National Bureau of Standards Test Confirm Energy Conserving ‘Thermal Mass Effect’ for Heavy (Log) Walls in Residential Construction

Summary of Test Findings

A study was conducted by the National Bureau of Standards (NBS) for the Department of Housing and Urban Development (HUD) and the Department of Energy (DOE) to determine the effects of thermal mass (the bulk of solid wood log walls, or brick and block walls) on a building’s energy consumption. For the test, six 20’x20′ test buildings were built on the grounds of the National Bureau of Standards, 20 miles north of Washington, DC, in the fall of 1980. Each structure was identical except for construction of its exterior walls. The buildings were maintained at the same temperature levels throughout the 28 week test period between 1981 and 1982. Energy consumption of each structure was precisely recorded by NBS technicians during this entire period.

Test Results

During the three week spring heating period, the log building used 46% less heating energy than the insulated wood frame building. During the eleven week summer cooling period, the log building used 24% less cooling energy than the insulated wood frame building. During the fourteen week winter heating period, the log building and the insulated wood frame building used virtually the same amounts of heating energy. The National Bureau of Standards technicians conducting the test calculated the R-value of the log building, which was constructed with a 7″ solid square log, at a nominal R-10. It rates the insulated wood frame building, with its 2’x4′ wall and 3-1/2″ of fiberglass insulation, at a nominal R-12, thus giving the wood frame structure a 17% higher R-value. Yet during the entire 28 week, three season test cycle, both buildings used virtually identical amounts of energy. This led the National Bureau of Standards to conclude that the thermal mass of log walls is an energy conserving feature in residential construction.

NBS Tests Confirm Energy-Conserving “Thermal Mass Effect” of Log Walls Full Report

In the first extensive field testing of its kind, researchers at the Commerce Department’s National Bureau of Standards (NBS) have confirmed that walls of heavyweight construction (such as those built with solid wood logs, concrete block or brick) exhibit an energy conserving “mass effect” in residential buildings during the summer and the intermediate heating season representative of fall or spring in a moderate climate. However, no mass effect was observed during the winter heating season. According to NBS researchers, these extensive field tests should help resolve a controversy over whether residences having heavyweight walls consume less energy for space heating and cooling than buildings having lightweight walls of equivalent thermal resistance. The National Bureau of Standards research team found that the heavyweight walls (including building number 5, the log structure) “did exhibit a thermal mass effect and thus save significant amounts of energy both in the summer cooling season and the intermediate heating season representative of fall or spring in this (Washington, DC) area.”

The Use of R-Values

Most state and local building codes require specific “R-Values,” or thermal resistance values, for the walls, ceilings, and floors of houses. The R-Values in these codes vary with geographical location and climate considerations. The Building Systems Councils’ technical staff and other industry professionals have often challenged the exclusive reliance on R-Values alone to rate the energy efficiency of a wall’s building materials while ignoring the thermal mass effect inherent in heavyweight (log) walls. R-Values are recognized by most professionals to be a reliable indication of the thermal performance of a material–under conditions of constant interior and exterior temperatures. The Building Systems Councils’ technical staff argues that these are not the conditions that exist in the “real world,” where outdoor temperatures vary widely during a typical day-night cycle. To obtain a true rating of building’s thermal efficiency in these conditions, building codes must also consider the “mass effect” of heavyweight (log) walls.

What Is “Mass Effect”?

According to NBS researchers, “the mass effect relates to the phenomenon in which heat transfer through the walls of a building is delayed by the high heat (retention) capacity of the wall mass. Consequently, the demand for heating or cooling energy to maintain indoor temperature may, under some circumstances, be pushed back until a time when wall heat transfer and equipment operating conditions are most favorable.” This heat retention phenomenon is also referred to as “thermal capacitance” or time lag–the resistance of a material (such as solid wood walls) over time to allow a change in temperature to go from one side to the other.

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