As described by Joseph Lstiburek, B.A.Sc., M.Eng., Ph.D., P.Eng., principal of Building Science Corporation, moisture accumulates in the building envelope when the rate of moisture entry into an assembly exceeds the rate of moisture removal. When moisture accumulation exceeds the ability of the assembly materials to store the moisture without significantly degrading performance or long-term service life, moisture problems result. But what happens when solar-reflective exterior surfaces, and airtight and high-R envelope assemblies are thrown into the mix? This question becomes more significant when anticipating expanded retrofit activity to improve existing building envelopes.
To better understand energy efficiency features and their interaction with moisture and ultimately durability, Oak Ridge National Laboratory (ORNL) has established a new experimental facility and initiated several studies, including an examination of the relationship between air tightness and moisture durability, hygrothermal performance of below grade construction as a function of soil type, and moisture levels associated with cool roofs.
These current studies are grounded in a history of building envelope moisture durability research at ORNL, sponsored by the US Department of Energy (DOE) Building Technologies Office and industry partners. “We became involved in moisture, and its effects on efficiency, shortly after a rash of lawsuits were filed in the late 1990’s. These suits centered on moisturerelated failures of energy efficient building envelopes,” explained Andre Desjarlais, Group Leader, Building Envelope Research Group at ORNL. “A general concern arose that as building envelopes became more airtight and energy efficient, they were also losing their drying potential, with moisture-related failures being the unintended consequence. There was push back against energy efficiency improvements because of concerns about durability.”
Could energy efficiency and durability be achieved at the same time? In 1998, ORNL began its partnership with the Fraunhofer Institute for Building Physics (IBP) to develop the capability to use computer simulation to evaluate the moisture durability of building envelope assemblies common in the US. “Our collaboration with Fraunhofer IBP was based on the fact that they already had a model, Wärme und Feuchte Instationär (WUFI), used primarily in Europe,” Desjarlais said. “In 2001-2002, we released our first revision to WUFI, adding typical US construction and materials, and weather files, to drive the model. Subsequently, through the American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE), we and many others helped develop the first standard associated with moisture durability, Standard 160.”
WUFI, today in it’s fifth major release version, is one of the most advanced commercially available hygrothermal simulation programs in use, with computations taking into account factors such as water absorption, liquid and vapor moisture transport, and night sky radiation. While its accuracy has been validated over a number of years, both by the Fraunhofer IBP and ORNL, against data from numerous full-scale field studies of building envelope assembly performance, ORNL’s latest projects will once again test WUFI’s mettle.
Air Tightness and Moisture
“Effects from the Reduction of Air Leakage on Energy Use and Material Durability,” a study led by Diana E. Hun, Ph.D., P.E., Building Technologies Research and Integration Center, ORNL, and currently in peer review, examined the impact of increased airtightness, indoor moisture sources, the moisture capacity of materials in the wall cavity, the thermal resistance of continuous exterior insulation, and the amount of winter solar radiation on 28 walls installed at the Building Envelope Systems Testing (BEST) laboratory in Syracuse, New York, Department of Energy (DOE) Climate Zone 5. Assessed walls had insulation with a thermal resistance of 3.7 K·m2/W (RUS = 21 h·ft2·°F/Btu), classifying them as High-R walls (~3.2 < RIS < ~7 K·m2/W, ~18 < RUS < ~40 h·ft2·°F/Btu). Additionally, wall panels in this study had approximate air leakage rates of 0.02 L/(s·m2) (Level 1), 0.2 L/(s·m2) at (Level 2), or ~ 1 L/(s·m2) (Level 3) at a pressure differential of 75 Pa. These leakage levels were selected from air barrier compliance options set by the 2012 International Energy Conservation Code (IECC) for commercial buildings.
Hun and her team monitored the installed walls over 12 months, from October 2011 to November 2012, logging temperature, relative humidity, humidity ratio, and water vapor pressures at the inner surface of the exterior sheathing; as well as heat flow through the interior drywall. Additionally, outdoor conditions were gathered from an onsite weather station (see Figure 1).
“The main question we wanted to address here is, if you improve air tightness, are you compromising moisture durability of the envelope?” said Desjarlais.
Notable among initial findings was the impact of air leakage on energy loads. These effects were evaluated by using heat fluxes through the Level 1 walls as the baselines; their low air leakage implied that fluxes through them were primarily due to conduction. Maximum temperature differences between the Level 3 and Level 1 walls occurred during December and January, and these months also saw Level 3 wall panel heat fluxes that were up to 54% higher than those for Level 1. Differences between Level 2 and Level 1 walls maxed out at 11%.
“The magnitude of increase in energy loss due to air infiltration was a surprise,” said Desjarlais. “The level of air leakage we were evaluating was below that set by IECC levels, and the walls were fairly efficient, with an approximate RUS-21 insulation rating. The large increase in heat flow between Level 3 and Level 1, and resultant energy implications, was somewhat startling.”
In relation to air tightness and moisture, humidity ratios at the inner surface of the exterior sheathing were tracked, given that this is typically a critical location for moisture problems. These ratios increased from winter to summer, and decreased from summer to fall. Level 1 panels, representing the tighter walls, typically had higher monthly average humidity ratio measurements during the winter. This was due primarily to the fact that tightly-built walls rely on diffusion, rather than airflow, to remove excess moisture. This limitation was exacerbated by wall orientation. “If walls are facing north, east, or west during winter they receive less solar radiation, and experience weaker solar-driven diffusion to dry toward the indoor space,” said Hun. “The same panels facing south had lower humidity ratios in the cavity.”
Nevertheless, as outdoor water vapor increased from winter to summer, infiltration raised humidity ratios in the leakier panels (those classified as Level 2 and 3) at a faster rate than in the tighter walls (Level 1), so that humidity ratio values in the Level 3 exterior sheathing became similar to the ratios in the Level 1 walls.
“We also found that, in terms of likelihood of mold growth, in addition to certain variables like relative humidity indoors and air tightness, the moisture capacity of materials played a part,” Hun commented. “There was a large difference between results from walls with wood exterior sheathing, and those with XPS rigid foam or glass mat gypsum sheathing. Walls framed with wood tended to have lower relative humidity values in the cavity as the wood materials served as a buffer.” By analyzing sorption isotherms, the capacity of wood to store water vapor helped maintain humidity ratios in wood framed walls at a lower level than in wall panels constructed using materials with a lower moisture storage capacity. The results imply that buildings located in cold climates and having envelope materials with low moisture capacity, which are more typically found in commercial construction, can be susceptible to moisture problems.
Other observations hit home in residential envelopes. “When we use rigid foam insulation as exterior sheathing, people have been leaning towards not putting oriented strand board (OSB) across the envelope, other than to reinforce the exterior foam at the corners from wind shear. This is the first data that suggests that if OSB is left out, you leave behind its moisture storage capacity, and you are potentially reducing the durability of your wall system,” Desjarlais stated. “In the walls that Hun evaluated, it is also apparent that when you move all of your insulation outboard, placing all of the R-value outside of the cavity of the wall, moisture performance increased. The higher the R-value of insulation applied outside of the wall cavity the warmer the temperatures in the cavity, which helps maintain a lower relative humidity.”
“With increased requirements on air tightness, there will be a change in moisture behavior in the envelope assembly,” summarized Manfred Kehrer, Building Envelopes Research, Building Technologies Research and Integration Center, ORNL. “The best envelope is totally airtight – no air often means less moisture, but this cannot be achieved in practice. To reduce energy leaks we increase air tightness but therefore we also reduce air speed through the wall, which means there is time for condensation to occur. So unless we are careful with the details of the envelope assembly, with increased air tightness, what were energy leaks have the potential to become moisture leaks.”
“What this field work shows is that you can achieve energy efficient, durable walls if you are more careful about material choice and wall assembly configuration,” Hun said.
Desjarlais agreed. “This is the reason for developing models and tools. If you double the R-value of a wall, suddenly, moisture isn’t so black and white. To borrow Kehrer’s analogy, you’re walking along the cliff, and if you take that one extra step, you’re in trouble. So many things are happening at once in a highly energy efficient wall. You’ve got water vapor, rain possibly penetrating the cladding, sunshine, the relative humidity you would pick first, and if you haven’t considered moisture your pick is at risk of a moisture-related failure. For new construction, use of accurate simulation tools enables forgiving, moisture-durable, and cost-effective envelope assemblies to be prescribed in energy codes. For retrofits, the tools can be used to verify that what you’re contemplating doing, will be moisture-durable, before you do it.”
This most recent field study, led by Hun, adds to a pedigree of field verification efforts that will eventually lead to enhancements of WUFI.
“Back in 2007, ORNL began using US data to validate WUFI, in partnership with the Exterior Insulation and Finish Systems (EIFS) Industry Members Association (EIMA)®,” Desjarlais explained. “The effort began in the very challenging hot and humid DOE Climate Zone 3 with the construction of a natural exposure test facility in Charleston, South Carolina where dozens of wall assemblies could be evaluated side-by-side. After the lawsuits in the late 1990s, EIMA was anxious to prove a family of exterior insulation and finish systems could effectively manage moisture and achieve energy efficiency without compromising durability. From there, we did a large number of building envelope simulations to expand this field-based hypothesis into other climates. Our research found that appropriately designed EIFS assemblies are working in every climate zone.”
“Compared with other tools, WUFI has been more successful with matching simulations to experimental field results,” added Kehrer. “Of course WUFI is not perfect either, but we know where the WUFI model works well, and where we have to improve our characterization of the behind-the-scenes complex physics.”
Below Grade Construction and Soil Moisture
In an effort to further increase WUFI’s scope, and thus enhance the simulation tool’s usefulness to the buildings industry, ORNL is expanding WUFI capability to handle below grade construction by adapting soil properties information into the database. While approaches exist to calculate heat transfer and some capillary action, ORNL’s team hopes to expand the below grade hygrothermal picture by determining the influences of soil moisture storage, liquid water transfer, vapor diffusion resistance, dry bulk and particle density, porosity, thermal conductivity, and specific heat capacity. The research should bring greater clarity to parameters such as liquid uptake of precipitation and liquid flow.
Figure 2. Oak Ridge National Laboratory research will expand WUFI capability to handle below grade construction by adapting soil properties information into the database. Graphic courtesy Oak Ridge National Laboratory.
ORNL’s work will also gauge the reliability of the soil parameters, by comparing simulation predictions against ongoing in-field measurements for temperature and soil moisture content, together with measured boundary conditions (see Figure 2).
“The final outcome of the study will be the evaluation of several soil types in several climate zones for a number of basement assembly types,” stated Kehrer. “The study will define the type of soil, together with the type of building construction considered most and least reliable with respect to energy consumption and moisture durability.”
Currently, WUFI simulation software does not allow for full absorption of precipitation at the ground surface if the outer element is already saturated with moisture, and better models are needed to more accurately simulate the moisture transfer mechanisms. Additionally, WUFI cannot simulate the impact of snow cover on heat transfer at the ground surfaces. ORNL research will try to fill in these gaps.
“These are examples of how we’re leveraging field evaluations and research to enhance WUFI, so that we are able to make more generalized conclusions and best practice recommendations,” stated Roderick K. Jackson, Ph.D., Whole-Building and Community Integration, Building Technologies Research and Integration Center, ORNL. “I think this type of coupled approach versus just taking a research approach with tools or experiments alone, will continue to be the direction going forward.”
Moisture in Cool Roofs
Besides verification investigations into moisture implications of air tight walls and below grade construction, ORNL also unveiled results evaluating possible increased condensation risk in cool roofs. According to a study by Ennis & Kehrer, 2011, cool roofs with a mechanically attached membrane (see Figure 3) have shown a higher risk of intermediate condensation in the materials below the membrane in certain climates and in comparison with similar constructions with a darker exterior surface. This risk is affected by climatic loads, indoor moisture sources, and air intrusion rates.
Figure 3. “There have been lots of articles in the last 1 or 2 years that just mention there is a problem with cool roofs in some applications, but offered no answers,” explained Kehrer. Graphic courtesy Oak Ridge National Laboratory.
“There have been lots of articles in the last 1 or 2 years that just mention there is a problem with cool roofs in some applications, but offered no answers,” explained Kehrer. “Our study is the first one that addresses what works and what doesn’t, and why. We looked at the influence of climate zone, air tightness of the roof, and interior conditions. The results really make sense.”
The results, compiled in Kehrer’s “Condensation Risk of Mechanically Attached Roof Systems in Cold Climate Zones,” emphasize the importance of solar reflectance at the roof surface. In cool roofs, the maximum condensate layer thickness is almost double that of a traditional black roof. In a cool climate, cool roofs run a much greater risk of condensation. Additionally, Kehrer’s work shows that a low or a high indoor moisture supply can cause a difference up to 10 times as great in condensation. If indoor moisture is kept at low levels, or air intrusions are kept low (a safe upper limit of air leakage at 50 Pa, Q50, is stated as 0.17 l/s,m2 for metal roofs (Hens et al., 2003)), there is little risk of intermediate condensation in a roof.
“Nothing really surprised us from the results,” said Kehrer. “Some of the wind flow assumptions could be weak, but no better data exists, so they are reasonable as best we can tell.”
“What we were seeing across the industry was effectively a condemnation of cool roofs,” added Desjarlais. “This, of course, is nonsense. Our message is simply that you do need to consider moisture when designing envelope assemblies, including cool roof assemblies. This study shows that you can have problems with traditional black roofs too, if indoor moisture levels are high, or there are high rates of air intrusion. Believe it or not, there had really never been a moisture durability study of mechanically attached roof systems even though they are the most widely used in this sector. Now that the tools are available industry seems eager to use them to reduce their risk.”
Taking Snapshots of ORNL’s Work
What are the most valuable lessons a builder can walk away with from this recent slate of ORNL work?
“When you have any kind of envelope, don’t make it vapor tight on both the indoor and outdoor sides, make the envelope a little forgiving in case of bad workmanship,” noted Kehrer. “If at all possible, allow a wall to dry in both directions, but at least one.”
“And by drying, this means by diffusion, not by air flow, otherwise you have a non-efficient envelope,” clarified Hughes.
“One time-honored, demonstrable way to protect the building envelope from moisture damage is to properly protect construction material from weather on the jobsite,” Desjarlais stated. “A major cause of failure is initial high levels of moisture in building materials. A builder will have substantially more issues if he doesn’t take care of building materials as they are shipped and stored on site. Reduce the amount of water in the structure when you first construct it.”
“My advice to homeowners is to control your sources of moisture indoors,” said Hun. “In general, this means properly ventilate bathrooms and the cooking area, which are among the major sources of moisture in a home. My advice to builders is to provide adequate mechanical ventilation in residences with tightlybuilt envelopes because this will help control indoor moisture levels in the winter and maintain acceptable indoor air quality throughout the year.”
“Certainly as people build or remodel, it is worthwhile to do a quick analysis. You would much rather find a problem in the computer than in your building,” commented Desjarlais. “This very goal, achieving energy efficiency without sacrificing moisture durability, is the intent behind ASHRAE 160 and WUFI. Our basic WUFI simulation makes these calculations easy and affordable in time and dollars to conduct a moisture durability analysis. You can’t expect a professional to spend a week to do analyses; the industry needs a straightforward tool that is easy to use and gets results in a reasonable period of time.”
“Call me practical or maybe just old, but after observing the fate of building energy simulation over the last 40 years, with the exception of a few production builders, I am somewhat skeptical that residential builders and remodelers will be embracing building moisture simulation on a project-by-project basis,” commented Hughes. “For new construction I see no reason why energy efficient, moisturedurable, and forgiving building envelopes cannot be prescribed in energy codes. However with retrofits, what is safe to do as an energy improvement depends on climate and the details of your existing envelope assembly. I think the materials supply chains need to step up with better technical market support tools to make the durability consideration as easy as possible for remodelers.”
Figure 4. The air and moisture penetration chamber, among the newest facilities at ORNL. Photo courtesy Oak Ridge National Laboratory.
Expanding its arsenal of experimental capabilities, the air and moisture penetration chamber is among the newest facilities at ORNL (refer to Figure 4). This apparatus will enable researchers to study the effects from air and moisture penetration through walls while sustaining selected temperature and humidity gradients across large-scale wall specimens, and simultaneously simulating positive and negative wind/gust pressures across the wall, as well as rain and/or solar radiation on the outdoor surface of the wall. The chamber also allows for accelerated tests that assess the long-term performance of new and retrofitted wall assemblies, enabling researchers to document cyclical effects of weather on airtightness and moisture content.
“We are beginning a project with Building America that will further evaluate the durability of energy efficient walls for new and existing residential construction in the new chamber,” Hun said. “While it will be somewhat similar to our Syracuse field test, evaluations are accelerated in the sense that we can control indoor and outdoor cycles without waiting for weather to happen. We can use the chamber to replicate the most challenging sequences of weather and monitor whether thresholds for moisturerelated failures are being reached in these walls.”
“Materials suppliers and large regional and national contracting organizations are also beginning to engage with ORNL to develop technical market support tools leveraging our envelope energy efficiency and moisture durability expertise,” said Hughes. “The goal of these tools is to make the durability consideration as easy as possible for those interested in increasing the energy efficiency of residential and light commercial building envelopes.”
Energy Design Update thanks Oak Ridge National Laboratory for sitting down with us and sharing their latest research. To visit ORNL Building Technologies online, go to /science-discovery/cleanenergy/ research-areas/buildings.
André Desjarlais is the Group Leader for the Building Envelopes Research Program at the Oak Ridge National Laboratory. He has been involved in building envelope and materials research for over 40 years, first as a consultant and, for the last 23 years, at ORNL. Areas of expertise include building envelope and material energy efficiency, moisture control, and durability. Desjarlais has been a Member of the American Society for Testing and Materials (ASTM) since 1987 and serves on Committees C16 on Thermal Insulation, E06 on Building Systems, and D08 on Roofing. He is the past Chairman of ASTM Committee C16. Desjarlais has been a member of ASHRAE since 1991 and serves on Technical Committees TC 4.4 on Thermal Insulation and Building Systems, TC 1.8 on Mechanical Insulation Systems, and TC 1.12 on Moisture Control in Buildings, and is past Chairman of TC 4.4.
Patrick Hughes, P.E., serves as Director of ORNL’s Building Technologies Research & Integration Center (BTRIC, /sci/ees/etsd/btric/). The BTRIC areas of focus are advanced building technologies, wholebuilding and community integration, and improved energy management in buildings and industrial facilities during their operational phase. BTRIC has a full-time staff of approximately 80 researchers and includes a Department of Energy (DOE)-designated National User Facility that supports ORNL research sponsored by DOE and others, in collaboration with industry, universities, associations, and utilities. Hughes served as an ORNL Group Leader (2001-04) and Senior Research Staff member (1989-2001) prior to his current position. Awards received by Hughes related to building energy efficiency include a DOE Energy Innovation Award in 1987; the International Ground Source Heat Pump Association’s “Outstanding Engineering Achievement Award” in 1998; and ORNL Significant Event Awards in 1993, 2010 and 2013.
Diana E. Hun, Ph.D., P.E., is an engineer for the Building Envelope Research group at ORNL. She is currently evaluating the performance of air barrier technologies and studying the optimisation of building energy demands and indoor air quality. To this end, she is collaborating with the Air Barrier Association of America (BAAA) and its participating research members, Building America, the US-China Clean Energy Research Center Building Energy Efficiency (CERB BEE) consortium, and various universities. Hun Received her Ph.D. from the University of Texas at Austin, where she studied human exposure to hazardous air pollutants in homes.
Roderick Jackson, Ph.D., R&D Staff, Oak Ridge National Laboratory, joined ORNL in 2009, and is currently on the R&D Staff of the Whole-Building and Community Integration group and is responsible for providing sub-programmatic management for residential building integration and deployment (RBID) research. Jackson’s research is primarily focused on the integration of technologies and best practices that maximize cost-effective energy efficiency in residential buildings. He leverages over six years of residential construction experience with four years served as president of a general contracting firm, where he managed custom-built new construction and retrofit residential projects. At ORNL, he also conducts energy policy research and analysis to facilitate comprehensive technical, social, and economic solutions to the energy and climate challenges currently facing the US. Dr. Jackson received his B.A., M.S., and Ph.D. in mechanical engineering from the Georgia Institute of Technology.
Manfred Kehrer is a senior staff member in the Building Technologies Research and Integration Center at Oak Ridge National Laboratory. Kehrer worked at Fraunhofer IBP Germany, for 10 years in the laboratory of the Hygrothermics Department. He then moved to the software development group where he worked on modeling, programming, and testing of the transient hygrothermal transportcalculation (WUFI), and eventually became the group manager. Since 2011, he has worked as a senior researcher within the Building Envelopes Group at Oak Ridge National Laboratory, where he is in charge of hygrothermal investigations.