also help in reducing
heating load by 15% during winter. Coolcolored
shingles with a low-perm underlayment
work as effectively as RB systems and
work better than ASV in reducing heat load
in summer.
Low-perm underlayments show some
benefit in reducing heat load and need further
investigation.
Measurements indicate that increasing
the ventilation ratio decreases the condensation
potential from 5.1% to 3.7% during
the winter months, whereas the ASV attic
reduces the potential to a mere 1.0%
throughout the year.
Modeling the impact of ventilation on
ceiling energy performance suggests that
only modest improvements are made by
increasing the ventilation rate. Doubling the
ventilation area, for example, yields a
reduction in peak heat flux of 7%. This is
only based on the heat transfer model without
taking moisture into consideration.
As expected, breathable underlayments
reduce potential for condensation by almost
33% as compared with nonbreathable systems.
BIBLIOGRAPHY
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Figure 11 – Average result of summer diurnal ceiling heat flux simulations.
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