Energy Modeling Compliance Path
COST-EFFECTIVE ENERGY EFFICIENT DESIGNS FOR SINGLE WYTHE MASONRY STRUCTURES
In v3.2, W Mark McGinley, PhD, PE, FASTM, explains how increasing energy code’s prescriptive (R-value tables) continuous insulation requirements does not have to exclude single wythe masonry as an option for cost-effective and energy efficient design for certain building types.
Energy Demand & Building Code
For most US climates, prescriptive building envelope requirements in energy code [ASHRAE 90.1-2010] mandate single wythe exterior masonry walls to have continuous insulation with R-values varying from 5.7 sf•°F•h/BTU to over 15 sf•°F•h/BTU. Although prescriptive thermal transmittance values (U-values) do not require continuous insulation, they do produce similar effective envelope R-value requirements and, thus, require external insulation of varying amounts. Even though masonry walls do not need to be covered to provide finish or protection, external insulation must be protected and thus covered. These coverings impact the cost of wall systems, are less durable than masonry and will produce higher maintenance costs.
Every increasing prescriptive insulation requirement often leads designers to falsely assume that building envelope high R-values are needed for the building to be considered energy efficient. Yet, increasing envelope insulation levels may have only a minimal effect on the overall building energy performance, especially for walls with a high thermal mass. Higher mass walls act to impede thermal movement through the building envelope when there are temperature fluctuations. Studies on building energy use have shown that improving efficiency of lighting, heating and cooling systems can result in a much greater reduction in energy consumption than simply increasing envelope thermal resistance, depending on building occupancy, operating schedules and climate zone.
Energy codes allow alternatives to the prescriptive path code compliance methods. The energy budget is one such method. This method more accurately predicts the impact of high thermal mass of exterior masonry walls and was used to develop more cost-effective design alternatives to simple prescriptive solutions for several building archetypes commonly constructed with single wythe concrete masonry unit (CMU) exterior wall systems.
For code compliance, the energy cost budget method requires that whole building simulations be used to verify that alternative building designs produce yearly energy costs no more than equivalent buildings configured to meet prescriptive code requirements modeled in the same cities, with the same set points, schedules, etc.
McGinley’s research found that for the building types studied—warehouse, supermarket and big box retail—holistic energy analysis and the energy budget code compliance paths produced alternative designs that did not require external insulation. They were more cost effective and energy efficient than designs developed from code prescriptive requirement configurations.
For details about the research and achieving code compliance with single wythe masonry in all climate zones, read more on pages 32-35 in SMART | dynamics of masonry v3.2 or online at dynamicsofmasonry.com/archives.