Design without compromise: Balancing durable and energy-efficient buildings

by Catherine Howlett | November 1, 2012 3:53 pm

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Two critical aspects of sustainable construction are long-term durability and energy efficiency. The latter is a measure of whole building performance, which depends on the envelope as well as the building systems and occupant behavior. This article addresses the enclosure’s contribution to energy efficiency—namely its impact on heating and cooling loads and how to balance this with a need for durability.

Updated energy codes—such as American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings, and International Energy Conservation Code (IECC)—have heightened criteria for minimizing the enclosure’s thermal loads and reducing heat flow through the following mechanisms:

• conduction (through materials), which
  is controlled with thermal insulation (i.e. materials resistant to heat flow);
• convection (through air currents), which is controlled with continuous air barriers (i.e. materials with high resistance to airflow); and
• radiation (through space), which is minimized with low-emissivity (low-e) surfaces (e.g. cool roofs or radiant barriers).

Durability of the building enclosure is especially critical because the expected service life is longer than mechanical, lighting, and water-heating systems. The major factors affecting the durability of multi-component building enclosure assemblies include heat, air, moisture transport, and material compatibility. The first two aspects are regulated through energy codes, but the potential impact and the unintended consequences of energy-efficient measures on moisture management are not well understood. A designer can significantly affect the service life of multi-component assemblies through selection of compatible materials, placement of materials within the building assembly, and design detailing.

This article examines recent energy code changes (excepting fenestration) for the opaque building enclosure, how these changes impact wall assembly design and long-term durability, and how moisture analysis tools can assist in selecting the optimum design option.

Trends in energy codes and wall assembly design
Building energy codes require increasing levels of thermal control for the building enclosure. Recent measures include increased insulation R-value, continuous insulation (ci), and continuous air barrier requirements. These measures reduce the envelope thermal loads and increase energy efficiency. However, the manner in which additional thermal insulation is added to framed wall assemblies is critical to their durability.

Prescriptive thermal insulation requirements for the last three versions of IECC and ASHRAE 90.1 are outlined in Figure 1 for non-residential steel-stud-framed wall construction. (Similar trends are common in residential occupancies, but the requirements are slightly more stringent.)

The main trends include increased overall R-value for thermal insulation as well as the expanding of ci requirements to more climate zones. The green-shaded cells in Figure 1 highlight the zone in which continuous insulation becomes mandatory for each code version. For example, the ci requirement begins in Climate Zone 5 for 2006 IECC, Zone 3 for 2009 IECC, and in all zones for 2012 IECC. ASHRAE 90.1 shows similar trends in ci requirements. In wood-stud construction, continuous insulation for the prescriptive compliance option is required in fewer climate zones.

In addition to increasing the R-value for the walls, ci is very important for reducing the thermal bridging that can significantly decrease the cavity insulations effective R-value. Thermal bridges are regions with high thermal conductance that bypass thermal insulation leading to large thermal losses, and are very significant for steel-stud construction.

Figure 2 shows a few examples of the impact of thermal bridging and the framing factor on the effective R-value of cavity insulation in steel-framed walls. As a result of these losses, ci is often required for prescriptive compliance options. The new requirements led to changes in wall assembly design for framed construction. The basic types of framed wall construction are shown schematically in Figure 3, but other combinations are also often used.

Various moisture analysis tools for durability assessment
Energy code criteria for reduced heat flow across the building enclosure contributes to increased building envelope efficiency. However, reduced heat flow also slows down the wall’s ability to dry, which may increase the risk of moisture-related issues under certain conditions. Building assemblies may start out, or periodically become, wet, but can have an acceptable performance and provide a long, useful service life when allowed to dry. Problems occur when buildings stay wet long enough under adverse conditions for materials to deteriorate.

Moisture analysis is often required to estimate potential durability risks due to changes in wall assembly design, construction practices, and new materials. The tools commonly used for moisture analysis include dewpoint calculation and Wärme und Feuchte Instationär (WUFI) analysis, which is German for ‘heat and moisture thermodynamics’. (WUFI[2] was developed in Germany by Fraunhofer Institute for Building Physics [IBP], in collaboration with Tennessee’s Oakridge National Laboratory [ORNL]. )

While determining dewpoint temperature and location within the building enclosure can provide useful design guidelines, relying solely on this calculation has significant limitations as it is based exclusively on vapor diffusion equations and cannot simulate other, more important moisture sources (e.g. air-transported moisture, incidental rainwater intrusion, and wet construction materials. The analysis also assumes steady-state, equilibrium conditions.

Dewpoint calculation is performed for the coldest temperature of the year while assuming interior and exterior conditions are fixed and associated with that particular point in time. These assumptions have serious limitations since equilibrium conditions are never reached in an open system (i.e. since the building assembly is not in a controlled environment, it is ‘open’ to continuously changing exterior climate parameters).

Unlike the dewpoint analysis that assumes steady-state conditions, WUFI is a dynamic simulation for coupled heat and moisture transport, using hourly climate data and transient parameters. Even though the WUFI model is based on vapor diffusion equations, it can simulate the impact of other moisture sources and design conditions such as ventilated cladding.

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Using moisture analysis to estimate long-term risks
The manner in which additional thermal insulation is added to framed wall assemblies for increased thermal performance is critical when it comes to durability. WUFI modeling can simulate different scenarios that may cause condensation and long-term durability risks for wall assemblies. Brief examples of WUFI analysis for estimating potential for winter or summer condensation, or drying rates following simulated wetting events, are described in this article. The wall assembly design and climate conditions (both interior and exterior) are specified for each example, and were selected as the best demonstration of a particular scenario.

The conclusions described for each example do not automatically extend to other conditions; however, the examples demonstrate how moisture analysis can be used for relative comparison between different design options to help select the lower risk options. John Straube, PhD, is among those who have published detailed studies on the use of WUFI moisture analysis for estimating the long-term durability risks of walls with high R-values. (For more information, see Research Report-0605[4], “Assessing the Durability Impacts of Energy-efficient Enclosure Upgrades Using Hygrothermal Modeling,” by John Straube and Christopher Schumacher. See also Research Report-0903, “Building America Special Research Project: High-R Walls Case Study Analysis[5],” by Straube and Jonathan Smegal. Finally, view Research Report 10-14[6], “High R-walls for the Pacific Northwest—A Hygrothermal Analysis of Various Exterior Wall Systems,” by Smegal and Straube.)

Due to inherent limitations of the WUFI model, simulation results represent relative performance of building assemblies, rather than quantitative predictions of the materials’ moisture content (MC). The trends in moisture accumulation and drying rates for different design options provide relative performance information.

Potential for winter condensation 
Condensation during winter can be caused by moisture vapor diffusion or moisture transported by air leakage. During winter, diffusion or air exfiltration from interior space with higher water vapor concentration to outside spaces with lower concentration, could lead to condensation and increased moisture content within cooler exterior layers, such as exterior sheathing.

To control winter diffusion, a vapor barrier or retarder is installed on the inside of the wall in certain climates. WUFI analysis can compare the diffusion condensation potential for different wall assemblies and climate conditions by estimating the moisture content of the exterior sheathing. The questions most often asked regarding winter condensation is whether to use a vapor retarder (or what kind) to control diffusion wetting.

Figure 4 compares the moisture content in the exterior sheathing for three hybrid/split insulation walls with different vapor retarders that will meet the 2009/2012 International Building Code (IBC) requirements: 0.1 perms, 5 perms, and 10 perms (i.e. Classes I and III vapor retarders).

All other components for the three wall assemblies in Figure 4 are the same. They comprised:

• interior gypsum board;
• vapor retarder (as indicated for each case);
• 89-mm (3.5-in.) fiberglass batt insulation within the 2×4 steel stud cavity;
• exterior gypsum board;
• water-resistive barriers (WRBs) with >25 perms, over exterior sheathing; and
• 38-mm (1.5-in) extruded polystyrene (XPS) exterior insulation.

WUFI analysis was performed for the exterior climate of Chicago, Illinois (Zone 5A), and medium internal moisture conditions (50 percent ± 10 percent relative humidity [RH] and 21 C ± 1 C [69.8 F ± 1.8 F]).

Figure 4 shows the moisture content in the exterior sheathing increases as the vapor permeance of the retarder increases from 0.1 to 5 to 10 perms. However, the winter moisture levels in the exterior sheathing are relatively low and decrease to the equilibrium levels during the summer for all three cases. This indicates under these conditions, there is low risk of winter condensation and long-term durability issues due to vapor diffusion.

Another source of winter condensation, which is unfortunately not always recognized, is the exfiltration of warm, moisture-loaded interior air that can transport larger amounts of moisture than vapor diffusion. In fact, air leakage may be the main source of winter condensation. The excess moisture in the exfiltration air could be deposited on cooler exterior surfaces, such as exterior sheathing, leading to condensation.

The number of hours of potential condensation depends on interior moisture loads and exterior temperatures. Due to air leakage’s unpredictability, it is difficult to define failure criteria for its condensation potential. However, the condensation potential due to air exfiltration could be estimated by comparing the hourly temperature of the condensation layer (e.g. exterior sheathing) with the interior air’s hourly dewpoint temperature. When the temperature of the condensation layer is below the dewpoint of the interior air, condensation can occur if air exfiltration reaches the condensation plane. The lower the dewpoint temperature of the sheathing, and the longer sheathing temperature is below the interior air dewpoint, the higher the condensation potential.

Figure 5 compares the air leakage condensation potential for traditional, split insulation and exterior insulation wall assemblies. For these simulations, Chicago’s exterior weather and medium internal moisture conditions (50 percent ± 10 percent RH and 21 C ± 1 C) were used. The number of hours of potential condensation for the year can be estimated for each case.

As exterior sheathing temperature for both traditional and split insulation walls is below the interior air dewpoint temperature during the winter months, there is potential for condensation from air leakage for both walls. This is highlighted by the red boxes in Figure 5. As expected, the exterior sheathing temperature for the split insulation wall is higher with fewer hours of potential condensation as compared to traditional wall design. However, there is no potential for condensation from air leakage in the exterior insulation wall, since the actual temperature of the sheathing is always above the interior air dewpoint.

Potential for summer condensation
During the summer, condensation can be caused by vapor diffusion or moisture transported by air leakage. The main concern is diffusion or air infiltration from warmer, more humid exterior environments into cooler interior layers. The summer diffusion condensation potential can be estimated by analyzing moisture content of cooler interior layers, such as interior sheathing.

The often-discussed inward solar-driven moisture diffusion can be simulated for a moisture storage cladding, such as brick or stucco. The air leakage condensation potential can be estimated by comparing the actual temperature in the potential condensation layer (interior sheathing) with the exterior air dewpoint temperature.

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Figures 6 and 7 show WUFI simulation results for diffusion and air infiltration summer condensation potential. The exterior climate and interior conditions for these examples were selected to simulate a scenario with a large diffusion driving force—such as Houston, Texas, with a Zone 2A exterior climate and low internal moisture conditions (45 percent ± 15 percent RH and 21 C ± 1 C [69.8 F ± 1.8 F]).

The two walls in these examples are hybrid/split insulation steel-framed walls with brick cladding, vapor-permeable air and water barriers (>25 perms), and no interior vapor retarders. Two ci materials were simulated to estimate the impact of vapor permeability of exterior layers on diffusion condensation potential:

• 25.4-mm (1-in.) XPS (0.8 perm-in.); and
• 31.75-mm (1.25-in.) mineral wool (>50 perm-in.).

The ci thickness was based on 2012 IECC requirements for Climate Zone 2A (R-13 + R-5 ci).

The simulation results in Figure 6 show practically no condensation potential from vapor diffusion—including inward solar-driven moisture—for either of the two wall assemblies. Even when exterior layers are vapor-permeable (e.g. mineral wool wall assembly), the amount of moisture from vapor diffusion is only very slightly higher than for the wall with XPS vapor-impermeable exterior layer wall, and remains below the equilibrium moisture content of the gypsum board sheathing throughout the year. The wall assemblies in these examples do not contain an interior vapor barrier/retarder, which is not required by code in this climate; drying to the interior was allowed during the summer months.

However, infiltration of warm, moisture-loaded exterior air which can transport significantly more moisture than vapor diffusion could lead to condensation within the interior sheathing as shown by the WUFI simulation results in Figure 7. These examples show the temperature of the condensation layer (interior sheathing) during summer months is below the dewpoint temperature of exterior air for both wall assemblies.

Under these conditions, condensation could occur if air infiltration reaches the condensation plane. While the number of hours of potential condensation for the year can be estimated, it depends on both interior moisture loads and exterior temperatures. Also, since air leakage is unpredictable, it is difficult to define failure criteria for the related condensation potential.

Based on WUFI simulations and field experiments, it is this author’s opinion that problems often attributed to solar-driven moisture, and associated with moisture storage cladding such as brick or stucco, are most likely due to air infiltration, not vapor diffusion. Poor air barrier continuity could contribute to air leakage condensation, and improper wall design—such as use of vapor-impermeable materials on the interior side of the wall in warm climates—could further aggravate the consequences of air leakage condensation. (To read more, see Research Report-1011[8], “Evaluation of Cladding and Water-resistive Barrier Performance in Hot-Humid Climates Using a Real-weather, Real-time Test Facility,” by Theresa Weston.)

The importance of a continuous air barrier in every climate cannot be emphasized enough. The impact of air leakage on moisture control can sometimes be overlooked if the justification for an air barrier focuses strictly on energy savings, which are lower in warmer climates than cold and mixed climates. However, anyone involved with remediation due to moisture problems understands that the time and cost involved outweigh any upfront expenses to properly implement an air barrier design, construction, and verification program.

Simulated drying rates 
WUFI simulations could be used to compare drying rates for different climate conditions and wall assembly design following a wetting event. Drying is very important for long-term durability. Good enclosure design minimizes the risk of wetting, but moisture intrusion can never be completely avoided, and drying potential must always be considered. If a wall is able to dry, it may experience some wetting without long-term durability risks.

The WUFI model has the ability to compare drying rates after a wetting event, which is a relative way to compare potential long-term durability of different design options for given conditions. The following examples illustrate the dependence of drying rates on wall assembly design, material choices, and ventilation behind exterior cladding.

Impact of wall assembly design on drying rates
The drying rates for three wall assembly design options—traditional, split insulation, and exterior insulation walls—are compared here.

The various WUFI simulations were performed for Chicago’s Zone 5A exterior climate and medium interior moisture conditions (50 percent ± 10 percent RH and 21 C ± 1 C [69.8 F ± 1.8 F]). The characteristics of the three walls were:

1. Traditional wall: interior gypsum board, 1-perm vapor retarder (per 2009/2012 IBC requirement, Climate Zone 5A), 2×6 steel-stud cavity with 140-mm (5.5-in.) fiberglass batt insulation, exterior gypsum board sheathing, >25 perms air and water barrier, air space, and exterior cladding.
2. Hybrid/split insulation wall: interior gypsum board, 1-perm vapor retarder, 2×4 steel-stud cavity with 89-mm (3.5-in.) fiberglass batt insulation, exterior gypsum board sheathing, >25 perms air and water barrier, 38-mm (1.5-in.) XPS exterior insulation (for R-13 +R 7.5 ci, per 2012 IECC), air space, and exterior cladding.
3. Exterior insulation wall: interior gypsum board, 2×4 steel stud cavity with no insulation, exterior gypsum board, >25 perms air and water barrier, 76-mm (3-in.) XPS exterior insulation, air space, and exterior cladding.

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To compare the drying rates for the three wall design options, the simulations were started with elevated moisture content in the exterior sheathing (e.g. 250 kg/m³[15.6 lb/cf]) for all three walls, and the drying curves of the wetted layer were observed. (Another way to simulate wetting in WUFI analysis is by allowing a certain fraction of rain to penetrate the wall assembly and to compare the drying rates, but this is also a relative analysis since it is impossible to predict how much, and when, rain will infiltrate.) WUFI simulations were performed for a three-year period, but only the first is plotted in Figure 8 which shows the comparative drying curves for the three wall assemblies.

The graphs in Figure 8 show the different drying rates for the three wall assemblies. The exterior insulation wall design dries the quickest, taking less than 500 hours to reduce the moisture content from 250 kg/m³ to equilibrium water content levels (e.g. 6.19 kg/m³[0.39 lb/cf]). The diffusion drying for the exterior insulation wall is less critical because the stud cavity is part of the interior conditioned space and the HVAC-aided drying occurs quickly. The traditional wall design also dries fast, taking less than 800 hours to reduce the moisture content from 250 kg/m³ to equilibrium levels.

The hybrid/split insulation wall assembly dries the slowest; even after a full year, the moisture content in the exterior gypsum board sheathing has not reached equilibrium values. The drying analysis should be interpreted as a comparison of relative drying potential since it is difficult to predict when and how much a wall will be wetted. In this example, the simulation began with very high moisture levels in exterior gypsum board to emphasize the differences in the drying rates of wall assemblies. Hopefully, such high moisture levels are very rarely experienced in practice.

The slow drying rates for the hybrid/split insulation wall in this example should certainly be of concern, since hybrid wall design is becoming the most common steel-framed wall construction to meet prescriptive energy code requirements, and XPS exterior insulation is often the default choice of continuous insulation.

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Impact of materials choices on drying rates
The slower drying rates for the hybrid/split insulation wall assembly in Figure 8 are partly attributed to the inherent lower drying rates of energy-efficient assemblies (drying needs heat, and there is less heat flow through energy efficient assemblies[11]). However, drying rates are significantly impacted by the choice of materials.

Diffusion drying for the hybrid wall assembly in this example in Figure 8 should occur almost exclusively to the outside, which is the preferential diffusion direction in climate Zone 5A. This wall was also simulated with an interior vapor retarder (1-perm, required by 2012 IBC), so there is little to no drying potential to the inside. Further, the exterior insulation was XPS, with a vapor permeance of 0.8 perm-in., which means the material served as an unintentional vapor retarder. In addition to placing the XPS on the ‘wrong’ side of the enclosure (the cold side), this wall assembly contains two vapor retarders: the intentional one to the inside required by code, and the additional XPS to the outside. Having double or multiple vapor retarders in a wall assembly is not a good practice for moisture management.

‘Diffusion-open’ pathways to the exterior (the preferred diffusion direction for Climate Zone 5A) are important for drying, as demonstrated by the following two examples. A vapor-permeable material provides a diffusion-open pathway, which favors diffusion drying. By comparison, a vapor barrier or retarder is diffusion-closed.

The first example in Figure 9 compares the drying rates for two hybrid/split insulation wall assemblies, one with vapor-impermeable XPS exterior insulation (38-mm [1.5-in.], 0.8 perms-in.), the other with vapor-permeable mineral wool exterior board insulation (50 mm [2 in.], >50 perms-in.). The ci thickness was based on 2012 IECC requirements for Zone 5A (R-13 + R-7.5 ci). The second example (Figure 10), compares the drying rates for two hybrid/split insulation wall assemblies with mineral wool exterior insulation (50 mm, >50 perms-in.), one with vapor-permeable WRB membrane (>25 perms), and the other with vapor-impermeable WRB (<0.1 perms). Both graphs also include the drying rates of a traditional wall design, for comparison purposes.

Both examples show that diffusion open pathways to the outside significantly increase the split insulation wall assembly’s drying rates. Even though the split insulation wall dries inherently slower than the traditional wall (the traditional wall is included in both graphs for comparison purposes), opening the diffusion drying pathway to the outside brings the split insulation wall assembly within acceptable drying rates. These examples demonstrate energy-efficient wall assemblies can be designed with low risk for long-term durability with the correct choice of materials.

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Effect of ventilation on drying rates
In some situations, diffusion[13] is the only practical mechanism available for drying,⁵ and WUFI simulations demonstrate its importance on the drying rates of different wall assemblies. However, drying rates could be further improved with ventilation behind cladding.

WUFI simulations in this section compare the drying rates for two hybrid/split insulation wall assemblies similar in all regards except for ventilation behind cladding. Both walls have diffusion-open pathways toward the outside to allow diffusion drying, but one is vented (i.e. 50 air changes per hour [ach]), and the other has no venting behind cladding (i.e. 0 ach).

Figure 11 shows the impact of venting on the drying rates for Chicago’s exterior climate Zone 5A, and medium interior moisture conditions (50 percent ± 10 percent RH and 21 C ± 1 C [69.8 F ± 1.8 F]).

A recent study by John Straube, PhD, and building science research engineer Graham Finch demonstrates the importance of vented cladding[14] on the drying rates of wall assemblies, and compares the moisture content predicted by WUFI simulations with the experimental MC in the exterior sheathing. The study found the measured MC in the exterior sheathing was generally lower than the content estimated through WUFI simulations, even at high ventilation rates behind the cladding (100 ach).

Based on the field results, the authors concluded when ventilation behind cladding is not included in WUFI simulations, MC could be overestimated by as much as 15 percent. All WUFI simulation examples in this paper used 50 ach ventilation rate (except for the example in Figure 11 which was performed with 0 ach).

Conclusions 
New energy-efficiency measures often require changes in traditional building envelope design, with the hybrid/split insulation wall design becoming the simplest steel-framed wall assembly to meet the prescriptive energy code requirements. The slow drying rates for the hybrid wall design are of special concern. While the more energy-efficient split insulation wall design has inherently slower drying rates (because drying needs heat and there is less heat flow through energy-efficient assemblies), the long-term durability risks could be significantly reduced with the correct material choices.

The most concerning practice for split insulation wall design is the default choice of vapor-impermeable insulation materials as the exterior ci, which introduces an unintended (and second) vapor barrier/retarder in the wall assembly. While less common, use of other vapor-impermeable exterior layers (such as low-perm WRBs) is also an inappropriate practice. Such choices of materials could impact long-term durability risks by significantly reducing the diffusion drying rates of wall assemblies.

The practice of using dewpoint calculation as a design guide provides a false sense of security. This is because the dewpoint in most hybrid wall assemblies is located to the exterior of WRB layer, which indicates there is no diffusion condensation potential in these assemblies. However, as discussed throughout this article, the dewpoint calculation alone provides no information on other important moisture sources and how different assemblies manage incidental moisture instruction (e.g. drying rates following a wetting event).

There is less experience with the design and performance of split insulation wall assembly, and experimental data for long-term durability takes many years to develop. In the meantime, moisture analysis could be used to estimate the long-term durability risks for different climates and material choices.

This article provided examples of how WUFI moisture analysis could be employed to compare different design options. Even though the conclusions only apply to the specific conditions for which WUFI simulations were performed, the methodology could be applied to any system or climate to better understand the long term performance of different design options, and to help make the best design decisions.

Maria Spinu, PhD, CSI, LEED AP, received her doctorate in polymer science from Virginia Tech and has worked with DuPont for two decades. She currently leads the Building Innovations group’s building science and sustainability initiatives. Spinu is a member of the American Society of Heating, Refrigerating, and Air-conditioning Engineers 90.1 Committee and envelope subcommittee, and has authored 15 patents. She can be contacted at maria.spinu@usa.dupont.com[15].

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