Introduction

Dynamic Thermal Performance and Energy Benefits of Using Massive Walls in Residential Buildings

INTRODUCTION:

Utilization of thermal mass in buildings can be one of the most effective ways of reducing building heating and cooling loads. Several massive modern building envelope technologies (masonry and concrete systems) have found their application in buildings in the last decade. They suffer from the lack of an accepted measure of their thermal performance. The steady-state R-value traditionally used as a wall thermal performance measure does not reflect the dynamic thermal performance of massive building envelope systems. To show the benefit of these systems, thermal performance analysis has to incorporate thermal mass effects.

A new measure of the wall thermal dynamic performance is proposed in this paper - Dynamic Benefit for Massive Systems (DBMS).The thermal mass benefit is a function of the material configuration and climate conditions. DBMS values are obtained by comparison of the thermal performance of the massive walls and light-weight wood frame walls. The product of DBMS and steady-state R-value is called “R-value Equivalent for Massive Systems.” It enables comparisons of massive walls. It does not have a physical meaning. It should be understood only as an answer to the question:

“What wall R-value should a house with wood frame walls have to obtain the same space heating and cooling loads as a similar house containing massive walls?”

The dynamic thermal performances for over twenty multilayer and homogenous wall material configurations were analyzed using thermal performance comparisons of massive walls and light-weight wood-frame walls. A one-story ranch-type house was used for these comparisons and they were performed using DOE-2.1E, a whole building energy computer code.

Application of the newly developed equivalent wall theory enabled whole building dynamic energy analysis for complex three-dimensional wall material configurations. Normally, complex building envelope components cannot be accurately analyzed using one-dimensional computer models like DOE-2. Typically, thermal modelers have to use simplified one-dimensional descriptions of complex walls. This significantly reduces the computer modeling accuracy because of the complicated 2 and 3-dimensional heat transfer which can be observed in most wall assemblies.

In this work, response factors, heat capacity, and R-value were computed for complex walls using finite difference computer modeling. They enabled a calculation of the wall thermal structure factors and estimation of the simplified one-dimensional equivalent wall configuration. Thermal structure factors reflect the thermal mass heat storage characteristics of the wall assembly. The equivalent wall has a simple multilayer structure and the same thermal properties as the complex wall. The equivalent wall and complex wall dynamic thermal behaviors are identical. The thermal and physical properties describing the equivalent wall can be used, very simply, in whole building energy simulation programs with hourly time steps. These whole building simulation programs require simple one-dimensional descriptions of the building envelope components. In this work, DOE-2.1E was used to calculate heating and cooling loads for six U.S. climates.