Energy Performance

Energy Performance

One of the reasons ESCS lightweight aggregate is considered a sustainable building material is because it optimizes the energy performance of concrete masonry. Improving the energy performance is an important part of a “green” concrete masonry building as it affects operating costs and occupant comfort for the life of a building.

Energy Efficiency: The following discussion taken from PCA Concrete Homes Newsletter (Nov/Dec 2009) covers this subject.

“Energy efficiency is a complex topic, and it’s important to look at all aspects that impact performance of a wall system. For instance, R-values are a quick way to rate insulation, but there are other properties of concrete masonry that contribute to energy savings. ASHRAE 90.1, the Energy Standard for Buildings Except Low-Rise Residential Buildings (2007), is able to consider the factors affecting energy use, three main ones being:

• Thermal mass
• Thermal bridging
• Insulation (thickness, rating, and location within the wall section).

Thermal Mass. Thermal mass can be described as the ability of a wall to store heat. One reason masonry (at any density) is effective with lower energy input is “thermal lag” which is sometimes referred to as the “flywheel effect.” [See figure below] As outdoor temperatures fluctuate, masonry
is slow to respond to changes. When air temperatures rise, masonry remains cooler. Alternately,
with dropping temperatures, masonry doesn’t cool off quickly. This allows HVAC systems to be sized smaller yet still heat and cool effectively, providing comfortable interior temperatures while saving energy.

Thermal Inertia. A concept related to thermal mass that’s a little less obvious is called thermal inertia. Thermal inertia is the combined effect of heat storage capacity within the wall and the thermal resistance to movement of heat through the wall. In concrete masonry, higher concrete densities increase thermal mass, but lower the wall’s thermal resistance. It is the combination of the two factors –thermal inertia– that determines how the wall contributes to building thermal performance in a dynamic environment. Even though lightweight masonry walls store less total heat, compared to normal weight masonry walls of the same thickness, they also release it more slowly, which can improve overall building thermal performance.

Even lightweight CMU provide substantial thermal mass compared to truly “lightweight” wall systems such as wood frame or steel stud. IECC 2009 contains exceptions to its R-value requirements, allowing lower R-values for above-grade mass walls compared to frame walls in commercial buildings. (International Energy Conservation Code (IECC) 2009, International Code Council Inc., Country Club Hills, IL, 2009, www.ICCsafe.org.) IECC 2009 recognizes that lightweight CMU have adequate weight to provide the required thermal mass to qualify for the masonry mass wall exceptions. And of course they provide improved R-values compared to normal weight masonry.

Thermal Bridging. Bridging can be thought of as the “path of least resistance.” If a CMU contains insulation in its cores but the concrete is very dense, the webs of the block can still act as thermal bridges. The Expanded Shale Clay and Slate Institute has termed their lightweight block walls as SmartWall Systems® to indicate that this approach balances thermal mass with low thermal bridging, thereby maximizing effectiveness of the core insulation. This leads to optimum energy performance.

Using ASHRAE as the criteria, a simple example shows that a 12-in. SmartWall System® with perlite insulation in the cores outperforms a steel stud wall with R-19 batt insulation. And a single-wythe masonry wall is a simpler system to construct than a steel stud frame wall with batt insulation, veneer, and an interior finish.

Insulation. As energy considerations become more and more of a factor, all wall systems will be required to contain increased insulation. The strategy is certainly beneficial. But depending on when and how this is enacted, it’s already possible to build energy-efficient masonry construction with current products.

In Summary

Lightweight block satisfies mandatory criteria for building structural walls, such as strength to support live and dead loads, and provides inherent fire resistance. Yet the lightweight aggregate enhances energy performance of the system and reduces costs associated with transportation, handling, and labor—all of which can contribute to increased sustainability. The reduced weight of the block offers other advantages as well. It has great sound absorption characteristics, low shrinkage behavior (which may lead to fewer cracks in the wall), and a reduced seismic loading, which can be beneficial where earthquake forces are considered in design.”

Thermal lag is sometimes called the “flywheel effect” because it offsets when the peaks and valleys occur, and reduces their magnitude. This is how masonry saves energy for heating and cooling.

Embodied Energy Payback Example

The embodied energy to manufacture rotary kiln structural lightweight aggregate includes mining, manufacturing, and transporting the material to the jobsite, or building product manufacturer. The cost of this embodied energy is often paid back in a very short period of time, because of the increased thermal performance, lower transportation costs, and reduction of labor costs associated with the building elements. For example, the following embodied energy payback using expanded shale, clay and slate in concrete masonry is less than one year.

Energy savings result from using ESCS aggregate in a typical lightweight concrete masonry unit, compared to using heavy normalweight aggregate for the same concrete masonry unit. These calculations assume that masonry is used in single wythe integrally insulated exterior building walls, which is a typical application.

The ESCS production BTU input and 1350 lb cu yd average density is per the Life Cycle Inventory analysis of ESCS performed for the Expanded Shale, Clay and Slate Institute by CTL (Construction Technology Laboratories.) Reference their February 17, 2000 report.

2,300,000 Btu / Ton to manufacture ESCS lightweight aggregate or 1150 Btu / lb.

A typical mix design for 8″ Lightweight cmu meeting ESCSI’s SmartWall specification density (93 pcf) and strength

This mix is expected to yield 75 8x8x16″ CMU with a wet weight of 25.5 lb and a cured weight of 24.0 lb. Each CMU has 15.1 lb of ESCS aggregate in it based on the expected yield.

This translates to 17,365 Btu per Block.

The energy saved in use in an exterior wall is documented in ESCSI Information Sheet #3530.2 for a “big box” building in Omaha, Nebraska. The difference in wall conductivity values between a 93 pcf lightweight CMU and a heavy normalweight 135 pcf CMU is shown as 0.157; using this value in the calculations the energy saving by using the lighter 93 pcf CMU is 20,769 Btu / Block / Yr.

Calculate the payback period by dividing the one time energy input to the ESCS aggregate in the lightweight CMU by the annual energy savings per block:17,365 Btu / Block divided by 20,769 Btu / Block / Yr and the pay back is 0.84 years or less then one year.

This example is conservative, as it does not include added savings in trucking cost, handling, lower dead loads etc. In other words life cycle energy savings realized from using ESCS will help to conserve valuable natural resources for future generations.

Thermal Performance Payback Example

Lowering the concrete density increases thermal resistance. For example concrete at 90 lb/ft3 has an R-value of 0.26/inch where the R-value for 135 lb/ft3 concrete is approximately .10/inch. In other words the 90 lb/ft3 concrete have a 260% better insulation factor then the 135 lb/ft3 material (ESCSI info sheet 3201, 1999). Also see the following table below:

Table 13.2

Source: National Concrete Masonry Association NCMA TEK 6-2A: R-Values for Single Wythe Concrete Masonry Walls

Henderson Engineering, Inc., Kansas City, MO, performed an energy cost study on a “big box retail” building for the Expanded Shale, Clay and Slate Institute to determine how lightweight concrete masonry at 90 lb/ft3 affected the LEED category EA1 when compared to normalweight concrete masonry at 135 lb/ft3. Several locations were evaluated with results for Omaha, NE (a central location) listed as follows:

Heating peak loads for exterior walls was 44% less.
Cooling peak loads for exterior walls was 51% less.
Total building heating peak load was 12% less.
Total building cooling peak load was 2% less.
Total building annual energy consumption was 2.2% less.

These savings translate into 5.5 cents per block per year. That savings is significant and extends over the life of the structure. This life cycle savings per block is many, many times greater than the potential higher first cost of the block. The peak load savings allow for smaller, more economical HVAC equipment to be used in many cases. This in turn lowers initial equipment cost and weight, and reduces peak demand on utility infrastructure. The annual energy consumption savings was calculated using flat rate energy cost. Additional benefits will result when using off-peak utility rates that are a consequence of longer time lags made possible with lightweight concrete.

Total Embodied energy

It is well documented that the total embodied energy to build a building is only 1 to 3% of the total occupant energy used by that building over its useful life (Construction Technology Laboratories report project no. 180028 conducted for ESCSI 2001). In light of the facts that approximately 97 to 99% of the energy used throughout the building life cycle is primarily a function of climate and occupant behavior, it becomes obvious that our biggest energy resource is efficiency.

Sustainability
Transportation Savings
Workforce Sustainability – Ergonomics