Increasing Energy-Efficiency with Lightweight Concrete Masonry Walls

Today, one of the design community’s major focuses is to lower the amount of energy used to heat, cool and operate buildings. To reduce this consumption, building teams often attempt to bolster thermal efficiency by packing more insulation in exterior walls, ceilings and floors. This means that if a wall is required to be R30, professionals may think that by adding more insulation to make it an R40 they will save more energy.  However, it is not a one to one proportional improvement. The Law of Diminishing Returns suggests that increasing the thickness (and additional R-value) of the same insulation only negligibly reduces heat loss. And while this does not deliver the desired results, it does incur significant financial and environmental costs.

To increase thermal resistance within a wall assembly, building teams can combine insulating techniques. For example, wall construction with lightweight concrete masonry unit (CMU) and high-performance insulation can be effective. When made with lightweight aggregate like expanded shale, clay or slate (ESCS), lightweight CMU are the concrete world’s version of a high-efficiency, energy-saving concrete product. The concrete in lightweight concrete masonry unit has roughly three times more resistance to heat flow than the regular concrete in a ‘normalweight’ CMU. This is one of the reasons why some say lightweight CMU qualifies as the concrete world’s version of insulation.

Concrete Masonry Construction with Next-Generation Lightweight CMU

Other than concrete density and the presence and placement of insulation, many factors influence the thermal performance of a concrete masonry wall. These include grouting, unit density and configuration of the webs and face shells. When lightweight concrete is used to make today’s Next-Generation CMU, the resulting wall assembly will be more energy efficient. Next-Generation CMU feature thinner and fewer webs —two-web units either in an ‘H-Block’ or ‘A-Block’ configuration.

Because CMU cross webs provide the direct means of transferring heat, making CMU with thinner and fewer webs will reduce thermal bridging, which results in higher CMU R-Values. This is achieved by today’s Next-Generation CMU. Adding in the insulating qualities of lightweight concrete Next-Generation LWT CMU will have a significant and direct impact on energy savings.

Compounding Energy Savings in the Masonry Wall

When today’s ‘Next Generation’ CMU are insulated with injection foam, which completely fills all the hollow cores of the wall and the tiny nooks and crannies between units, the net effect is a wall assembly with 2-3 times higher R-Values than what is possible with normalweight CMU. (See Table 1)

As a result, when the entire lifespan of a building is considered, an insulated lightweight CMU wall has a higher R-value and saves significant amounts of heating and cooling energy. So much energy is saved that even though a normalweight CMU’s environmental product declaration (EPD) is better than a lightweight CMU’s EPD, the cumulative energy saved with insulated lightweight CMU creates a net positive impact for a building.

_{Table 1— Comparison of 12″ Concrete Masonry Assembly R-Values (hr-ft2-°F/BTU) (Source: Smaller Webs = Higher R-Values by Jason Thompson, Masonry Edge / Story Pole Magazine, Vol. 7, No.2)}

Looking at the Bigger Picture

In addition to enabling more energy-efficient wall assemblies, Next-Generation CMU offers multiple advantages for those involved in its production and installation. Fewer webs allow producers to manufacture block with less material. It is also lighter in weight, which makes it possible to transport more blocks in fewer truckloads. Furthermore, its lightweight properties and easy-to-maneuver design help masonry contractors with faster construction and reduces the risk of injury and fatigue. Learn more about the benefits of lightweight CMU for mason contractors in ESCSI’s latest article.