By Claude Goguen, P.E., LEED AP
We spend a lot of time in buildings. The Environmental Protection Agency estimates more than 90% of the average day is spent indoors.1 Most of us wake up in a building and go to other buildings for work, school or shopping. We continue in and out of buildings all day long, never stopping to contemplate the comfort provided by building enclosures.
These partitions provide an efficient barrier between the serene and comfortable inside and the sometimes chaotic and inclement outside. Building enclosures are designed to be aesthetically pleasing while providing security, safety and comfort to occupants. That includes keeping moisture out and possessing thermal properties that help maintain a comfortable climate inside regardless of conditions outside. That can be a tall order depending on climate, location, orientation and other factors.
As the green building trend continues to grow, the methods and materials used to manufacture these building enclosures have become a primary aspect of energy-efficient design. Romans long ago recognized concrete as an excellent building material not only for its durability, but also for its ability to absorb, store and release ambient heat to even out interior temperatures. Thousands of years later, for these same reasons and more, concrete continues to be a primary material for building construction in today’s age of sustainable building.
Precast wall types
There are myriad precast wall systems in North America, but most of the them fall into three main categories: solid wall, thin wall or sandwich panels. They can be an architectural veneer, a structural wall or a combination of both. They can also be conventionally reinforced or prestressed and come in a wide variety of finishes and colors.
Solid walls are simply wall sections of solid concrete and reinforcing. Thin wall panels are thinner sections with a framing system attached. These are often used for veneers and can be attached to a concrete or steel structural system. Some of these thin wall panels will have insulation installed at the manufacturing facility so utilities can begin work on-site immediately.
The sandwich panel, or insulated precast concrete wall panel, has two layers of concrete separated by a layer of rigid insulation. Wythe connectors are installed so that they connect both layers of concrete through the rigid insulation. This helps ensure all layers will act as a monolithic system when exposed to loads. Concrete wythes can vary in thickness depending on the structural and architectural requirements of the project. Typical wythe thicknesses range from 2 inches to 6 inches. Wythe connector materials include carbon steel, carbon fiber and plastic.
Each type of wall section will perform differently. However, they are all precast concrete which means they possess inherent qualities when it comes to thermal- and moisture-related characteristics.
Thermal mass and concrete
Concrete is a dense material. This density, along with concrete’s specific heat, gives it a high thermal mass. Thermal mass is defined as a material’s ability to store heat energy. Precast concrete walls and roofs absorb heat, keeping the interior of a building cooler throughout the day. That heat is then released at night when temperatures are cooler. Think of it as a thermal battery that recharges every day and releases at night (see Figure 1). Also, thermal mass on the interior building surface will help absorb heat gains in the interior space. Thermal mass is useful, especially in areas with higher differences in daytime and nighttime temperatures, to dampen the fluctuations of indoor temperatures. This results in a lower demand on HVAC systems and consequently can save money on equipment and energy bills.
Insulation and concrete
While precast concrete excels in thermal mass, it is less effective as an insulator. A material’s ability to have insulative qualities depend on its thermal resistance, or its ability to resist heat flow.<
We measure this as R-value. Precast concrete has low R-values which vary based on the concrete’s density. For a concrete density of 150 pounds per cubic foot, the R-value is approximately 0.7 per inch. As concrete density decreases, the R-value increases. However, if rigid insulation is added, these numbers go up dramatically. The types of rigid insulation generally used with precast concrete wall panels are:
- Expanded Polystyrene: R-values typically 3.8-to-4.4 per inch
- Extruded Polystyrene: R-values typically around 5 per inch
- Polyisocyanurate: R-values typically 6-to-8 per inch
A precast concrete building is good at regulating its own temperature. When rigid insulation is added to precast panels, such as sandwich panels or thin wall panels, it creates an ideal building envelope that provides high R-values while regulating temperature fluctuations.
American Society of Heating, Refrigerating and Air Conditioning Engineers Standard 90.1, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” is an international standard that provides minimum requirements for energy-efficient designs for buildings. The standard acknowledges the benefits of concrete walls.2
Watching for thermal bridges
Precast concrete thermal properties will remain constant if the section characteristics are constant. Thermal bridging refers to areas in the precast concrete section where the effective thermal R-value is reduced due to a change in section. This could be because of a gap in insulation due to a stud or wythe connector. This can be minimized by using continuous insulation and, when necessary, low conductivity wythe connectors.
Thermal imaging (see Figure 2) is a useful tool for evaluating the performance of building enclosures. A well-constructed precast concrete building will show very few signs of thermal bridging.
The infiltration of moisture into a finished building is one of the biggest challenges. Some wall assemblies incorporate many layers of different materials, including an air gap where a leak can form or condensation can take place. Moisture forming on the interior of a building is unsightly and can cause damage to the building and its contents, with the damage sometimes hidden from view for extended periods of time. Moisture accumulation can cause mold to form, wood to rot and metal to corrode. These are three things precast concrete won’t do. Mold needs certain conditions to grow and since precast concrete is inorganic, it will not enable mold to form and grow like it does on wood and Sheetrock. Precast concrete will not rot or corrode in the presence of moisture, but this may be a moot point since moisture is less likely to be present with precast concrete enclosure systems.
Precast concrete’s permeability, or the speed at which liquids and gases can penetrate it, depends primarily on its porosity. The porosity of concrete is a function of the quality of the mix and the manufacturing and curing process. Since precast concrete is manufactured in a controlled environment, quality control is enhanced and porosity is minimized, making the wall virtually impenetrable by moisture.
The key to managing moisture on a building project is to get it enclosed quickly. Precast concrete enclosures can be erected rapidly, reducing the exposure of the building’s interior to humidity and moisture. Also, precast wall assemblies are panelized, which means fewer joints than other types of enclosures. This means less points of potential moisture penetration.
The construction team will benefit from certain attributes of precast concrete building enclosure systems such as ease of installation. Following construction, air quality, comfort and energy efficiency are benefits occupants and owners of precast concrete buildings get to enjoy. Add in the inherent durability and resiliency of precast and all will enjoy a quality precast concrete enclosure for decades to come.
Claude Goguen, P.E., LEED AP, is NPCA’s director of sustainability and technical education.
1. U.S. Environmental Protection Agency. 1989. Report to Congress on indoor air quality: Volume 2. EPA/400/1-89/001C. Washington, DC.
2. ASHRAE 90.1 – 2016, Energy Standard for Buildings Except Low-Rise Residential Buildings, 2016. Atlanta, GA