Continuous Insulation: Wall Assembly Case Study Comparisons

Continuous Insulation:
Wall Assembly Case Study Comparisons
G. David Schoenhard, RRO, AIA, BECxA, CSI, CDT, LEED AP
DSS-Philly | Narberth, PA
dschoenhard@dssphilly.com
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ABSTRACT
Continuous insulation presents challenges with regard to the overall wall assembly’s design, performance, and
construction. This program at the advanced level will evaluate the comparative differences in eight commercial steelframed
wall assembly designs standardized to an NFPA 285-compliant assembly under ICC 2018 in Climate Zone
5 (due to the requirement for an interior vapor retarder). The presentation will begin with an overview of the eight
designs’ components and each assembly’s benefits and challenges. The presenter will explains the rationale for the
choice and location of the continuous and total insulation, sheathing (if any), and weather and vapor control layers,
and highlight concerns regarding condensation management and thermal breaks. The eight assemblies will then be
compared for their NFPA 285 compliance and evaluated through WUFI hygrothermal modeling over four climate
seasons. The models will also evaluated for their structural cladding load and wind-load resistances, and, finally, they
will be comparatively ranked by cost of construction and speed of production. Rather than focusing on the benefits
of one construction material or assembly, this program comparatively reviews eight distinctive entire wall assemblies.
The presentation will use three-dimensional modeled imagery to help illustrate the designs’ integrated assemblies.
G. David Schoenhard, RRO, AIA, BECxA, CSI, CDT, LEED AP
DSS-Philly | Narberth, PA
G. David Schoenhard, RRO, AIA, BECxA, CSI, CDT, LEED AP, has over 40 years
of experience in the design, detailing, and construction of institutional, commercial,
research, and multifamily residential architecture. As the principal and manager of DSSPhilly,
an architectural building enclosure consulting firm in the Philadelphia region,
he brings his decades of experience and discipline to design, implement, and remediate
building enclosures on behalf of other architects, owners, and contractors. He also
actively researches comparative building enclosure assemblies and is an AIA Education
Provider. He is certified as a BECxA, RRO, CSI, LEED AP, and CIT.
SPEAKER
While many professional presentations tend to focus on one construction material or one building enclosure strategy in isolation, this leaves vast gaps when trying to understand and compare entire wall assemblies. The everyday challenge of integrating multiple wall materials while meeting building and energy codes, climatic restraints, and construction budgets can become daunting and overwhelming. The result, unfortunately, is for the designer to rely on office standards or to follow precedents set by previous projects. “This is the way we have done it, so that is how we will do it.” For better or for worse, and unfortunately often the latter, this results in constructed walls that age prematurely, fail progressively, or perform less efficiently than expected.
EIGHT WALL ASSEMBLIES
MEET A COMMON BENCHMARK
This paper presents and evaluates eight substantially different commercial wall assemblies by describing and comparing their components, conducting hygrothermal analyses, assessing their ability to support cladding loading, and ranking them by cost and speed of production. To create a “level playing field” of comparison, all eight wall assemblies were intentionally noncombustible with steel-stud framing, in accordance with the 2018 International Building Code1 and the 2018 International Energy Conservation Code2 (IECC), and located in Climate Zone 5, which by code requires an interior vapor retarder. They all incorporated code-compliant continuous insulation, total insulation, an air barrier, water-resistant barrier, as well as the aforementioned interior vapor retarder. They were all subjected to 30 psf of wind loading, and, for construction takeoff purposes, each wall assembly was theoretically 12 ft high by 300 ft long. Finally, each wall was compliant with the vertical and lateral fire propagation requirements of NFPA 285,3 which requires compliance of the entire wall assembly, not just one compliant material (Fig. 1). Therefore, for this comparative analysis, each wall assembly included an exterior ¼ in. thick fiber-cement wall panel, which is considered a lightweight, noncombustible cladding. Finally, the interior of each assembly was completed with ⅝ in. thick fire-rated gypsum drywall. As an additional parameter, the vertical cladding support tracks were intentionally decoupled from the locations of the vertical studs, thus allowing the cladding to have left-right layout adjustability.
CONTINUOUS INSULATION
SETS THE THEME
The requirement for continuous insulation is driven by the energy code because of its nearly 100% efficiency, whereas batt insulation located between the studs is barely 50% effective. Using the IECC prescriptive R-value method in Climate Zone 5 requires a minimum of R-7.5ci of continuous insulation and R-13 of between-stud batt insulation. As a second compliance route, the IECC U-factor method requires an assemblage of values. The sum of each of the individual R-values for each layer and air films, along with the tabulated effective R-value for the between-stud batt insulation (if any), results in a minimum requirement of U-0.064, which as a reciprocal equates back to an R-value of R-15.625. Both methods are used within the eight wall assemblies to ensure their minimum code compliance and their options for additional enhanced higher performance.
Because continuous insulation is so central to this comparative study, it is important to understand its current definition. Section C202 of the 2018 IECC states that continuous insulation is “… continuous across all structural members without thermal bridges other than fasteners and service openings.” It also states that it may be located on either the exterior or interior, or be integral to the assembly. This study of eight wall assemblies explores continuous insulation in all three locations (Fig. 2).
While continuous insulation is required and impacts the composition and performance of the overall wall assembly, it is not the single most important component. All the components come together synergistically to form a
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Continuous Insulation:
Wall Assembly Case Study Comparisons
Figure 1. Stock image of NFPA 285 vertical and lateral fire propagation testing.
Figure 2. Overview of the eight wall assemblies and location of continuous and total insulation in relation to the structure.
composite whole, as discussed in the following sections.
WALL ASSEMBLY #1: TYPICAL HORIZONTAL Z GIRT WITH MINERAL WOOL CONTINUOUS AND BETWEEN-STUD BATT INSULATION
Wall assembly #1 (Fig. 3) begins like all of the eight assemblies: with 6 in. steel-stud framing at 16 or 24 in. on center. Gypsum sheathing is located on the exterior, to which a vapor permeable, air- and water-resistant barrier (AWB) with a permeability rating greater than 10 perm is applied. The assembly is Class A-rated with a flame spread (FS) ≤ 25, with smoke developed ≤ 450. The AWB can be self-adhering sheet-applied, fluid-applied, or preapplied to the sheathing panels. This exercise does not consider mechanically fastened, nonadhered sheet membranes. The figures in this paper show AWB preapplied to the sheathing panels with their joints and fasteners capped with a fluid-applied flashing sealant. In wall assembly #1, galvanized Z girts are horizontally applied directly atop the AWB and fastened through the upturned leg and the sheathing to the stud flanges behind. Semirigid, water-resistant-treated mineral wool insulation panels are friction fit between the girts to provide the continuous insulation. As in all eight assemblies, the ¼ in. fiber-cement panels are face-fastened to ¾ in. deep vertical cladding tracks fastened to the horizontal girts, which also provides necessary rainscreen back ventilation. This assembly uses FS 25, R-15 fiberglass or mineral wool batt insulation with a code-required Class II vapor-retardant foil facing that is friction fit between the steel studs. Finally, ⅝ in. type-X gypsum drywall is applied to the interior, which in this exercise intentionally used mold-resistant panels.
Why is this assembly so often used as the “office standard”? It uses common materials that are readily available, the wall encloses quickly to become construction weathertight, and this assembly is free of NFPA 285 compliance because it does not contain any plastic foam insulation.
What, however, are its challenges that somehow do not disqualify it from being the “office standard”? Each steel horizontal girt extends through the depth of the continuous insulation, thereby forming a thermal bridge. Even with the batt insulation blanketing its inward side, the cavity-side face of the gypsum sheathing at each girt is close to the same exterior cold winter temperature, thus risking localized condensation buildup. In winter, moisture vapor and liquid condensation within the stud cavity can become trapped by the interior vapor retarder and remain there until the wall’s exterior warms up, allowing the wall to dry by revaporizing outward. Where does the moisture come from if there is an interior vapor retarder? It comes from leaks in the vapor retarder and from occasional seasonal inward vapor drive from the exterior, such as when the rained-on wet wall is warmed by the sun and the moisture vapor drives inward toward the cool, dry interior, where it becomes trapped by the vapor retarder within the stud cavity. This inward vapor drive is also common during the warmer humid seasons and results in the exterior continuous insulation zone being actively wet with condensation and rainscreen back-drainage. The WUFI hygrothermic models, which will be discussed later, illustrate these condensation risks and moisture buildup issues. While new mineral wool is excellent at draining moisture, each horizontal girt acts as a stop or resting surface that encourages absorption into the lower fibers of the insulation. Atmospheric moisture is also dust-laden, so over time the mineral wool’s filtering adds dust to the felted voids. When moisture is present, it reduces the insulation’s efficiency and hastens the eventual corrosion of the galvanized Z girt. This assembly is not compatible with the superinsulation approach of Passive House design. Doing so with batt insulation is problematic because it creates a receptor for moisture and the differential temperature is insufficient for the moisture to be driven back out of the wall. Moisture therefore tends to accumulate, thus risking mold formation on organic paper surfaces.
With all these challenges, what is so appealing about this typical zee assembly? It is perceived as being cheaper, but this report’s cost and production assessment will show that this is unfounded. This assembly requires multiple construction steps on the exterior side of the wall’s sheathing, and the risk of damage to the AWB and of resultant water leakage is quite high. Moisture can also accumulate between the back leg of the Z girt and the AWB. The self-sealing tests of the AWB are laboratory tests, not actual field tests; accumulated water can accelerate the sheathing’s deterioration and be sucked in under differential air pressure during high windstorm events.
WALL ASSEMBLY #2: ENHANCED ZEE WITH BACK-SPRAYED POLYURETHANE FOAM
Having presented most of the wall components in the previous section, this section focuses on the unique qualities of wall assembly #2, which are intended to improve or enhance the deficiencies of assembly #1 (Fig. 4). Instead of galvanized Z girts, this wall uses fiberglass Z girts, which removes the corrosion and thermal bridging issues of assembly #1. Instead of between-stud batt insulation, this assembly uses at least 2 in. thick
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Figure 3. Wall assembly #1: Typical horizontal galvanized Z girt, continuous exterior mineral wool insulation, and between-stud foil-faced batt insulation as interior vapor retarder.
Figure 4. Wall assembly #2: Enhanced fiberglass Z girt, continuous exterior mineral wool insulation, and between-studs urethane back-spray as dual-face vapor retarder.
closed-cell, back-sprayed polyurethane foam (ccSPF). The rigid foam acts as a Class II vapor retarder from both the inward and outward vapor-drive directions, and it functions as an additional air barrier. The foam insulation outgasses before building occupancy, and the newest generation of blowing agents is more environmentally friendly.4 The back-insulation can also be superinsulated up to 3⅝ in. thick for a total wall R-value of R-32.28 (using the R-value method). The reason for the 3⅝ in. SPF and not a full 6 in. stud depth is to comply with NFPA 285 certification requirements and to allow cavity space for back-wiring.
This assembly approach has some benefits, but the cost analysis given in this paper shows that the benefits come at a cost. The exterior-side mineral wool is still prone to active-seasonal condensation and rainscreen back-drainage. The back leg of the Z girt can be protected from water accumulation with an additional cap bead of sealant.
WALL ASSEMBLY #3: PHENOLIC 100% OUTBOARD AS APPLIED RIGID INSULATION
Instead of between-stud insulation of any form or mineral wool exterior insulation and girts, assembly #3 places 100% of the insulation outboard of the AWB-clad sheathing (Fig. 5). It uses rigid phenolic foam insulation panels instead of the more common rigid extruded polystyrene (XPS) to be NFPA 285-compliant while still allowing lightweight exterior wall cladding panels (Fig. 1). The insulation is non-moisture-absorbing, and, with a small bead of spray foam around the perimeter edges, the insulation becomes almost 100% effective except for the through-fasteners of the horizontally oriented hat-channel furring. The insulation panels are intentionally shimmed away from the AWB by ⅛ in. to allow for downward flowing condensation and back-drainage, while not being large enough to encourage upward air drafts. The horizontal hat-channel furring is factory-perforated to allow through-drainage as well as upward back-venting. An optional additional 3⅝ in. or R-24.28 of SPF may be added to the stud cavity to superinsulate.
There are several challenges to remember with assembly #3. As stated, the rigid foam boards need to be phenolic and not XPS to be NFPA 285-compliant while using lightweight exterior claddings. It is important to maintain the ⅛ in. back-drainage cavity with kickout flashings so as not to rely on the outward face of the rigid insulation. The wall encloses quickly for temporary construction purposes, but it still requires multiple construction steps on the outward side of the wall, which affects the wall assembly’s overall cost and production speed.
WALL ASSEMBLY #4: ISOCYANURATE ALL-IN-ONE, 100% OUTBOARD AS DURABLE INSULATED SHEATHING
Like assembly #3 , assembly #4 places 100% of the rigid insulation outboard of the structure, except that the insulation panel is durable enough to be an all-in-one panel with no gypsum sheathing (Fig. 6). This means that it becomes the sheathing, air barrier, water-resistant barrier, and vapor-retardant barrier from both the inward and outward directions, and it provides 100% of the insulation as continuous insulation. Only the low thermal conductivity stainless-steel fasteners of the panels and hat channels penetrate the panels into the backup studs. As an additional option, it is possible to apply closed-cell hand-sprayed foam to the projecting screw shanks within the stud cavity to remove that condensation risk.
Rigid isocyanurate (ISO) insulation panels are factory-clad with durable prefinished aluminum facers front and back, which function as Class I vapor retarders from both inward and outward vapor directions. In addition to the panel’s horizontal shiplap joints and the optional use of spray foam in all the joints, the exterior side’s joints and fasteners are capped with fluid-applied flashing sealants. In this paper, exterior tape is discouraged, and the higher-performance silicone version or the hybrid STPE versions of fluid-applied flashings is preferred to the acrylic or urethane versions. The exterior facer is more durable than the interior’s; it is intended to be exposed to the back-drainage cavity and becomes the direct surface to which the horizontal hat-channel furring and claddings are applied. Once the panel is installed and cap-sealed and the horizontal furring applied, the exterior side can be successfully quality control water-tested as no further penetrations will occur to the panel.
While this sounds simple and direct, there are several challenges to be aware of. The stud spacing needs to be 16 in. on center, and without the gypsum sheathing, the assembly is best suited for lower-rise construction without fire-rated assemblies. It is important to use the punched horizontal furring to allow through-drainage and upward ventilation. It is also important to take extra precautions during construction to protect the exterior panel’s AWB facing and at corners. While the facing is considered durable, it is still vulnerable to construction damage. This paper recommends treated wood blocking at door and window openings clad in flashing tape, and as an additional option, application of SPF to just the steel wall framing webs at perimeters and wall
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Figure 5. Wall assembly #3: Rigid phenolic continuous and total insulation, located 100% outboard on air- and water-resistant barrier, vapor retarder, and back-drainage.
Figure 6. Wall assembly #4: Rigid and durable foil-clad isocyanurate board sheathing as 100% continuous and total insulation, with integral air- and water-resistant barrier and dual-faced vapor retarder.
openings to improve the thermal performance at these vulnerable locations.
WALL ASSEMBLY #5: ISO
100% CONTINUOUS INSULATION WITH ADDED AWB FACING
Assembly #5 is identical to #4, except that instead of using a more durably clad ISO panel, this assembly uses a less expensive foil-clad panel that requires a field-applied separate AWB facing (Fig. 7). While the panel is cheaper, the installation of the AWB requires an extra material and step. However, the AWB provides an additional opportunity to ensure that the wall assembly is weathertight.
WALL ASSEMBLY #6: BONDED ALL-IN-ONE, 100% OUTBOARD, STRUCTURAL INSULATED SHEATHING
Instead of the rigid ISO of assemblies #4 and #5, assembly #6 uses high-density urethane rigid insulation factory-bonded to a magnesium oxide exterior sheathing panel with factory-applied AWB precoating (Fig. 8). This is referred to as structural insulated sheathing (SIS). Once panels are fastened back to the studs with stainless-steel fasteners and the joints are spray-foam filled, the wall is temporarily construction weathertight. Because of the factory-applied AWB coating, once the joints and fastener heads are capped with fluid-applied flashing sealant, the wall is permanently durable. As an option, spot-spraying SPF on the inward screw shanks omits the small condensation risk at the fasteners. The magnesium oxide exterior sheathing panel is structurally considered a fastener base with fastener pullout and shear-testing. Additional horizontal cladding girts are therefore not necessary and the exterior cladding with its vertical support and ventilation tracks may be applied directly to the fastener-base SIS panel face. Quality control water testing can be conducted efficiently once the vertical cladding tracks are applied and before the cladding. The combination of high-density urethane and exterior magnesium oxide provides a panel that is both NFPA-certified as well as being fire-rated. From a construction standpoint, it assembles and closes in very quickly, as well as being well suited to panelization. Fire-rated and midrise projects are well suited to this type of assembly.
With all these advantages, what are this assembly’s challenges? It is manufactured by one company, and thus it is considered sole-sourced. Also, the material cost of the panels is at a premium, although the construction time savings is an offsetting benefit. Extra care needs to be taken to not over-torque the fasteners and strip the panel threads. It requires some effort to research, understand, locate distribution, and document this assembly outside of the more familiar “office standard.”
WALL ASSEMBLY #7: CLIP AND RAIL, WITH OUTWARD MINERAL WOOL AND INWARD CCSPF
Semirigid mineral wool located as exterior-side continuous insulation is integral to assembly #7 (Fig. 9). The use of thermally broken structural brackets fastened through to the studs atop the AWB and sheathing, along with infinitely adjustable structural cladding rails provides the ideal venue for continuous mineral wool insulation. The cladding system’s adjustability can accommodate designs with intricate wall profiles and changes in cladding materials. Consequently, this assembly is common in high-design projects. Back-sprayed SPF completes the insulation and provides the code-required, outward-driving Class II vapor retarder. Assembly #7 is NFPA 285-compliant and may be as fire-rated as necessary. Superinsulating is also possible with additional back-sprayed ccSPF.
What are some of this assembly’s challenges? As shown by the WUFI modeling, the outward framing system and mineral wool are subjected to active multiseason inward vapor-driving condensation as well as back-cavity rainscreen drainage. The back-cavity spatial voids need to be limited in size so that they do not inadvertently become flue stacks. The construction wall encloses quickly, yet there are still multiple exterior construction steps that each requires high levels of skill and quality control supervision, and each step is prone to wind damage until fully completed. Assembly #7 is a finicky assembly, yet it provides the greatest assembly flexibility along with high
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Figure 7. Wall assembly #5: Rigid and foil-clad iso board sheathing as 100% continuous and total insulation, with integral dual-faced vapor retarder and separate durable air- and water-resistant barrier.
Figure 8. Wall assembly #6: Structural insulated sheathing with 100% outward rigid urethane insulation, fastener-base sheathing with precoated air- and water-resistant barrier as dual-faced vapor retarder.
Figure 9. Wall assembly #7: Clip-and-rail cladding support fastened outboard of air- and water-resistant barrier with continuous exterior mineral wool insulation and between-stud urethane back-spray insulation as dual-faced vapor retarder.
levels of fire safety. However, according to the
WUFI model, the outward zone is actively wet
with condensation and rainscreen back-drainage,
which will degrade the assembly’s performance
over time, and the regular exposure to
freeze-thaw cycles weakens the integrity of the
water-tight connections.
WALL ASSEMBLY #8:
BACK-LATTICE CONTINUOUS
INSULATION, 100% INWARD
Assembly #8 takes the vertical stud framing
concept and turns it on its side (Fig. 10).
While the vertical studs may be stretched out
to 24 in. centers, there is a crisscross backlattice
of 1 in. Z- or J-shaped horizontal backfurring
as horizontal furring located at 16 in.
centers. The horizontal furring is separated
at each stud intersection with a ½ in. thick,
R-1.55, high-density, urethane thermal isolation
pad. The exterior gypsum sheathing is
applied horizontally instead of vertically to the
horizontal back-furring. None of the structural
fastener levels carries through completely; thus
there is no through-fastener thermal bridging.
Combining the 1 in. Z- or J-shaped backfurring
and the ½ in. thermal isolation pad
results in a 1½ in. gap between the vertical stud
flange to the sheathing. The entire backside of
the sheathing is then back-sprayed with 3⅝ in.
thick ccSPF, taking care to spray behind the
stud flanges, into the belly and over-capping
the back-furring, into the outward flanges of
the studs, and onto the webs of any perimeter
and wall-opening framing. The ccSPF at a
minimum 1½ in. provides an additional
air barrier, as well as the Class II vapor
retarder, from both outward and inward
vapor-drive directions. The entire 3⅝
in. of ccSPF meets the total insulation
value of the prescriptive R-value method,
and it has been proven to more than
comply with the energy code’s U-factor
requirement by 3D Therm modeling. The
required 1½ in. of continuous SPF insulation
functions as an air and vapor retarder
outboard of the stud flange, while also
continuously inboard of the back-furring.
The R-1.55 of the thermal isolation pads
equates in the energy code to not being a
thermal break and thus a thermal isolator.
This wall assembly is unique and has
achieved NFPA 285 engineering evaluation
compliance and has completed
ASTM E3315 and ASTM E23576 testing,
and ASTM E119 one-hour fire testing..
With the complete 3⅝ in. of backsprayed
ccSPF, which functions as the
required vapor retarder and continuous insulation,
this wall assembly performed extremely
well in WUFI hygrothermic modeling, which
will be reviewed in the following section. The
wall assembly is resistant to both inward and
outward vapor drive, and the seasonal rainscreen
back-drainage and exterior-side condensation
is free to collect and drain without
wetting any outward insulation.
There are fewer construction
components on the
exterior side of the assembly.
Once the AWB coated sheathing
panels are capped with
fluid-applied flashing sealants,
the construction is complete
and a permanent weathertight
enclosure is ensured. The only
remaining outward activity is
the installation of the finished
cladding which can occur
when the weather is favorable.
Because the horizontal furring
is located behind the sheathing,
the exterior vertical cladding
may be applied anywhere and
fastened directly through the
AWB-faced sheathing, where
quality control water testing
can be conducted efficiently.
Because the ccSPF is entirely on
the interior side, it is weatherand
season-independent. This
assembly is also well suited to
prefabricated panelization.
What are this assembly’s challenges? It
requires a designer, a detailer, and the contractor’s
understanding of the back-lattice concept
because it effectively places the wall on its side
and uses a complete ccSPF back-spray. The
high-density urethane thermal isolation pads
and the specialized Z- and J-shaped backfurring
for the framing intersections need to
be specially ordered, and special detailing is
needed at wall openings and panelized perimeters.
Typically, a simple galvanized sheet
metal angle frames the vertical edges, and flexflash-
protected, fire-retardant-treated, 2 × 6
in. wood blocking rings the window and door
openings to ensure the fire assembly and the
thermal efficiency at the openings.
HYGROTHERMIC MODELING
OVER FOUR CLIMATE SEASONS
FOR CLIMATE ZONE 5
For the purposes of this hypothetical exercise,
Pittsburg, Pa., located in Climate Zone
5, was chosen as the location for the WUFI
models because it contains a wealth of climate
data. Again, this study intentionally chose
Climate Zone 5 because of the code’s requirement
for an interior vapor retarder. Each of
the eight assemblies was comparatively WUFI
modeled over a data period of three years with
examples chosen to illustrate each of the four
climate seasons. Some introductory explana-
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Figure 10. Wall assembly #8: Back-lattice of
crisscross framing with thermal isolation pad at
intersections, exterior air- and water-resistant
barrier, and back-spray of closed-cell urethane for
100% continuous and total insulation, and as dualfaced
vapor retarder.
While continuous
insulation is required and
impacts the composition
and performance of the
overall wall assembly, it
is not the single most
important component.
All the components
come together
synergistically to form a
composite whole.
tions are helpful when looking at the WUFI data graphics. Where the pink and purple areas touch, this corresponds to the dew point or risk for condensation formation on surfaces dense enough to support the change of state from a vapor to a liquid. The accompanying green and blue chart shows the moisture buildup, if any. The direction of the seasonal outward or inward vapor drive is indicated by the direction of the downward sloping pink line.
Wall assembly #1, Typical Zee, shows two high-risk areas (Fig. 11). In March, there is significant condensation buildup on the cladding hardware, leading to nighttime ice formation on the face and back and within the continuous mineral wool insulation. In July, another risk area is located deep within the wall at the cavity side of the vapor retarder layer: the summertime vapor drive is so intense that moisture nearly condenses at this location or on the nearest dense surface, such as the face of the adjacent studs or sheathing.
Wall assembly #2, Enhanced Zee, shows one location of moisture buildup in July at the outer face of the AWB-clad sheathing and behind the continuous mineral wool insulation (Fig. 12). This area must drain freely and be flashed efficiently, but each horizontal Z girt is a shelf that collects moisture before it continues its downward path.
Wall assemblies #3, #4, and #5, with 100% continuous insulation outboard of the structure, shows no risks except that moisture will form on the outward side of the insulation panel and cladding hardware as long as the moisture is free to flow downward and outward, though freezing and thawing cycles can weaken the fastener to panel water-tighness over time (Fig. 13). The inward and outward vapor drive over four seasons is devoid of condensation risks because of the dual-sided,
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Figure 11. WUFI for wall assembly #1: Note active condensation in outward cladding support and mineral wool and potential for captured moisture at the interior vapor retarder during summer.
Figure 13. WUFI for wall assemblies #3, #4, and #5: These assemblies perform extremely well in all four seasons.
Figure 12. WUFI for wall assembly #2: Note active condensation at the exterior cladding support and mineral wool.
vapor-retardant-clad, rigid insulation panels, which prevents the passage and collection of moisture-laden air.
Wall assembly #6, Bonded All-In-One (SIS), shows a small amount of moisture buildup in March within the magnesium oxide sheathing panel as vapor is blocked during its inward drive by the rigid urethane foam; however, the quantity of moisture buildup is insignificant (Fig. 14). It is important to ensure that the AWB on the panel’s face is vapor permeable to allow outward revaporization.
Wall assembly #7, Clip and Rail, shows significant moisture buildup and activity in March at the outer face of the AWB-clad sheathing, which is behind the continuous mineral wool insulation (Fig. 15). The insulation becomes temporarily wet but then dries to become effective again. It is unclear how the buildup of atmospheric dust in rainwater affects the long-term performance of mineral wool.
Wall assembly #8, Back-Lattice, similar to assembly #6, performs splendidly. It shows a small amount of moisture buildup in March within the gypsum sheathing panel as vapor is blocked during its inward drive by the rigid spray-urethane foam, but the quantity of moisture is insignificant (Fig. 16). To test the moisture performance of this wall assembly further, assembly #8 was also subjected to WUFI modeling for Houston, Tex., (Climate Zone 2, hot/humid) and for Green Bay, Wisc., (Climate Zone 6, cold/humid) (Fig. 17).
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Figure 14. WUFI for wall assembly #6: This assembly performs extremely well in all four seasons with insignificant moisture in the exterior sheathing.
Figure 15. WUFI for wall assembly #7: Note active condensation in outward cladding support and mineral wool on face of the air- and water-resistant barrier.
Figure 16. WUFI for wall assembly #8: This assembly performs extremely well in all four seasons with insignificant moisture in the exterior sheathing.
HOW DO THE
EIGHT WALL ASSEMBLIES COMPARE WITH SUPPORTING CLADDING LOADINGS
The ability of the eight wall assemblies to support cladding loading varies from a low of 3 psf to a high of 20 psf. The theoretical 12 ft high wall has been engineered for 25 to 30 psf of wind loading (both positive and negative) (Fig. 18). The Z girts in the #1 and #2 assemblies, by their deflection-prone depth and shape, limit the load carrying capacity to 3 psf, which is still sufficient for lightweight fiber cement and aluminum composite material claddings. The back leg of the Z girt tries to pry the fasteners out of the studs. The use of the horizontal hat channel fastened atop the rigid insulation in assemblies #3, #4, and #5 holds steady by compressing against the rigid insulation and supports about 15 psf of cladding loading, which is sufficient for terra-cotta claddings. The #3 phenolic foam assembly has a compressive strength of 15 psf as opposed to 25 psf for rigid ISO, and therefore results in slightly less supporting capacity. The #7 clip-and-rail assembly in its most basic form can support about 10 psf, but with additional fasteners and stronger brackets it can be upgraded to 20 psf. Finally, the #8 back-lattice assembly is capable of 20 psf and even higher with adjustments to the horizontal back-furring gauge and spacing.
TIME IS MONEY; HOW DO
THE ASSEMBLIES RANK BY COST AND PRODUCTION SPEED
With the assistance of a professional construction estimator, the eight wall assemblies for takeoff purposes were compared, with each being 12 ft high by 300 ft long. We first compared and ranked all eight assemblies by cost per square foot to purchase and construct, and found that the four cheapest assemblies, from least to most expensive, are the #4 ISO, the #5 ISO, the #6 Bonded All-in-One, and the “office standard” #1 Typical Zee (Fig. 19).
We then ranked all eight assemblies according to their speed of production or how long it would take to build a 12 × 300 ft wall assembly of each type. We found that the top four assemblies, in order of the fastest to the slowest to build, are the #6 Bonded All-in-One, the #4 ISO, the #8 Back-Lattice, and the #5 ISO (with the separate AWB).
Finally, combining these two rankings gives a ranking of the cheapest and fastest assemblies: the #4 ISO, the #6 Bonded All-in-One, the #5 ISO (with the separate AWB), and the #8 Back-Lattice.
For lower-rise projects, the ISO panels, such as assemblies #4 and #5, are both
158 | Schoenhard IIiiBEC International Convention & Trade Show | September 15-20, 2021
Figure 17. WUFI for wall assembly #8 in Climate Zones 2 and 6: This assembly performs as well as that modeled for Climate Zone 5.
Figure 18. Comparison of cladding support loadings ranging from 3 to 20 psf.
cheaper and faster. However, on higher and more demanding projects, the #6 Bonded All-in-One and the #8 Back-Lattice are the two cheapest and fastest solutions.
Where does this leave the #1 Typical Zee, which is so often used as the “office standard”? While its components may be less expensive, it takes longer to construct. Then how about the #7 Clip and Rail, which is often selected for high-design projects? That assembly is both more expensive and more time-consuming to construct. As a proviso, contractors in other regions will discover different ranking results, so before deciding on one or another assembly, reconfirm your own cost and time rankings.
CONCLUSION
It is hoped that this paper has been found interesting and useful. Full-size mock-up laboratory testing is continuing for three of the wall assemblies: #1 Typical Zee, #2 Enhance Zee, and #8 Back-Lattice (Fig. 20). To date, the ASTM E331,5 ASTM E2357,6 and ASTM E283,7 testing has been conducted with all three assemblies passing the code minimums of < 0.04 cfm/ft2 @ 75(Pa) ≈ 1.6 psf. In addition, for the ASTM E331 testing, assembly #8 excelled up to 30 psf, which ≈ 108 mph wind speed. Assembly #8 has also passed one-hour E119 fire-testing, as well as 3D Therm analysis. The next round of testing will further analyze all three assemblies by subjecting full-size mock-ups in real time using microanalyzed thermal chambers with heat, cold, and moisture to conduct ASTM E14248 and ASTM E22689 thermal testing. This testing will study the resultant effectiveness of the insulation when hot, cold, and wet, and will study the AWB and flashings by comparing the three assemblies including those with SPF, mineral wool, and the #8 Back-Lattice arrangement.
REFERENCES
1. International Code Council (ICC). 2017. 2018 International Building Code. Country Club Hills, IL: ICC.
2. ICC. 2017. 2018 International Energy Conservation Code. Country Club Hills, IL: ICC.
3. NFPA (National Fire Protection Association). Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Wall Assemblies Containing Combustible Components. NFPA 285. Quincy, MA: NFPA
4. BASF. “WALLTITE LWP Series.” spf.basf.com <Accessed August 7, 2021.>
5. ASTM Subcommittee E06.51. 2016. Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference. ASTM E331-00(2016). West Conshohocken, PA: ASTM International.
6. ASTM Subcommittee E06.41. 2018. Standard Test Method for Determining Air Leakage Rate of Air Barrier Assemblies. ASTM E2357. West Conshohocken, PA: ASTM International.
7. ASTM Subcommittee E06.51. 2019. Standard Test Method for Determining Rate of Air Leakage Through Exterior Windows, Skylights, Curtain Walls, and Doors Under Specified Pressure Differences Across the Specimen. ASTM E283/E283M. West Conshohocken, PA: ASTM International.
8. ASTM Subcommittee E06.51. 2016. Standard Test Method for Determining the Rate of Air Leakage Through Exterior Windows, Curtain Walls, and Doors Under Specified Pressure and Temperature Differences Across the Specimen. ASTM E1424-91(2016). West Conshohocken, PA: ASTM International.
9. ASTM Subcommittee E06.51. 2016. Standard Test Method for Water Penetration of Exterior Windows, Skylights, and Doors by Rapid Pulsed
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Figure 19. Comparison of assemblies based on cost of construction, by speed of construction, and by combined cost and speed of construction.
Figure 20. View of performance testing of wall assembly #8.
Air Pressure Difference. ASTM E2268. West Conshohocken, PA: ASTM International.
ADDITIONAL RESOURCES
• “Thermafiber – Technical Bulletin, What happens if Thermafiber gets wet?.” Thermafiber Inc. TF-TB-021103/Rev 032103.
• Deg Priest and Javier Trevino. “Engineering Evaluation, Engineering Extensions based on NFPA 285 Testing, BASF WallTite with ACM Cladding, Project No. 10403J, Revision # as Prepared for BASF Corporation.” Priest & Associates Consulting. Pleasanton, TX. November 30, 2017.
• Deg Priest and Javier Trevino. “Engineering Evaluation, NFPA 285 Assemblies – Back Lattice Wall Assembly, Project No. 10884, Revision 1, as Prepared for DSS-Philly.” Priest & Associates Consulting. Pleasanton, TX. June 24, 2020.
160 | Schoenhard IIiiBEC International Convention & Trade Show | September 15-20, 2021