Article No: 186

2006-06-23 09:11:02
Tech Talk - Energy performance of RCF concrete homes Part I

Concrete homes have numerous benefits. They are quiet, low-maintenance, durable, and resistant to storms, pests, and abuse. Perhaps one of the most underrated benefits of concrete homes, however, is their energy performance. Performance of concrete homes is not just a factor of insulation. It is also enhanced by the effect of thermal mass, lower infiltration rates, and reductions in size (and cost) of mechanical equipment. This article will discuss insulation systems for RCF (Removable Concrete Form) houses, the benefits of thermal mass, and systems for insulating these structures. The next issue will discuss infiltration, air quality, moisture control, and mechanical equipment.

Thermal mass
Why is thermal mass beneficial? Thermal mass is directly related to the weight and density of concrete and it contributes substantially the energy performance of a structure through its heat-storage properties. The magnitude of the actual impact depends on the climate in which the structure is located, orientation and glazing, the thickness of the concrete, and the position of the concrete relative to the insulation and conditioned space. Almost all materials have some heat storage capacity, but the combination of high capacity, availability, strength, low cost, and formability makes concrete a great choice when compared to other building materials.

Several studies have documented the impact of concrete homes on energy consumption. The Department of Energy’s Oak Ridge labs has developed models for calculating the impact of thermal mass on a structure. It is often expressed in equivalent R-Values with the mass of the wall delaying heat transfer though the insulation. In some moderate climates, where it gets too hot during the day and too cold at night, a structure with the appropriate amount of thermal mass can perform admirably without insulation.

The Portland Cement Association produced a tool available on a reference CD called “Thermal Mass Comparison of Wall Systems” which takes a 2,450-square-foot home with 12 types of wall systems (two frame construction and 10 different mass walls) and models them at 25 locations across the United States and Canada. The analysis also includes modeling of infiltration rates and calculation of the impact of mass walls on mechanical equipment sizing. The study model uses the DOE 2.1-E calculation engine.

Another good source of information for assessing performance of concrete and other high thermal mass housing systems is the “Thermal Mass Handbook, Concrete and Masonry Provisions Using ASHRAE/IES 90.1-1989.” It covers a wide variety of insulated and uninsulated concrete wall assemblies.

There is considerable research on thermal mass and insulation performance that has been undertaken since the first ASHRAE energy standard was developed. Consistently these reports demonstrate the effectiveness of thermal mass to enhance the material R-value of the purchased assembly. As an example, research conducted a Oak Ridge National Laboratory (ORNL) documented a structure with 6 inches of concrete on the interior side of 2 inches (R10) of rigid extruded polystyrene built in a range of regions. For instance, in Ft. Worth, Texas, this wall design achieves actual energy performance ratings (DMBS) equivalent to wood frame walls with R30+ insulation. The chart below shows the impact of this wall in different regions of the country as an excerpt from this research.

Graph provided by Oak Ridge National Labs and Polish Academy of Sciences, 2001

Insulation systems
The position of the thermal mass relative to the insulation is important with regard to performance. If the insulation is positioned on the inside face of the wall, the contribution of the mass is considerably reduced because the mass is isolated from the space to be conditioned. Systems that position the insulation on the exterior or sandwich it between two layers of concrete are best with respect to improved performance attributed to thermal mass. The systems must be designed to eliminate steel and concrete traversing the insulation layer as these will compromise the insulation layer. 

Homes with significant amounts of thermal mass positioned inside the insulation recover quickly if the conditioned air escapes from the interior. The heat energy stored in the mass walls and/or floors quickly reheats the interior space. It’s as though the entire structure is a high-capacity, low-level, heat-emitting radiator. A 4-inch thickness of concrete is considered optimal. Thicker walls will still work but they will not reach as high a temperature since the amount of mass that needs to be heated is greater.

Exterior insulation
Systems which utilize exterior insulation must have a finish material applied to, or over, the exterior of the insulation. This material will usually double as a weather-resistant finish for the exterior and a protection system for the insulation. Stucco or other cement-based systems can be applied directly to the exterior while traditional systems, such as wood or synthetic siding, are attached through an attachment fin or plate cast into the wall or positioned at the outside face of the insulation.
The interior of this type of system is typically coated with a skim coat of plaster and painted. Electrical wiring is enclosed in conduit cast into the walls.

The exterior surface of sandwich wall systems can be cast-in-place concrete (texture determined by the form or form liner), cast-in elements (such as thin brick) or a post-applied system of stucco, lap siding, or any other traditional exterior finish material. The interior is typically a skim coat of plaster with paint. Most sandwich walls consist of two 4-inch layers of concrete with insulation thicknesses ranging from 2 to 4 inches sandwiched between. The two layers of concrete are held together with non-conductive structural composite ties. Finishes for concrete housing systems will be the subject of a later article in this series.

This technology is here and it is proven. The contractors who have been building foundations for decades don’t need to learn any new skills or purchase new equipment. They simply apply their expertise to above grade structures.