November 15, 2013

Abstract: 

This report explains the moisture-related concerns for high R-value wall assemblies and discusses past Building America research work that informs this study. Hygrothermal simulations were prepared for several common approaches to high R-value wall construction in six U.S. cities (Houston, Atlanta, Seattle, St. Louis, Chicago, and International Falls) representing a range of climate zones (2, 3, 4C, 4, 5A, and 7, respectively). The simulations are informed by experience gained from past research in this area and validated by field measurement and forensic experience.

Executive Summary

In recent years, rising energy costs, energy security concerns, and social change have generated increased demand for better thermal performance. Building standards and construction codes have required higher minimum R-values. High-R assemblies, however, can be more susceptible to moisture problems. Different design considerations, construction techniques, and strategies are needed to ensure long-term service and durability of these assemblies.

The following report explains the moisture-related concerns for high R-value wall assemblies and discusses past Building America research work that informs this study. Hygrothermal simulations were prepared for several common approaches to high R-value wall construction in six U.S. cities (Houston, Atlanta, Seattle, St. Louis, Chicago, and International Falls) representing a range of climate zones (2, 3, 4C, 4, 5A, and 7, respectively). The simulations are informed by experience gained from past research in this area and validated by field measurement and forensic experience.

The modeling program was developed to assess the moisture durability of the wall assemblies based on three primary sources of moisture: construction moisture, air leakage condensation, and bulk water leakage. The peak annual moisture content of the wood-based exterior sheathing was used to comparatively analyze the response to the moisture loads for each of the walls in each given city. Walls that experienced sheathing moisture contents between 20% and 28% were identified as risky, whereas those exceeding 28% were identified as very high risk.

All of the wall assemblies perform well under idealized conditions. However, only the walls with exterior insulation, or cavity insulation that provides a hygrothermal function similar to exterior insulation, perform adequately when exposed to moisture loads. Walls with only cavity insulation are particularly susceptible to air leakage condensation. None of the walls performed well when a precipitation-based bulk water leak was introduced to the backside of the sheathing, emphasizing the importance of proper flashing details.

This report is intended for designers and builders who are concerned about best practices for moisture management in high R-value walls, and for researchers who may need to assess other high R-value assemblies.

1 Introduction

1.1 Problem Statement

Rising costs of energy, concerns relating to climate change, and demands for increased comfort have led to the desire for increased insulation levels in many new buildings. However, increasing the insulation used in new construction may lead to increased problems in managing moisture. Depending on the insulation strategy, new construction techniques and strategies may need to be employed to ensure that external and internal moisture sources are properly handled, such that moisture-sensitive materials are protected and maintained at safe levels.

Reducing the heat flow across an enclosure (by increasing insulation levels) may decrease its durability relative to standard construction, depending on how that heat flow reduction is achieved. High R-value walls are no different. By adding insulation inside of wood sheathing or cladding, the moisture content (MC) of the sheathing or cladding will rise in cold weather, the risk of condensation increases significantly, and outward drying potential is reduced. Adding insulation also increases the risk of condensation in the summertime only if cooling is present, whether by natural heating, ventilation, and air conditioning (HVAC) systems or natural cooling, but in this circumstance on the exterior side of the interior finish, especially if the interior finish is vapor impermeable (vapor barriers, cabinets, mirrors, etc.). In short, pressure to increase energy efficiency has a potential “systems effect” on the moisture-related performance of new and existing housing, and this impact must be understood to mitigate unexpected and unintended performance and durability problems.

The risk of moisture damage depends on a number of factors, including climate (seasonal changes, orientation, exposure) and interior conditions (temperature, relative humidity [RH], pressurization), as well as particulars of the wall assembly such as cladding type, the presence or absence of a ventilation and drainage gap behind the cladding, the sheathing material, the type and location of insulation material, the vapor permeance of various layers (including vapor control layers and finishes), and the sensitivity of the assembly to workmanship errors, movement over time, and environmental changes. The range of factors involved makes understanding and predicting moisture-related performance a complicated activity.

A significant amount of laboratory and field research has been conducted to better understand the moisture performance of materials, subassemblies, and enclosure systems. A significant amount of research is still underway; however, research is increasingly conducted by the private sector and is not immediately made available to those who are designing and building. At the same time, insulation and airtightness standards continue to become more stringent while the number of available building materials and systems continues to increase. Designers and builders, faced with greater demands and more options, are now seeking more information and guidance from manufacturers, consultants, and standards organizations; however, real physical testing, analysis, and reporting have not kept up with the industry demand for information and guidance.

For interim guidance, the fundamental physics of moisture properties and motion in building components and systems, complemented with empirical evidence and observations, can be applied to infer the moisture-related problems in existing and proposed buildings. Sophisticated hygrothermal simulation tools have been developed. When the limitations of the simulations are understood, and the simulation results are calibrated against field and laboratory measurements, these tools can extend our ability to make recommendations.

This paper builds on past research work and building science theory and uses hygrothermal simulations to examine the moisture sensitivity of select high-R wall systems to boundary conditions and design decisions. Section 1 provides the background for the study and presents the research questions addressed by the work. Section 2 explains how the range of potential factors was limited to significant cases and describes the approach to the hygrothermal simulations. Section 3 presents the results of the study and Section 4 provides recommendations, including climate-specific guidance and drawings to describe appropriate construction assemblies. 

1.1.1 Definition and Classification of High R-Value Walls

The term high R-value enclosure attempts to bring together what is known about delivering exceptionally good control of heat flow through walls, roofs, windows, and foundations. High R- value enclosures are more than just assemblies with an increased amount of insulation. These enclosures are systems that are airtight, have little thermal bridging, manage solar heat gain, ensure human comfort, are buildable at production scale, and provide moisture control to ensure durability and health expectations are met.

There are no widely accepted definitions of the terms high R and high R-value, but they are usually understood as providing higher thermal control than the building code mandates. For the purposes of this report, a high-R enclosure provides an effective R-value that meets or exceeds those listed in Table 1, but also meets high standards for buildability, durability, health, and comfort.

R-value is commonly used in reference to the thermal resistance of insulation products. However, this metric does not account for the impacts of thermal bridging, air leakage, installation quality, and thermal mass—i.e., it does not account for many of the factors that affect thermal performance in real-world structures. It is this multitude of factors that work together to deliver good thermal control. Oak Ridge National Laboratory has proposed “whole-wall R- value,” which is the R-value for the whole opaque wall including the thermal performance of not only the “clear wall” area, but also all typical envelope interface details. Although this does not account for all of the impacts listed above, it is a better indicator of performance. 

In 2009, Building Science Corporation (BSC) was tasked with preparing a Building America white paper on high thermal performance enclosures (Straube, 2010). This paper defined performance requirements, reviewed past and current research, and outlined the research gaps in this area, including the need to demonstrate and document methods to achieve high levels of thermal performance and airtightness. Table 1 below provides BSC’s recommended “whole- wall” minimum R-value for different enclosure components for each climate zone. The column highlighted in red shows the minimum “whole-wall” R-values that are used in this report as the current minimum standard for high R-value wall assemblies.

Climate
Zone
WallVented
Attic
Compact
Roof
Basement
Wall
Exposed
Floor
Slab
Edgeb
Windows
(U/SHGC)
Sub-
slabc
1104035510noneyesnone
2155040102050.35/<.25none
320504510207.50.30/<.35
425604515307.50.30/<.357.5
53065501530100.24/<.507.5
63575602040100.18/—10
74090652545150.15/—15
850100753550200.15/—20

 Table 1. Current Recommended "Whole-Wall" Minimum R-Valuea Including Thermal Bridging (Straube 2010)

a These are recommended values based on experience - see economics section

b Slab edge insulation includes all of stem wall or monolithic slab edge

c Full area coverage of slabs

A broad classification of the approaches to high R-value enclosures for cold climate residential buildings was suggested in a recent Building America study (Lukachko et al. 2012). Figure 1 below describes two common approaches: adding insulation to the exterior of the building structure (i.e., the “exterior” approach), which may include insulation materials inside the structural cavity or none at all; and adding more insulation inside the structural cavity (i.e., the “inside” approach), which uses different types of insulation material and an increased width of the cavity to reach higher R-value levels. Walls are illustrated in Figure 1, but the classification could also apply to other enclosure components with a few modifications.

For each of the approaches in Figure 1, there are additional modifications depending on choice of insulation material and the thickness of the wall or of various layers in the assembly. Different enclosure assemblies will have different requirements to meet or exceed current durability expectations, but recommendations can be made at this level of classification. In general, drained and ventilated claddings, exterior insulating sheathing, and high airtightness combine to provide an enclosure that is more durable even when insulated to high levels. 

Figure 1. Common approaches to high R-value enclosures

Notes for Figure 1:

  1. The left-hand side is the “outside climate” and the right-hand side is the “inside climate.”
  2. Dark gray is rigid insulation, medium gray is spray foam insulation, light gray is cavity fill insulation, and white is open structure.

1.1.2 Sources of Moisture for High R-Value Walls

Enclosure assemblies are subject to moisture loads from a number of sources including bulk water (introduced by leakage), built-in moisture, water vapor (introduced by vapor diffusion or air leakage), and capillary transport through materials in contact with water or in contact with the ground. Different approaches to high R-value construction are affected differently by each source. The moisture sources are described below.

1.1.2.1 Bulk Water

The largest potential moisture source in wall assemblies is bulk water leakage. Bulk water is introduced at the exterior of wall assemblies in the form of rainwater and meltwater (from ice and snow). The means and methods to prevent the bulk water penetration and moisture damage are well developed and understood. Roof overhangs and wall surface features prevent rainwater from pooling or standing on the exterior surface. Flashings prevent bulk water penetration at interfaces, at openings (e.g., windows and hatches), and at service penetrations (plumbing and electrical stacks, air intake and exhaust vents, etc.) (Lstiburek 2006). Exposure to bulk water can also be indirect: splash-back from hard surfaces at the base of the wall and surface runoff from grade or roof areas sloping toward the wall are common problems.

1.1.2.2 Built-in Moisture

Moisture is said to be “built-in” when damp or wet materials are enclosed in an assembly during construction. Built-in moisture can be introduced through the use of wet materials or through unprotected materials that are wet by rain or meltwater during construction. Builders in areas with high hours of annual rainfall are likely to have a high level of awareness of this issue and in some areas (the Pacific Norwest coast, for example), building regulations require spot measurements to verify that the MC of the wood framing is below critical levels before construction is allowed to be closed in.

1.1.2.3 Water Vapor

Another moisture source, water vapor, is often considered but not as well understood. Through the winter months in cold and mixed climates the indoor air can be a significant source of water vapor. Water vapor moves through and into the assembly by two mechanisms: vapor diffusion and airflow. Methods to control vapor diffusion and air movement are well documented (Latta 1973; Hutcheon and Handegord 1985; Quirouette 1985; Straube and Burnett 2005) but are unfortunately rarely well executed. Airflow is capable of transporting hundreds of times more moisture than vapor diffusion (Wilson 1961); hence, it is important to control airflow to prevent moisture problems and ensure the durability of the building enclosure.

1.1.2.4 Capillary Transport

Movement of moisture by capillary action occurs through the interconnected network of pores in a hygroscopic material or between two adjoining hydrophilic materials due to the attractive force of surface tension. Capillary transport through joints is significant only in gaps of less than about ⅛ in. (3 mm) but can occur in a broad range of building materials such as concrete, clay brick masonry, and wood. Wall assemblies in direct contact with a concrete foundation can be at risk of wetting by this mechanism unless protected by a capillary break created by a nonporous or hydrophobic material.

Solutions exist to control these moisture sources and maximize assembly durability. Solutions include: use insulation exterior to any sheathing, use lower permeance insulating exterior sheathing, build a ventilation space outside of the sheathing or behind the cladding, build a more airtight enclosure, provide better rainwater management (e.g., drained subsill flashing), etc. These solutions are considered and explained in Sections 3 and 4 of this report.

1.2 Past Building America Research

The increased risk for moisture damage in insulated wall assemblies is well understood by researchers (Rose 2005; Straube and Burnett 2005; Hutcheon and Handegord 1985), but it is not well understood by the code and building communities.

When the thermal resistance of a wall assembly is increased, wood-based sheathings and some sidings (particularly wood and fiber cement) are placed at a higher risk of moisture damage (Lstiburek 2010). Field experience with certain types of high-R enclosures (e.g., structural insulated panels (SIPs) and double stud walls and dense-pack roof assemblies) have shown that wetting due to small errors (for example, rain leaks or convective loops) can occur and, since drying is very slow (due to increased airtightness, decreased heat flux, and the introduction of vapor impermeable layers), high RH and moisture content (MC) persist for longer periods and there is a heightened risk of damage (Straube and Burnett, 2005). 

In 2009 and 2010, BSC conducted a series of studies, each focusing on a different part of the building enclosure. These included reports for high R-value walls (Smegal and Straube 2009), high R-value foundations (Straube and Smegal 2010), and high R-value roofs (Straube and Grin 2010). Each report looked at thermal control, but also moisture control, durability, buildability, cost, and material use, for common high R-value assembly designs. Analysis conducted for these reports sought to identify high R-value assemblies that were likely to be implemented at a production scale and that were also designed to minimize durability risks.

A study conducted by IBACOS in 2010 (Broniek et al. 2010) also evaluated different approaches to the construction of high R-value wall assemblies. This study included a comparison of simulation results using a whole-house energy model and collected experience with construction issues through consultation with builders and manufacturers, and through the construction of full-scale mockups. In addition to some of the same performance criteria examined in the BSC study above, IBACOS looked at architectural flexibility (i.e., the ability of the wall system to accommodate a wide range of floor plans and finishes) and scalability to mass production in multiple climate zones.

Field research projects have been conducted by Building America teams to assess different approaches to high R-value enclosures in number of climate zones. Some recent BSC examples include:

  • The Westford Habitat for Humanity project in Westford, Massachusetts 
  • Research with Transformations, Inc. at three developments in Massachusetts 
  • The NIST Net Zero Energy Lab House in Gaithersburg, Maryland
  • The Neighborhood Stabilization Program 2 community in Wyandotte, Michigan.

The Westford Habitat project used a 4-in. layer of insulating sheathing outside of advance framed 2 × 6 walls and 4 in. of insulating sheathing over engineered wood rafters (i.e., the “exterior” insulating sheathing approach, see Figure 1). With the cavity fill insulation included, this prototype house has nominal R-44 walls, R-66 roof insulation, R-26 basement wall insulation, R-10 under the basement floor slab, and a whole-house airtightness of 1.5 ACH50. Important lessons were learned during the construction of wall and roof assemblies with  thick layers of exterior insulation, including special details for window and door installation (Lstiburek 2009; BSC 2010a).

The Transformations, Inc. project involved three communities of houses that employ a 12-in. thick double stud wall assembly to achieve a high R-value enclosure (i.e., the “inside” double stud approach, see Figure 1). The double stud approach is favored by some builders because it allows for the use of low-cost cavity fill insulation materials (instead of more expensive board foam and spray foam insulation materials). Double stud walls, however, are at a higher risk of moisture-related problems than walls constructed using the insulating sheathing (exterior) approach. The moisture risks associated with this approach have been documented as part of the high R-value Wall study (Straube and Smegal, 2009a) and the Transformations project continues to assess this issue with long-term field measurement of a side-by-side comparison between full- cavity ocSPF and full-cavity netted and dry blown-in cellulose (BSC 2010b; Ueno et al. 2012).

The NIST Net Zero Energy Lab House was designed primarily to test mechanical and renewable energy systems inside an ultra-low load enclosure. The enclosure was designed using current best practices for thermal control and airtightness. The walls and roof assemblies were fully clad in oriented strand board (OSB) sheathing to support a continuous self-adhered membrane. This membrane was detailed as the air barrier system, which was continuous over the roof/wall interface and integrated with windows and other enclosure penetrations. Insulating sheathing was added over this membrane: 4 in. of polyisocyanurate (PIC) (R-26) for the walls and 6 in. of PIC (R-39) for the roof (i.e., the “exterior” insulating sheathing approach). The final airtightness  test result for this assembly was 0.61 ACH50. An extensive set of detail drawings was prepared for this project and construction and quality control processes were documented (Lukachko  et al. 2011).

In Wyandotte, 18 houses were constructed using a hybrid insulation approach consisting of 2 in. of extruded polystyrene (XPS) insulating sheathing (R-10), with 2 in. of closed cell spray polyurethane foam (ccSPF) sprayed to the interior of the insulating sheathing (R-12), fiberglass batt insulation was used to fill the balance of the 2 × 6 wood stud cavity (R-12). There were two primary outcomes from this research. First, the airtightness measurements demonstrated that builders having little previous experience with energy-efficient construction techniques were able to achieve consistent results that are < 1.5 ACH50. Second, the process changes implemented to help secure these results were straightforward and ended up encouraging better communication between designer, builder, and the officials supervising the project (Lukachko  et al. 2012).

1.3 Research Questions

Researchers and builders have gained experience with the detailing and construction of high R- value wall assemblies. Past work described in the section above has identified the moisture risks for high R-value walls; however, more work is needed to quantify the risk. Furthermore, a number of variables affect the risk (climate, cladding type, insulation type and location, etc.) and more information is needed to assess the impact of each of these. Finally, with the continued introduction of new materials, changing indoor environmental conditions, enhanced expectations from the occupants, and higher performance standards, there are a myriad of factors that need to be considered before moisture guidelines can be developed for high R-value wall assemblies. To further develop our understanding of these factors, the following research questions are addressed in this report.

  1. What is the role of insulation levels on the risk in different climates?
  2. How resistant are the walls to air leakage and vapor diffusion in different climates?
  3. What are the drying rate capacities of the proposed wall assemblies?
  4. What are the high-level steps necessary to build moisture-resistant high-R wall assemblies?

To answer these questions, hygrothermal simulations were prepared to assess the performance of representative high R-value walls in a range of climate zones. The study was structured to assess the sensitivity and response to different factors. The simulations were calibrated against field experience with and laboratory research on similar high-R wall assemblies. The approach to this work is described in Section 2 below. 

2 Modeling Methods

2.1 Technical Approach

In this project, hygrothermal modeling tools, field experience, and building science theory were used to address the research questions. These research questions can be divided in to two main sections: (1) what are the limits for water vapor diffusion and air leakage in select climates, and (2) what are the moisture performance characteristics of the proposed high-R wall assemblies in a range of climates? Each section is considered separately from a modeling perspective.

To assess the limits of water vapor diffusion and air leakage in wall assemblies, a parametric study was devised. The study compares the response of two wall assemblies, one that is particularly sensitive to moisture loads and another that is more tolerant based on experience, to bound the limits of the problem. These walls were simulated in a range of climates (very cold, cold-humid, hot-humid, and hot-dry) and subjected to differing levels of interior RH and air leakage rates. Certain climate zones required specialized treatment for vapor or thermal control to maintain code compliance. In all cases, the walls were created to comply with the 2012 International Residential Code (IRC).

To assess the moisture performance characteristics of the proposed high-R walls, a comparative modeling approach was used. Select cities, representative of a range of climate zones, were chosen to expose the proposed walls to a range of environmental conditions. A baseline simulation was then conducted to better compare the walls with added moisture loads. The proposed walls were then subjected to a series of moisture loadings from three major sources of moisture: construction moisture, air leakage condensation, and bulk water leakage. The degree of moisture loading was based primarily on experience, but also refers to published literature (i.e., ASHRAE 160P-09) (ASHRAE 2009). The results were recorded and analyzed.

The reader is cautioned that the research contained within this report is based largely on simulations and has not been verified empirically. It is based on assumptions derived from significant field experience and published literature and the results were compared with known performances of high-R buildings. However, to validate the models, empirical research on small- and full-scale assembly mockups and buildings, with monitored boundary conditions and instrumented with temperature and moisture sensors, needs to be undertaken to examine and fully quantify these risks—this will be identified in the conclusions as a future research need. . .

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