Building America Project Team: Oak Ridge National Laboratory

As modern buildings become more airtight, with higher insulation levels, the building envelope becomes more sensitive to unmanaged moisture. To minimize the risk of moisture accumulation, the hygrothermal performance of high-R walls must be better understood and assessed under many different conditions, such as variations in material properties, workmanship, indoor conditions, and the outdoor environment. These variations can only be predicted probabilistically, using multidimensional risk analysis to evaluate possible parameter variations that influence moisture durability of a given building assembly in any given climate. The most efficient way to conduct comprehensive moisture durability assessments for large numbers of wall systems and variables is to leverage limited field evaluations (conducted by Building America teams) with targeted lab chamber tests, which validate and improve probabilistic simulation models. Better lab and field data, calibrated models, and expert knowledge can then be used to develop user-friendly guidance tools to help building designers make better envelope design decisions to optimize moisture management and thermal performance. For more information on the ORNL moisture-managed high performance wall design tool, see the Building Science Advisor.

In fiscal year 2017, ORNL will conduct probabilistic risk analysis on high performance wall systems by simulating the varying installation, material properties, and environmental parameters that occur in buildings. Field evaluations of the most common high-R wall assemblies will be conducted by Building America teams in the climate zones with the greatest moisture performance risk/uncertainty. These field data will be used to validate the simulation models. However, only one set of installation practices, material properties, and environmental parameters can be evaluated per year in each test home, which severely limits the ability to adequately validate the probabilistic moisture durability model. To enable a more streamlined and comprehensive development process, ORNL will complement the field evaluations with controlled laboratory tests conducted in the ORNL HAM (Heat, Air, and Moisture) chamber. Unlike traditional wall assembly experimental tests that use a hot box apparatus, the HAM chamber is capable of evaluating the simultaneous flow of heat, air, and moisture through wall systems. Assessing the impact of heat, air, and moisture simultaneously provides an experimental environment consistent with the "real world," but under tightly controlled laboratory conditions.

This project will provide quantitative risk analysis of high-R wall systems for the Building Science Advisor (BSA) tool, based on coordinated field and lab experiments, and synthesized with the knowledge-base of top building scientists in the field. Consequently, BSA users will be able to easily access credible building science expertise to make informed decisions about moisture risk management and real-world performance of high-R walls.

ORNL is developing an innovative new design tool that will put building science expert advice in the hands of every building designer who cares to manage moisture risk in high-R walls. As modern buildings become increasingly more airtight, constructed with modern labor-saving materials, and equipped with air-conditioning, they have become less forgiving to moisture intrusion. They may not dry out as easily as old, leaky structures used to, and cooler interior surface temperatures from air-conditioning increases condensation potential during many hours of the year. To remain durable, modern buildings need to be engineered to last, which requires a level of building science expertise that is not common in the housing industry. The Building Science Advisor (BSA) will change that.

The BSA will build on the success of the Building America Solution Center by providing a user-friendly online tool that takes the guesswork out of complex high performance wall design decisions. The BSA will provide building science knowledge and advice based on expert experience, field measurements, laboratory tests, and probabilistic computer simulations.

ORNL has enlisted the support of leading building science experts to begin developing a database of high performance building envelope assemblies that effectively manage moisture risks. This expert wisdom is being supplemented and extended with field and laboratory testing of relatively new envelope assemblies and material combinations, with insufficient field data available to confidently assess today. Finally, computer simulations are used to fill in the balance of the envelope systems not covered earlier. Again, the field and laboratory testing is used to validate the simulation results.

How the BSA will work: Users will be prompted to enter relevant information about the building location, design, and material selection options like cladding, structural system, and insulation. Influential factors such as climate, building air tightness, material properties, and internal moisture loads (e.g., people) are also considered to estimate and compare the moisture durability performance of several design options. From there, the user will be given two options:

• View a detailed material description of satisfactory wall designs (based on previous selections); or
• Provide a detailed description of the complete wall assembly. If the BSA deems the assembly’s performance unsatisfactory, it will provide the reasons why and suggest necessary changes to wall design to ensure more robust performance.

Using the BSA will enable users to confidently select assembly design characteristics that achieve their design goals with the least moisture durability risk. Links to design-specific guidance will also be provided to help users manage any remaining risk. This tool will further enable DOE’s Building Technologies Office to meet its long-term energy goal of a 50% reduction in building energy consumption.

For more information, visit the Building America Envelope and Advanced HVAC Research webpage.

NREL and ORNL will work together to evaluate factors that can contribute to high relative humidity in low-load homes. This research will inform development of HVAC design guidance and standards, the Building Science Advisor (ORNL) and Building America Comfort Roadmap priorities.

Occupant-related internal gains are a significant source of uncertainty when it comes to selecting space-conditioning equipment and, as the efficiency of the building thermal enclosure improves, internal gains account for an increasing percentage of the total building conditioning load. In addition, equipment controls, airflow rates, and the chosen capacity can all affect indoor relative humidity. This project will use EnergyPlus to quantify the impact that variations in internal moisture loads, and air conditioner configuration can have on indoor relative humidity and the recommended equipment capacity. The project will:

• Use stochastically derived latent and sensible internal gain profiles to determine the sensitivity of internal gains on indoor relative humidity
• Evaluate the impact that air conditioner oversizing and air flow rate have on indoor relative humidity
• Determine recommended blower-off delays to better control indoor relative humidity in low-load homes

In addition, HVAC contractors are wary of installing low-capacity equipment in low-load homes because of concern that there may be insufficient capacity to handle unique "party" and setpoint recovery loads. This project will also investigate how well appropriately-sized equipment in a low-load home can handle these atypical loads that are not accounted for by current equipment selection procedures.