By Jody Wall, P.E., LEED AP, Carolina Stalite Company
Concrete masonry units (CMU) had not changed much since the original ASTM C90 Standard Specification for Hollow Load bearing Concrete Masonry Units was published in 1931. ASTM C90 – 2011b now allows for, but does not require, a unit’s webs to be configured differently. As a result, CMU designs may be altered in order to utilize fewer raw materials in production, reduce unit weight and/or increase energy efficiency of concrete masonry construction. ASTM C90 changed from requiring a minimum web thickness to requiring a minimum web area of 6.5 square inches per square foot of face area. No longer are manufacturers required to have three 1 inch thick webs on 8 inch block; instead an 8x8x16 block could have one web as thin as 0.75 inch and still meet ASTM C90’s new minimum web area requirements.
This reduced web area can greatly increase the R-value of concrete masonry wall systems. The R-value is a measure of thermal resistance used in the building and construction industry. Under uniform conditions it is the ratio of the temperature difference across an insulator and the heat flux (heat transfer per unit area per unit time). The key to maximizing the R-value of concrete masonry is minimizing the thermal bridges within the masonry and increasing the thermal resistance of the materials used to produce the concrete masonry unit.
The need to increase the thermal resistance of the materials used to produce the concrete masonry unit is where lightweight aggregate plays a huge role. The thermal resistance of lightweight aggregate is almost 3 times that of typical normal weight aggregates. This increase in thermal resistance yields much higher R-values in the New C90 configurations than can be achieved with normal weight aggregates. The table below shows the comparison between normal weight and lightweight aggregates R-values.
As you can see, the new C90 shape can improve the R-value by 66% and 80% compared to the typical normal weight block. But the use of lightweight aggregate increases the R-values from the 66% to 226% for 8 inch units and from 80% to 264% for the 12 inch units. This increase in R-value is directly related to the higher thermal resistance of the lightweight aggregate used.
The higher R-values now achievable using the combination of the New C90 and lightweight aggregates makes compliance with energy codes more achievable, and will save the owner energy use dollars through the life of the building. Whether you are using ComCheck or whole building analysis the higher R-values are a benefit. In ComCheck, often building envelopes that would not meet the energy code using typical units can comply using the New C90 units made with lightweight aggregates. Even in buildings where the New C90 units made with lightweight aggregate do not in themselves bring the building envelope into compliance, they can minimize the trade-offs required to bring the building envelope into compliance.
There have been several publications highlighting how increased R-values save energy dollars over the life of a building. In a study published in ESCSI information sheet 3530, performance testing showed that the use of 90 pcf masonry units can save $1.15 per block over the first 10 years of a building and $2.86 over a 30 year time period. The energy use analysis compared 90 pcf lightweight masonry units to 135 pcf concrete masonry units in an example building design in Omaha, Nebraska. The use of lightweight aggregates in the New C90 units along with the improved energy performance of the web configuration will yield even greater benefits.