Assessing the Performance, Application, and Cost of Retrofit Wall Systems for Residential Buildings

IIBEC Interface February 2023
Feature
IN THE US, 39% of total energy is consumed
by the building sector, and 20% of that total
is attributed to residential buildings.1 Newly
constructed houses built to meet modern
energy codes incorporate a combination
of tight, well-insulated building enclosure
components, high-performing windows,
controlled mechanical ventilation, and other
efficient components that deliver comfort,
adequate airflow, and moisture control
in addition to significantly lower energy
consumption than ever before.
Older houses (those built before 1992
when the US Department of Energy [DOE] Building Energy Codes Program was
established) represent approximately 68% of
the US residential building stock,2,3 and these
structures often have significant air leakage
and inadequate insulation. In residences with
little to no air sealing or insulation, heating
and cooling losses can represent a substantial
portion of utility bills.
The residential remodeling market continues
to grow, amounting to $424 billion in 2017 (up
50% from 2010). In 2017, approximately 50% of
home improvement projects included upgrades
to mechanical and enclosure systems in aging
housing stock (made up of approximately 93%
wood-framed walls, 5% masonry, and 2% steel
framing).4 These upgrades include replacement
of windows and doors; siding and roofing;
heating, ventilation, and air-conditioning (HVAC)
systems; and insulation. Approximately one
in five homeowners has invested in energy
efficiency retrofits.4 Even so, the number of
existing residential buildings with little to no
insulation is staggering. An estimated 34.5
million houses with wood studs have no wall
insulation,5 representing approximately 38%
of existing single-family detached houses in
the US. Similarly, 71% of existing houses have
air leakage rates of 10 or more air changes
per hour at 1.04 lb/ft2 (50 Pa) of pressure,
indicating a significant amount of air leakage
through the building enclosure.4
There is a significant need for cost-effective
methods of increasing wall insulation and
reducing air infiltration for existing houses. In
current practice, wall retrofits seldom include
the air, moisture, and vapor controls that are
considered best practices for high-performance
new home construction, and the lack of such
controls could potentially create problems
that put the building materials or occupants at
risk. Well-tested and documented retrofit wall
systems can help save substantial amounts of
energy and improve home durability, comfort,
health, and resilience. Done correctly, deep
energy retrofits (DERs) can significantly
improve the energy and air-barrier performance
of a building’s thermal enclosure, help manage
indoor environmental pollutants, improve
the building’s aesthetics, and increase
homeowner comfort.
This article describes a three-year DOEfunded
project to identify high-performing
wall retrofit systems and provide a real-world
context for their thermal, moisture, and
economic performance that can aid decision
makers in balancing various goals for DERs.
INDUSTRY INPUT AND
LITERATURE SURVEY
As an initial step in this project, the research
team invited experts from industry, academia,
the national laboratories, and other research
organizations to join an expert advisory
committee and participate in an expert meeting
to help identify and characterize candidate
wall systems. The meeting was held on April
19, 2019, in Arlington, Va., with 33 experts in
attendance. A report summarizing this meeting
was published.6
The objectives of this meeting were to bring
together leading researchers and innovators
to review the research methodology and to
encourage suggestions, information sharing, and
collaboration. The meeting’s outcomes would
inform potential retrofit systems to be developed
and tested. Specific topics discussed in detail
included data characterization for proposed wall
selections, wall selection for subsequent in-situ
testing, and techno-economic study criteria.
The literature review7 was conducted and
published in June 2019. It provides an overview
of the thermal and moisture performance of
wall assemblies, identifies relevant research,
and summarizes current practices for exterior
wall retrofits for existing houses, focusing on
retrofit applications to the exterior side of a
wall assembly. Given that the vast majority of
residential wall systems in the US are wood
framing, the report focused on this construction
practice.
In addition to investigating wall assemblies,
the literature review explores various innovative
Assessing the Performance,
Application, and Cost
of Retrofit Wall Systems
for Residential Buildings
By André Desjarlais, F-ASTM
This paper was originally presented at the 2022 IIBEC International Convention and Trade Show.
Interface articles m ay c ite t rade, b rand, o r
product names to specify or describe adequately
materials, experimental procedures, and/or
equipment. In no case does such identification
imply recommendation or endorsement by the
International Institute of Building Enclosure
Consultants (IIBEC).
www.iibec.org IIBEC Interface • 27
WOOD WALL FRAMING STEEL WALL FRAMING
WEST
GUARD
BAY
EAST
GUARD

insulation materials and provides background
for a techno-economic analysis, and the use of
such analyses in building construction. A review
of literature on the modeling and simulation of
hygrothermal wall assembly performance is also
presented, and references and links for a variety
of sources of relevant information are included.
FIELD TESTING
Test Facility and Test Panels
The experimental portion of this project was
carried out by the University of Minnesota at the
Cloquet Residential Research Facility (CRRF),
which is located on the Cloquet Forestry Center
near Cloquet, Minn., approximately 20 miles
(32 km) west of Duluth and in DOE Climate
Zone 7. The CRRF building (Fig. 1 and 2) is
elongated along an east-west axis to maximize
the northern and southern exposures. It sits on a
full basement with 12 independent above-grade
test bays protected by two end-guard bays. The
eight test bays that have both north and south
exposures (Bays 1 to 4 and 9 to 12) were selected
to conduct in situ testing for this project.
Baseline Test Panels
Two series of in situ experiments were conducted
during this three-year project. The first series
of test walls (Phase 1), which were developed
in response to the activities associated with the
literature survey and the expert meeting, were
deployed in the CRRF in December 2019 and
evaluated for two winter periods. After studying
the results of these first tests, the research team
proposed a second series of wall assemblies
(Phase 2) in consultation with an advisory
committee that oversaw the research project.
These wall assemblies were installed in the CRRF
in December 2020.
Phase 1 of this project was conducted in Bays
1 to 4 and Phase 2 used Bays 9 to 12. Each test
bay has a north-facing and a south-facing wall
opening. These openings are approximately
8 ft (2.4 m) wide and 7 ft (2.1 m) high, and
for this project, they were divided in half to
support two different test panels. Each test
panel was mirrored on both the north and south
orientations so eight pairs of wall assemblies
were studied during each phase.
The test panels are approximately 4 ft (1.2 m)
wide by 7 ft (2.1 m) high. Each test panel was
divided into three wall cavities at approximately
16 in. (0.4 m) on center (oc) to represent older
wood-frame construction. The center cavity
of each test panel was a true 16 in. (0.4 m) oc
and was designated as the test cavity. All the
monitoring sensors were installed within this
test cavity. The wall cavities on each side of the
test cavity were designed as guard cavities. They
received the exact same insulation treatment
to mitigate any differential horizontal heat
flows between the test and guard cavities. Both
horizontal and vertical moisture flows between
the test panels and test opening were controlled
with the use of low-permeability membrane
tapes.
To assess the impact of wall retrofits, a
baseline wall assembly was designed and used
as the starting point for each wall assembly and
16 identical test walls were constructed for each
phase. The baseline test walls were constructed
of 2 × 4 in. (51 × 102 mm) spruce, pine, or fir
wood studs with 1 × 6 in. (25 × 152 mm) pine
board exterior sheathing. The pine sheathing
was loosely fit to reflect older construction.
The sheathing was covered with a heavy no.
30 building paper lapped and stapled to the
sheathing followed by 8 in. (203 mm) cedar lap
siding finished with an oil-based primer, vaporretarder
primer, and latex topcoat. This exterior
finish was selected to represent an older house
with several coats of oil-based paints. Once
the test panel was installed in the test opening
and the instrumentation array was installed,
an interior finish of 5/8-in.-thick (16-mm-thick)
gypsum board with a vapor-retarder primer
was added. The interior finish was selected to
Figure 1. The Cloquet
Residential Research
Facility was used for the
in situ testing of retrofit
wall assemblies.
Figure 2. Floor plan of the Cloquet Residential Research Facility.
28 • IIBEC Interface February 2023
represent an older house with heavy drywall
or plaster and several coats of paint. The southfacing
baseline walls from Phase 2 are shown in
Fig. 3. Team members familiar with construction
practices in the local climates indicated that
vapor retarders were not historically included in
construction practices for the time period that
was being considered for initial constructions.
Since the majority of retrofits were to be
performed on the exterior side of the wall
assembly, access to the interior side of the cavity
was unavailable and therefore vapor retarders
were not included in most of the retrofits.
Instrumentation
Depending on the specific construction, each
test cavity had between 15 and 20 sensors
installed. Sensors for temperature
(type-T thermocouples), relative
humidity (capacitance type), heat flux
(heat flux transducers), and moisture
content (brass nails coated with
enamel) were deployed in each test
panel. Generally, temperature sensors
were installed on the interior and
exterior surfaces of the drywall, the
interior and exterior surfaces of the
sheathing, and the exterior surface of
the siding. Relative humidity sensors
were placed on the cavity-side surface
of the drywall and the interior and
exterior surfaces of the sheathing. The
heat flux transducer was located on
the interior surface of the drywall. The
moisture content pins were inserted
from the cavity side to measure
the moisture content of the interior
and exterior surfaces of the pine sheathing
as well as the middle of the cedar siding.
Figure 4 presents a schematic of a typical
instrumentation array.
The data acquisition system for this
experiment was based on the Campbell
Scientific CR-1000X data logger. The centrally
located logger collected data from modules
located in each test bay. The data acquisition
system was also set up to collect interior
and exterior boundary conditions. The
interior temperature and relative humidity
were measured in each test bay. In Phase 1,
the exterior temperature, humidity, wind,
and precipitation data were gathered from
local weather stations. For Phase 2, a local
weather station was added to the CRRF with
temperature, relative humidity, wind speed
and direction, rain gauge, and horizontal solar
radiation instruments. Additional pyranometers
were used to measure the solar radiation of
the vertical wall surface on both the north
and south exposures. Data were continuously
collected throughout the winter periods. These
data were used to validate both thermal and
hygrothermal models as described in the
following.
Wall Retrofits
Over the course of the three-year project, 16
baseline/retrofit strategies were evaluated.
Walls A through H were instrumented and
installed in the CRRF in December 2019, and
Walls I through P were set up in December
2020. Data collection on each wall has been
ongoing continuously since their installation. A
brief description of each retrofit follows.
Wall A: Base Case Wall #1
Wall A is the baseline wall without any retrofit
treatment.
Wall B: Drill and Fill (Cellulose)
For Wall B, the siding was removed in two
locations just below the midpoint and near the
top of the cavity, and holes were drilled through
the building paper and sheathing. The cellulose
was installed by a certified contractor with a
target density between 3.5 to 4.0 lb/ft3 (56 to
64 kg/m3). The holes in the sheathing were
sealed with spray foam, tape was used to repair
the building paper, and the siding was replaced.
Figure 3. Exterior view of baseline walls depicting cedar siding before wall retrofits.

Figure 4. Typical layout of instrumentation in test panels.
www.iibec.org IIBEC Interface • 29
Wall C: Minimally Invasive
Cavity Spray Foam
This treatment is a foam installed from
the interior. The foam manufacturer’s
representatives managed all formulation and
installation techniques, including the injection
of the proprietary closed-cell polyurethane
liquid foam through very small holes in the
drywall. Infrared imaging was used to ensure
the cavities were completely filled, and the holes
in the drywall were sealed with the spray foam.
Wall D: Exterior Expanded
Polystyrene Foam Panel
(Siding Remains)
This wall treatment used a commercially available
expanded polystyrene (EPS) insulation product
that includes built-in drainage capabilities and
an embedded structural ladder for attachment.
A low-density fiberglass board was installed over
the existing siding to remove the air channels that
would be created between the existing lapped
siding and the rigid EPS panel. A housewrap was
stretched over the fiberglass board to provide a
new air- and water-control layer. Two layers of EPS
(2- and 2.5-in.-thick [51- and 64-mm-thick]) were
installed to the existing wall with screws using
the integral fastening ladder. Vinyl siding was
installed with screws to the integral fastening
ladder in the second panel.
Wall E: Drill and Fill (Cellulose)
with Exterior Extruded Polystyrene
(Siding Removed)
For Wall E, dense-pack cellulose was installed as
described for Wall B. In this case, the cedar lap
siding and building paper were removed and
housewrap was installed as a new air- and watercontrol
layer. Also, 2 in. (51 mm) of extruded
polystyrene foam (XPS) were held in place, and 1
× 4 in. (25 ×102 mm) furring strips were fastened
to the framing through the insulation layer with
washer head screws. A ¾-in.-thick (19-mm-thick)
XPS layer was placed between the furring strips
to support the vinyl siding cladding that was
attached to the furring strips.
Wall F: Drill and Fill (Cellulose) with
Exterior Vacuum Insulation Panel/
Vinyl Siding (Siding Removed)
For Wall F, dense-pack cellulose was installed
as described for Wall B. The cedar lap siding
and building paper were removed, and a
housewrap was installed as a new air- and
water-control layer. A vacuum insulation panel/
vinyl siding composite panel was installed to
the exterior sheathing.
Wall G: Exterior Mineral
Fiberboard (Siding Remains)
For Wall G, a vapor-permeable liquidapplied
membrane was applied over the
existing lapped siding to provide a more
robust water-control layer. A 2-in.-thick
(51-mm-thick) mineral wool panel was held
in place, while a second 2-in-thick mineral
wool layer was installed with staggered
joints. Also, 1 × 4 in. (25 × 102 mm) furring
strips were installed with washer head
screws. A semirigid fiberglass board was
installed between the furring strips to act
as an insect screen that allows drainage and
drying, and fiber-cement siding was fastened
to the furring strips.
Wall H: Exterior Structural
Graphite-Impregnated EPS
Panel (Siding Remains)
For Wall H, a low-density fiberglass board
was installed over existing siding to fill
potential air voids between the existing
lapped siding and the retrofit panel. A 1.5
in. (38 mm) structural oriented strand board
(OSB) sheet was fastened with screws to the
wall framing and covered with a fully adhered
peel-and-stick membrane. Two layers
of 21/8-in.-thick (54-mm-thick) graphiteimpregnated
EPS were installed using a
limited number of cap nails, and 1 × 4 in.
(25 ×102 mm) furring strips were installed
with washer head screws. A semirigid
fiberglass board was installed between
the furring strips to act as an insect screen
that allows drainage and drying, and both
fiber-cement siding and a metal panel siding
were fastened to the furring strips. This wall
treatment was envisioned to be an off-site
fabricated panel, but for this study, it was
installed in layers onto the existing wall.
Wall I: Base Case Wall #2
Wall I is a baseline wall without any retrofit
treatment, identical to Wall A.
Wall J: Drill and Fill (Fiberglass)
For Wall J, the siding was removed in one
location just below the midpoint and near the
middle of the cavities, and holes were drilled
through the building paper and sheathing.
The fiberglass was installed by a certified
contractor with a target density of 1.5 lb/ft3
(24 kg/m3). The holes in the sheathing were
sealed with spray foam, a piece of building
paper was used to repair the water-control
layer, and the siding was replaced.
Wall K: Interior Polyiso Insulation
with Fiberglass Batt
For Wall K, the drywall was removed and an
unfaced fiberglass batt with an R-value of 13
(RSI 2.3) was carefully installed in the existing
cavity. A 1-in.-thick (25-mm-thick) foil-faced
polyisocyanurate foam board was installed
over the studs. The drywall was reinstalled, and
a sealant was used to ensure airtightness.
Wall L: Drill and Fill (Fiberglass)
with Exterior Polyiso Insulation
(Siding Removed)
For this wall, fiberglass was installed as
described for Wall J. In this instance, the cedar
lap siding and building paper were removed
and the holes were filled with spray foam.
A housewrap was applied and a 1-in.-thick
(25-mm-thick) foil-faced polyisocyanurate foam
board was installed with 1 × 4 in. (25 × 102
mm) furring strips fastened to the framing with
washer head screws. A prefinished lap wood
composite siding was fastened to the furring
strips.
Wall M: Exterior Insulation and
Finish System Panel (Siding
Removed)
This treatment used a 6-in.-thick
(152-mm-thick) piece of EPS foam finished
on all six sides with a stucco material and was
intended to be prefabricated. The existing
siding and building paper were removed, and a
coat of liquid-applied membrane was applied.
All gaps and nail holes in the sheathing were
filled with a proprietary caulk, and a second
coat of membrane was applied. The prefinished
exterior insulation and finish system (EIFS)
panels were fixed in place using a gun-grade
adhesive, and a temporary shelf at the bottom
edge of the test panel supported the weight
as the adhesive cured. The shelf supports were
removed approximately 24 hours later.
Wall N: Prefabricated
Polyurethane Blocks
For this prefabricated wall treatment, a
housewrap was installed over the existing siding
to serve as a new air- and backup water-control
layer. A base plate was installed to receive the
custom trim pieces at the top and both sides of
the assembly. The custom metal starter strip
was installed to receive the first polyurethane
foam block, which was mechanically attached.
Subsequent blocks engage the block below with
a large tongue-and-groove shape in the foam
extrusion.
30 • IIBEC Interface February 2023
Wall O: Drill and Fill (Fiberglass)
with Exterior Fiberglass Board
Insulation
This wall treatment uses fiberglass installed as
described for Wall J. The siding was repaired,
but touch-up was not required, and a sheet of
housewrap was draped from the top of the panel.
Two-inch-thick (51-mm-thick) semirigid fiberglass
boards were installed and held in place with 1 ×
4 in. (25 × 102 mm) furring strips fastened to the
framing with washer head screws. A fiber-cement
siding was installed on the furring strips.
Wall P: Thermal Break Shear Wall
(Siding and Sheathing Removed)
For Wall P, the existing siding, building paper,
and sheathing were removed and an unfaced
fiberglass batt with an R-value of 13 (RSI 2.3)
was installed in the existing cavity, followed by
a 1-in.-thick (25-mm-thick) XPS board installed
over the studs. A ¾-in.-thick (19-mm-thick)
OSB sheet was installed over the XPS and
fastened securely to the studs with 4-in.-long
(102-mm-long) screws. A housewrap was
installed, followed by a typical installation of
vinyl siding.
ENERGY MODELING
Energy modeling have been used in many
studies to evaluate enclosure performance.8
Laboratory and field evaluations of building
enclosure performance are expensive. In the
past decade, modeling software programs for
building energy and enclosure performance
have become more robust, and the value of
findings from these programs is recognized by
the research community and industry. Most
building modeling tools are based on solving
physics-based energy and mass equations; they
can provide detailed outputs on many aspects of
building performance.
To capture annual energy cost savings for
houses after the DERs, whole building energy
modeling (BEM) tools were used. They simulate
whole building energy consumption using
hourly modeling of thermal loads and HVAC
systems. BEM tools account for all the energy
interactions involving indoor space, outdoor
environment conditions, HVAC, lighting,
service water heating, other appliances and
equipment, and occupancy behavior. In such
analyses, the energy flow through enclosure
elements such as the walls, roof, and windows
is treated as one dimensional, and mass
flow of moisture and air and phase changes
of moisture are not well captured. Among
these tools, the DOE-sponsored EnergyPlus
is a popular model because of its continuous
research and development supported by DOE
and the modeling community.
A reference set of residential building models
representative of the existing national residential
building stock was created to quantify the
energy performance of the proposed walls. The
DOE’s Building Energy Codes Program has used
residential prototype buildings to evaluate the
energy and economic performance of residential
energy codes, and to develop proposed code
changes.9 However, the prototypes represent the
new construction stock and minimal compliance
with the residential prescriptive and mandatory
requirements of the 2018 International Energy
Conservation Code (IECC).10 Thus, these
prototype models were modified to represent
the existing building stock, and the inputs for
these modifications were taken from the National
Renewable Energy Laboratory’s ResStock
database (a large-scale housing stock database
developed by combining public and private
data sources, statistical sampling, and detailed
building simulations).11,12
The baseline house was
created for this study with
modifications using the ResStock
data to better represent the
existing building stock. Based
on US Census Bureau data,3
the baseline house is a singlefamily,
two-story house with
a gross floor area of 2400 ft2
(223 m2) with a slab-on-grade
foundation type and either an
electric resistance or gas-furnace
heating system type. Details
about the model can be found in
the technical support document
by Mendon, Lucas, and Goel.13
Based on ResStock data,
a baseline energy model was
constructed with the following
assumptions:
1. The uninsulated walls were
framed with wood 2 × 4s
at 16 in. (0.4 m) oc, and the
insulated, vented ceilings
had R-value 30 (RSI-5.3)
insulation.
2. Natural gas heating system
with an efficiency of 80%
Figure 5. Energy modeling outputs compared with measured experimental data for Wall A.
Figure 6. Energy modeling outputs compared with measured experimental data for Wall J.
www.iibec.org IIBEC Interface • 31
annual fuel utilization efficiency, and a
cooling system with an efficiency of seasonal
energy efficiency ratio of 10.
3. Ducting inside of the conditioned space,
eliminating the need for duct leakage
modeling.
4. Standard electric water heater for Climate
Zone 1 and Climate Zone 2 and gas water
heaters for all other climate zones.
5. Clear single-pane windows with a U-factor
of 1.22 Btu/h-ft2-°F (6.92 W/m2•K) and a
solar heat gain coefficient (SHGC) of 0.39 for
Climate Zones 1–3 and clear double-pane
windows with a U-factor of 0.62 Btu/h-ft2-°F
(3.52 W/m2•K) and SHGC of 0.39 for Climate
Zones 4–8.
6. Whole house infiltration rates of 15 air
changes per hour at 1.04 lb/ft2 (50 Pa) of
pressure for the baseline house.
The baseline house was modified to create a
set of models representing each of the climate
zones as defined by the IECC. Each baseline
model was then simulated with all 14 wall
retrofit options using EnergyPlus Version 8.6.
However, because EnergyPlus uses a simplified
one-dimensional calculation approach for
conduction heat transfer through the building
enclosure, the research team applied THERM,14
a two-dimensional conduction heat-transfer
analysis program developed by Lawrence
Berkeley National Laboratory, to capture the
multidimensional effects of thermal bridging.
A THERM model was developed for each wall
section using the as-built layout and thermal
properties of the wall assemblies, and overall
section U-values were obtained from THERM
and applied to the respective EnergyPlus
models.
To use energy modeling to analyze wall
performance on a national scale, it is first
necessary to benchmark model results
against measured data. Within this project,
all 14 candidate wall retrofit assemblies were
constructed and instrumented with sensors
at the CRRF. To validate the energy models’
enclosure calculations, multiple energy models
were constructed, each representing a residential
building containing the candidate retrofit wall
assemblies. These energy models were run
using the site-measured weather data, and the
results of each of these models were compared
against measured temperature and heat-flux
measurements. Interior-facing wall surface
temperatures, exterior-facing wall surface
temperatures, and interior-facing heat fluxes were
compared between the measured and modeled
assemblies to validate model performance.
Figures 5 and 6 present benchmarking
plot examples. In Fig. 5, the exterior surface
temperature and interior surface heat-flux values
for Wall A, the baseline wall, are displayed,
and the measured and modeled data can be
compared. For the displayed data set, the root
mean square error values are 4.7°F (2.6°C) and
1.10 Btu/hr-ft2 (3.47 W/m2) for exterior surface
temperature and interior heat-flux comparisons,
respectively. Similar data are depicted in Fig. 6 for
Wall J, the dense-packed fiberglass drill-and-fill
wall.Although the test assemblies at the CRRF give
insight into the real-world moisture and energy
performance of the proposed retrofit assemblies,
physical experiments only provide context for the
climate in which the experiment was conducted.
Therefore, to improve understanding of the
energy-saving potential of these candidate
retrofit assemblies, researchers also performed
simulations on the assemblies for the following
cities selected from the IECC 2015 climate zones
to represent a diverse set of climates: Miami, Fla.
(Climate Zone 1); Houston, Tex. (Climate Zone
2); Memphis, Tenn. (Climate Zone 3); Baltimore,
Md. (Climate Zone 4); Chicago, Ill. (Climate Zone
5); Burlington, Vt. (Climate Zone 6); Duluth,
Minn. (Climate Zone 7); and Fairbanks, Alaska
(Climate Zone 8). National energy prices were
also assumed for this analysis. Energy cost
values of $0.1013/kWh and $1.00/Therm were
applied nationally for electricity and heating fuel,
respectively.
Figures 7 and 8 depict the annual energy
costs for the simulated prototype house for
Phase 1 and Phase 2 walls, respectively. Broad
conclusions related to the potential savings
and cost-effectiveness of climate zones can be
Figure 7. The annual energy costs for the modeled residential prototype building with the
Phase 1 wall retrofitted assemblies.
Figure 8. The annual energy costs for the modeled residential prototype building with
Phase 2 wall retrofitted assemblies.
32 • IIBEC Interface February 2023
drawn. For Climate Zone 1, the average savings
for all simulated retrofit options is 12%. Wall
performance for this climate zone is led by Wall
H, which is also the assembly with the highest
effective R-value. Average cost savings continue
to increase from Climate Zones 1 to 8, with
Climate Zone 8 having an average savings of
31%. From a national scale, these results suggest
that the most influential climates for enclosure
retrofits are those that are heating dominated
(Climate Zones 5 through 8).
HYGROTHERMAL MODELING
Hygrothermal modeling is used to evaluate the
condensation potential, moisture content, and
drying capacity of the assembly, as well as the
potential for mold growth and freezing-andthawing
damage. During the last two decades,
several computer simulation tools have been
developed to predict thermal and moisture
conditions in buildings and the building
enclosure. In addition to their use as forensic
tools in the investigation of building failures,
these computer models are increasingly used to
make recommendations for building design in
various climates.
WUFI modeling is a commonly used
research tool in the building industry.15–18 WUFI
is an acronym for the German phrase Wärme
Und Feuchte Instationär, which means “heat
and moisture transiency.” The WUFI model is
based on a state-of-the-art understanding of
the physics regarding sorption and suction
isotherms, vapor diffusion, liquid transport,
and phase changes. The model is well
documented and has been validated by many
comparisons between calculated and field
performance data.
Hygrothermal modeling is used to verify
that the proposed energy efficiency retrofit
measures do not create a durability issue. The use
of transient hygrothermal models for moisture
control is well established in the building
industry in its codes, standards, and building
insulation design principles. Building enclosures
are designed to naturally shed liquid water and
attempt to minimize its entry into the building
structure. Building enclosures should also be
constructed to facilitate vapor transport so that
moisture does not accumulate within the building
enclosure and lead to moisture accumulation and
its subsequent failure mechanisms.
Hygrothermal simulations were carried
out using WUFI Pro (version 6.4). Two types
of hygrothermal modeling were undertaken
for this project. First, the model outputs were
compared with the field measurements to verify
that the models were correctly capturing all
the transport phenomena occurring in the field
experiments. Once the model was validated,
it was employed to generalize the findings for
other climate zones.
In instances where certain materials used
in the wall assembly constructions were not
available in the model’s material property
database, the thermal conductivity and water
vapor permeance were measured in accordance
with, respectively, ASTM C518, Standard Test
Method for Steady-State Thermal Transmission
Properties by Means of the Heat Flow Meter
Apparatus,19 and ASTM E96, Standard Test
Methods for Water Vapor Transmission of
Materials.20 The material properties were
compared to those in the model’s materials
database, and modifications were made
accordingly. In some cases, there were no
material properties, so a new material property
entry was created.
Figure 9. Comparison of measured relative humidity and temperature with calculated values using WUFI Pro (version 6.4) for Wall A (Phase
1). The simulated results are represented by pos_#, where # represents the probe position for temperature and relative humidity in the wall
assembly. The measured temperature and relative humidity are represented by TC_# and RH_#, respectively, where # represents the probe