Highly Insulated Wall Systems with Exterior Insulation of Polyisocyanurate under Different Facer Materials: Material Characterization and Long-Term Hygrothermal Performance Assessment

The application of exterior insulation in both new construction and retrofits is a common practice to enhance the energy efficiency of buildings. In addition to increased thermal performance, the rigid insulation can serve to keep the sheathing board warm and serve as a water-resistive barrier to keep moisture-related problems due to condensation and wind-driven rain. Polyisocyanurate (PIR) rigid boards have a higher thermal resistance in comparison to other commonly used exterior insulation boards. However, because of its perceived lower permeance, its use as exterior insulation is not very common. In this study, the hygrothermal property of PIR boards with different facer types and thicknesses is characterized. The material data obtained through experimental test and extrapolation is used in a long term hygrothermal performance assessment of a wood frame wall with PIR boards as exterior insulation. Results show that PIR with no facer has the smallest accumulated moisture on the sheathing board in comparison to other insulation boards. Walls with a bigger thickness of exterior insulation perform better when no vapor barrier is used. The PIR exterior insulation supports the moisture control strategy well in colder climates in perfect wall scenarios, where there is no air leakage and moisture intrusion. In cases where there is trapped moisture, the sheathing board has a higher moisture content with PIR boards with both aluminum or fiberglass type facers. An innovative facer material development for PIR boards can help efforts targeting improved energy-efficient and durable wall systems.


Introduction
The ever-increasing demand from building codes for improved energy efficiency and society's increasing awareness of environmental sustainability is driving the building construction and manufacturing industries to develop innovative solutions for durable, high-performance buildings. It is

Hygrothermal Property Characterization of PIR Foam with Different Facers
To study the long-term performance of hygrothermal simulation performance of PIR as an exterior insulation under different climates, a hygrothermal material property characterization of three types of PIR products is conducted: fiberglass-faced PIR, PIR with aluminum facer and an unfaced PIR insulation. A series of laboratory measurements are carried out to characterize the hygrothermal properties of a rigid PIR insulation board with different facing materials. The hygrothermal properties measured are density, thermal conductivity, specific heat capacity, sorption isotherm, water vapor permeability, water absorption coefficient, and porosity. Measurements are conducted in accordance with the ASTM Standards [25][26][27][28]. The measurement procedures, standards used and material characterization results are shown in the section below. The PIR hygrothermal property characterization under different facer materials for PIR of thickness 12.7 mm (0.5 in.), 25.4 mm (1 in.) and 38.1 mm (1.5 in.) are measured through a laboratory test. The obtained properties are extrapolated for other thickness sizes. The measured hygrothermal properties of PIR core insulation, PIR with fiberglass facer, and PIR with foil-faced PIR insulation are presented below.
The density is determined from physical dimensions and oven-dry mass measurements of six specimens for all three PIR samples, as shown in Table 1. The thermal conductivity of the PIR core board and with foil and fiberglass facers is carried out according to ASTM Standard C518: "Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus "using four -30 cm × 30 cm specimens (average thickness 24.86 mm). The measurements are done at a temperature difference of 20 • C (68 • F) and a mean temperature of 24 • C (75.2 • F). Table 2 shows the average thermal conductivity and resistance values of the four PIR board samples for all types of facers. In addition, the maximum and the minimum measured values are also presented in the table.  Sorption Isotherm: PIR boards with fiberglass facer and no facer are used under this study. Because of its low permeability, the aluminum-faced PIR board is not investigated for its hygrothermal property characterization. The equilibrium moisture contents at 50%, 70%, 80%, 90% and 95% relative humidity (RH) conditions and saturation moisture content in 100% RH are determined according to ASTM Standard C1498: "Standard Test Method for Hygroscopic Sorption Isotherms of Building Materials". For each test point, three specimens with dimensions of 100 mm × 100 mm with a thickness of 24 mm are used. The measurements are carried out in controlled climatic chambers that are maintained at a constant temperature of 23 • C and the desired relative humidity set point. A water immersion test is used to determine the capillary saturation moisture contents of the samples. Table 4 shows the measured equilibrium moisture contents of PIR board specimens at different relative humidity. The water vapor permeability of PIR insulation is determined according to ASTM Standard E-96: "Standard Test Methods for Water Vapor Transmission of Materials". Using climatic chambers with wet cups and dry cups methods, the water vapor transmission rates of PIR samples are determined. The climatic chambers are set at 50%, 70% and 90% relative humidity and 23 • C temperature. The dry (0%) and wet (100%) relative humidity conditions in the test caps are provided by calcium chloride (CaCl 2 ) desiccant and distilled water, respectively. For each test, three circular specimens of 11.94 cm in diameter are used as a replica. The average water vapor permeability values of PIR boards at different mean sample relative humidity are shown in Table 5. The water absorption coefficient of the PIR samples is determined according to ASTM Standard C1794: "Standard Test Methods for Determination of the Water Absorption Coefficient by Partial Immersion." Three test specimens, 100 mm × 100 mm with a thickness of 37 mm each, are used for the measurements. The lab conditions are 33.7 ± 0.2 • C temperature and 39.8 ± 3% relative humidity. Water was maintained at 20.6 ± 0.5 • C. The measured water absorption coefficients of the PIR are found to be 0.0004 kg/(m 2 ·s 1/2 ) and 0.0007 kg/(m 2 ·s 1/2 ) for fiberglass facer and no facer PIR boards, respectively.
The porosity of PIR samples are obtained by determining the full saturation weight of the samples following the water immersion test procedure. Based on these measurements of three replica specimens, the measured porosity values of no facer and fiberglass-faced of PIR boards are 31.37 ± 1.34% and 20.51 ± 0.39%, respectively.
Since the hygrothermal property of the PIR with fiberglass is dependent on both the hygrothermal properties of PIR core and the facer material, a hygrothermal property of fiberglass-faced PIR and the PIR core is measured. By subtracting the water vapor resistance of the fiberglass-faced PIR from PIR core, the permeability of the fiberglass facer can be easily calculated. Once the independent permeability values of the facer material and PIR core are found, the overall permeability values of the fiberglass-faced PIR is computed. Figure 1 shows the water vapor transmission rate of fiberglass-faced PIR and PIR core at different thicknesses.
Materials 2020, 13, x FOR PEER REVIEW 5 of 15 the measurements. The lab conditions are 33.7 ± 0.2 °C temperature and 39.8 ± 3% relative humidity.
Water was maintained at 20.6 ± 0.5 °C. The measured water absorption coefficients of the PIR are found to be 0.0004 kg/(m 2 ·s 1/2 ) and 0.0007 kg/(m 2 ·s 1/2 ) for fiberglass facer and no facer PIR boards, respectively. The porosity of PIR samples are obtained by determining the full saturation weight of the samples following the water immersion test procedure. Based on these measurements of three replica specimens, the measured porosity values of no facer and fiberglass-faced of PIR boards are 31.37 ± 1.34% and 20.51 ± 0.39%, respectively.
Since the hygrothermal property of the PIR with fiberglass is dependent on both the hygrothermal properties of PIR core and the facer material, a hygrothermal property of fiberglassfaced PIR and the PIR core is measured. By subtracting the water vapor resistance of the fiberglassfaced PIR from PIR core, the permeability of the fiberglass facer can be easily calculated. Once the independent permeability values of the facer material and PIR core are found, the overall permeability values of the fiberglass-faced PIR is computed. Figure 1 shows the water vapor transmission rate of fiberglass-faced PIR and PIR core at different thicknesses.

Hygrothermal Simulation
To study how the facing parts of rigid PIR exterior insulation affect the hygrothermal performance of a building envelope, two-year WUFI® simulations for a highly insulated rainscreen wall systems are conducted. This section discusses the simulation setup, initial and boundary conditions, simulation assumptions.

simulation Set up
The indoor relative humidity and temperature conditions are set based on the ASHRAE standard 160 P Intermediate model. The input parameters varied to simulate the dynamic response of the wall system in different climates, with different types and thicknesses of exterior insulations, rain infiltration, and vapor barrier. The wall system that is considered for the study comprises the following layers of materials:

Hygrothermal Simulation
To study how the facing parts of rigid PIR exterior insulation affect the hygrothermal performance of a building envelope, two-year WUFI®simulations for a highly insulated rainscreen wall systems are conducted. This section discusses the simulation setup, initial and boundary conditions, simulation assumptions.

Simulation Set up
The indoor relative humidity and temperature conditions are set based on the ASHRAE standard 160 P Intermediate model. The input parameters varied to simulate the dynamic response of the wall system in different climates, with different types and thicknesses of exterior insulations, rain infiltration, and vapor barrier. The wall system that is considered for the study comprises the following layers of materials: • regular Portland Stucco as an exterior cladding; • 19 mm ( 3 4 in.) rainscreen air gap; • different types and thicknesses of exterior insulations; • spun-bonded polyolefin as a sheathing membrane (as the second plane of protection from precipitation and water intrusion); • plywood sheathing board; • fiberglass insulation in 2 × 6 wood frame studs; • 6 mm (1/4 in.) polyethylene sheet as a vapor and air barrier; • gypsum board as an interior finishing layer Table 6 shows the variation in simulation parameters during this modeling work. Three types of facing options (aluminum foil, fiberglass facer, and no facer) and two insulation thicknesses (2 in. and 4 in.) are considered in the study to evaluate the effect of PIR thickness and different facer materials on the overall hygrothermal performance of a wall system. Even though using a PIR product without facers is uncommon, incorporating this parameter shows how an innovative vapor-open facer product can enhance the hygrothermal performance of an envelope. While the material properties of the extruded polystyrene (XPS) were obtained from the WUFI ® database, the properties for the PIR insulation with different facer material properties are measured.

Boundary and Initial Conditions
The indoor conditions of relative humidity and temperature are set using the ASHRAE standard 160 P intermediate model. The external surface is exposed to the weather conditions of Vancouver, BC, to study the PIR performance with different facers and thickness under different moisture infiltration and combined effects of vapor barrier and exterior insulation. Weather data of two North American cities (Winnipeg, MB, Canada and Vancouver, BC, Canada) are used to study how a PIR exterior insulation of different facers performs from a hygrothermal perspective. The initial conditions of 20 • C and 80% RH are used for all wall component members and the simulation ran for two consecutive years. WUFI's weather data for cities of Winnipeg, MB, and Vancouver is used and a cold year data is selected. Figure 2 shows the wall assembly considered in the study. Continuity of vapor/air barrier (polyethylene sheet) is assumed to be maintained in the modeling. Therefore, there is no airflow through the wall system. The PIR boards with fiberglass and aluminum facers hygrothermal properties are lumped together based on the measured values of the core and facer material. The wall system was assumed to be with no deficiency and the layers of materials to be in perfect contact, exhibiting no dimensional physical change with time. The material properties of the wall components other than the PIR boards which are used in the simulation are shown in Table 7.  Table 7.

Hygrothermal Simulation Results and Discussions
In this section, based on the hygrothermal properties characterization of the PIR material, the long-term performance of a wood frame wall with PIR as exterior insulation under different thickness and facer materials for different climate zones is presented and discussed.
This section presents the results of a two-year hygrothermal assessment of four different types of exterior insulations, namely PIR with aluminum foil; PIR with fiberglass facer, PIR with no facer and XPS.
The effects of wind-driven rain, application of vapor barrier, different climates, and the overall performance of wall assemblies under different types and thicknesses of exterior insulations are presented.

Effect of Wind-Driven Rain
The Vancouver weather, a climate zone known for its heavy annual rainfall, is used to analyze how moisture infiltrated into an interior wall system due to wind-driven rain can affect the overall performance of a wood frame wall with exterior insulation.

Hygrothermal Simulation Results and Discussions
In this section, based on the hygrothermal properties characterization of the PIR material, the long-term performance of a wood frame wall with PIR as exterior insulation under different thickness and facer materials for different climate zones is presented and discussed.
This section presents the results of a two-year hygrothermal assessment of four different types of exterior insulations, namely PIR with aluminum foil; PIR with fiberglass facer, PIR with no facer and XPS.
The effects of wind-driven rain, application of vapor barrier, different climates, and the overall performance of wall assemblies under different types and thicknesses of exterior insulations are presented.

Effect of Wind-Driven Rain
The Vancouver weather, a climate zone known for its heavy annual rainfall, is used to analyze how moisture infiltrated into an interior wall system due to wind-driven rain can affect the overall performance of a wood frame wall with exterior insulation.
Rain infiltration percentages of 0%, 0.1%, and 0.5% are assumed to reach the sheathing board (plywood). Figures 3a, 4a and 5a show the dynamic moisture content of plywood in wall systems with two-inch exterior insulation with different rain infiltration percentages. Similarly, Figures 3b, 4b and 5b represent moisture content values of plywood in wall systems with four-inch exterior insulation at 0%, 0.1%, and 0.5% infiltration percentages. In all cases, constant cavity ventilation of 100 ACH is assumed.    In most cases, the performance ranking from the least to the most accumulation of moisture on the plywood during the simulation period was a PIR with no facer, XPS, PIR with fiberglass facer, and PIR with aluminum foil. In cases without rain infiltration, starting from the last quarter of the second year, the wall with PIR exterior insulations without facer outperformed the other walls with different exterior insulation by a bigger margin.
In cases where the rain infiltration was not the dominant moisture source, for all wall systems and exterior insulation types, envelope systems with four-inch insulation performed better than In most cases, the performance ranking from the least to the most accumulation of moisture on the plywood during the simulation period was a PIR with no facer, XPS, PIR with fiberglass facer, and PIR with aluminum foil. In cases without rain infiltration, starting from the last quarter of the second year, the wall with PIR exterior insulations without facer outperformed the other walls with different exterior insulation by a bigger margin.
In cases where the rain infiltration was not the dominant moisture source, for all wall systems and exterior insulation types, envelope systems with four-inch insulation performed better than envelope systems with two-inch insulation. This is because the thicker exterior insulation keeps the plywood warmer, minimizing the potential for condensation.
The moisture content of the plywood remained at an acceptable level for all wall systems when there is no rain infiltration. In simulation cases where 0.1% of the wind-driven rain was added in the plywood, all wall systems, except the wall with PIR with aluminum foil insulation, remained below the critical moisture level of 18%, for both 2 in and 4 in continuous simulation as shown in Figure 4a,b, respectively. This result shows if the moisture infiltration is managed to an acceptable limit most foam insulations with the exception of vapor impermeable foam insulation can be applied.
To simulate an extreme case scenario, a rain infiltration of 0.5% is assumed. As Figure 5a,b show, most of the wall systems were subjected to very high moisture accumulation in the sheathing board. In this case the moisture control performance of the wall systems has failed in both 2-in and 4-in continuous insulations of the all simulated foam boards.

Combined Effects of Vapor Barrier and Exterior Insulation
Many building codes require an application of a vapor barrier to be used in building envelopes to enhance the moisture durability of wall systems. This study examines if the PIR exterior insulation can serve as a vapor barrier and what are the combined effects of vapor barrier and exterior insulation in wall systems. Figure 6a,b show the effect of the vapor barrier on the hygrothermal performance of a wall system under different exterior insulations. Figure 6a shows when the wall includes a two-inch thick exterior insulation, the wall systems with vapor barrier performed better in all four exterior insulation cases. The walls with no vapor barrier and either exterior insulation of PIR with aluminum foil, PIR with fiberglass facing, or XPS registered a high moisture content. From the wall systems with the vapor barrier, the only wall exposed to a beyond-critical moisture content was the wall with PIR with aluminum foil insulation.
Materials 2020, 13, x FOR PEER REVIEW 11 of 15 envelope systems with two-inch insulation. This is because the thicker exterior insulation keeps the plywood warmer, minimizing the potential for condensation. The moisture content of the plywood remained at an acceptable level for all wall systems when there is no rain infiltration. In simulation cases where 0.1% of the wind-driven rain was added in the plywood, all wall systems, except the wall with PIR with aluminum foil insulation, remained below the critical moisture level of 18%, for both 2 in and 4 in continuous simulation as shown in Figure  4a,b, respectively. This result shows if the moisture infiltration is managed to an acceptable limit most foam insulations with the exception of vapor impermeable foam insulation can be applied.
To simulate an extreme case scenario, a rain infiltration of 0.5% is assumed. As Figure 5a,b show, most of the wall systems were subjected to very high moisture accumulation in the sheathing board. In this case the moisture control performance of the wall systems has failed in both 2-in and 4-in continuous insulations of the all simulated foam boards.

Combined Effects of Vapor Barrier and Exterior Insulation
Many building codes require an application of a vapor barrier to be used in building envelopes to enhance the moisture durability of wall systems. This study examines if the PIR exterior insulation can serve as a vapor barrier and what are the combined effects of vapor barrier and exterior insulation in wall systems. Figure 6a,b show the effect of the vapor barrier on the hygrothermal performance of a wall system under different exterior insulations. Figure 6a shows when the wall includes a two-inch thick exterior insulation, the wall systems with vapor barrier performed better in all four exterior insulation cases. The walls with no vapor barrier and either exterior insulation of PIR with aluminum foil, PIR with fiberglass facing, or XPS registered a high moisture content. From the wall systems with the vapor barrier, the only wall exposed to a beyond-critical moisture content was the wall with PIR with aluminum foil insulation.
(a)  Figure 6b shows the insulation thickness increased from two inches to four inches for all four types of exterior insulations. In all types of exterior insulations, the wall systems without vapor barrier perform better than walls with a vapor barrier. This due to the thick exterior insulation facilitating a warm condition for the plywood, discouraging condensation. The lack of a vapor barrier provides a route for moisture from rain infiltration to dry to the inside. The plywood moisture content stays below 18% in all wall systems without a vapor barrier.

Hygrothermal Performance of Exterior Insulations under Different Climates
This study looked at the hygrothermal performance of the exterior insulation under the different climates of the cities of Winnipeg, MB and Vancouver, BC. These locations represent wet-coastal and cold-dry climates, respectively. This simulation assumed 0.1% rain infiltration reaching the plywood surface and constant air cavity ventilation of 100 ACH. Figure 7a,b show the moisture content of plywood for the different types of exterior insulations of two-inch and four-inch thicknesses, respectively. In all four exterior insulation cases, the plywood moisture content is lower in Winnipeg cases than those in Vancouver. This is due to Vancouver's higher annual rainfall. In both two-inch and four-inch thicknesses of PIR insulation with aluminum foil, the moisture content of the plywood exceeded 18%. As shown in Figure 7a, the maximum moisture content of the plywood in wall systems with two-inch fiberglass-faced PIR for Winnipeg and Vancouver was 17.78% and 19.71%, respectively. However, in the case of four-inch-thick fiberglass-faced PIR, the maximum moisture content of the plywood for Winnipeg and Vancouver cases was 16.70% and 18.47%, respectively, as shown in Figure 7b. The moisture content of the plywood in all wall systems with no-facer PIR insulation remained below 15% throughout the simulation period.  Figure 6b shows the insulation thickness increased from two inches to four inches for all four types of exterior insulations. In all types of exterior insulations, the wall systems without vapor barrier perform better than walls with a vapor barrier. This due to the thick exterior insulation facilitating a warm condition for the plywood, discouraging condensation. The lack of a vapor barrier provides a route for moisture from rain infiltration to dry to the inside. The plywood moisture content stays below 18% in all wall systems without a vapor barrier.

Hygrothermal Performance of Exterior Insulations under Different Climates
This study looked at the hygrothermal performance of the exterior insulation under the different climates of the cities of Winnipeg, MB and Vancouver, BC. These locations represent wet-coastal and cold-dry climates, respectively. This simulation assumed 0.1% rain infiltration reaching the plywood surface and constant air cavity ventilation of 100 ACH. Figure 7a,b show the moisture content of plywood for the different types of exterior insulations of two-inch and four-inch thicknesses, respectively. In all four exterior insulation cases, the plywood moisture content is lower in Winnipeg cases than those in Vancouver. This is due to Vancouver's higher annual rainfall. In both two-inch and four-inch thicknesses of PIR insulation with aluminum foil, the moisture content of the plywood exceeded 18%. As shown in Figure 7a, the maximum moisture content of the plywood in wall systems with two-inch fiberglass-faced PIR for Winnipeg and Vancouver was 17.78% and 19.71%, respectively. However, in the case of four-inch-thick fiberglass-faced PIR, the maximum moisture content of the plywood for Winnipeg and Vancouver cases was 16.70% and 18.47%, respectively, as shown in Figure 7b. The moisture content of the plywood in all wall systems with no-facer PIR insulation remained below 15% throughout the simulation period.

Conclusions
This study characterized the thermal and hygrothermal property of a PIR insulation with different facers. Based on the measured material property data, the long-term hygrothermal performance assessment of PIR insulation as rigid exterior insulation was examined. The experimental measurements show that most properties (such as density, water permeability and sorption isotherm) vary significantly as the thickness increases due to PIR being a composite of facer

Conclusions
This study characterized the thermal and hygrothermal property of a PIR insulation with different facers. Based on the measured material property data, the long-term hygrothermal performance assessment of PIR insulation as rigid exterior insulation was examined. The experimental measurements show that most properties (such as density, water permeability and sorption isotherm) vary significantly as the thickness increases due to PIR being a composite of facer and PIR core. The material data presented here can be used in future modeling works to accurately simulate the hygrothermal property of PIR board with different thicknesses and facer materials.
In addition, this study examined specific application parameters such as thickness and facer types of the insulation core, simulations with varied rain infiltration rates, vapor barrier applications, and climates. Results show that thicker insulation provided a better moisture control strategy because it helped the sheathing board to stay warm. Facer materials used in the PIR insulation significantly affected the hygrothermal performance of wall systems. The hygrothermal performance of the PIR board PIR with unfaced insulation outperformed the XPS and the PIR Boards with aluminum and fiberglass facers. In light of its superior thermal performance in comparison to that of most foam insulation boards, PIR insulation with no facer or vapor-open facer material could contribute significantly to the current demands of high-performance, durable building construction practices.