Article No: 227

2008-01-28 17:56:03
Tech Talk: Concrete Homes: Selections for Conditioning
By: Jim Baty, CFA Technical Director


Provided by the U.S. Department of Energy, this chart provides the areas of a standard framed home where energy is consumed.  Air-infiltration accounts for 36 percent  not counting losses associated with the framed envelope which adds another 31 percent.  Combined, 67 percent  of a home’s energy loss occurs through the decision to build a framed home. 
 
Part VI:  This six-part series focuses on details of today’s concrete homes. Over the past two years, this column has provided considerable information on the general benefits of concrete homes, the various methods used for construction and the performance characteristics. These articles provide focus on the decisions, details and results that can affect the quality achieved in the above-grade concrete home industry delivered by removable concrete forms (RCFs). Since strongly entering the market nearly a decade ago, this method of construction offers an ever-expanding variety of architectural and practical construction solutions for today’s homeowner and designer. This final article in the series summarizes the key areas of conditioning for the home that require considerable thought and follow-through to assure effective and efficient operation. 

A primary reason so many customers are interested in concrete homes today is the escalation in utility costs. We feel the impact of rising energy costs throughout our environment, but perhaps these are most evident in the built environment. Although gasoline for transportation receives the national media’s attention, it is the doubling, tripling and even more that occurs in the monthly utility bill that impacts Americans’ cash flow the most.

Energy efficiency begins with mass

Misconception No.1: Simply increasing the amount of insulation present between the studs will not help a home feel like it has a constant temperature from one end to the other.

Energy efficiency for the homeowner can be understood from different perspectives. Certainly, efficiency is an important factor to homeowners who buy RCF houses. RCF homes have specific, measurable differences from wood frame homes that affect the comfort of the people inside. RCF homeowners quickly note that their houses have a much more even temperature—more even over the course of a day and more even from place to place all around the house. This is a direct result of the physical trait known as “thermal mass.”

It is important to understand that R-value is not the only measure of energy efficiency. The traditional measurement of R-value, thermal resistance, is the resistance of a plane of material or a sandwich of different materials to the conduction of heat from one side to the other. The problem with R-value is that it does not consider temperature changes over the course of a long span of time. In the equation above, time is a constant measurement (often one hour). Since temperature changes happen throughout a 24-hour day, it is important to take time into consideration. This is where the great mass of concrete becomes important. Below is an explanation of the difference between thermal mass, and thermal transfer. Understanding these two characteristics and how they work in unison can help you, and the customer, understand the efficiency of an RCF home. 

Energy efficiency is maintained
without thermal transfer

Misconception No. 2: Concrete is cold and damp because it pulls energy from the room.
A concrete house retains energy as described above. Lighter materials cannot retain that energy for an extended time; however, many of the ecological attempts to build sensitive housing also fall short due to the opposite physical characteristic known as “thermal transfer.” Thermal transfer is how quickly energy (heat or cold) can be transferred from one side of the wall to the other. This is known as the material U-value (like windows), which is the inverse of the R-value. It is true that concrete has a relatively high U-value, along with wood and steel.

The insulation of a framed wall exists between the studs. However, everywhere there is a stud, there is no insulation. In fact, if you add up the area of the typical frame wall that is wood—and un-insulated—it is about 25 percent of the total area. When you’re near a large un-insulated area in the winter you are inclined to feel a chill—this is what some people call a cold spot. You may also feel uncomfortably warm near one of these spots in the summer. This kind of thing can occur at a corner or around a window or door. These places often have more wood and less insulation due to the structural requirements.

RCF housing gains significant advantage in a comparison of thermal transfer due to the ability to separate structure from insulation.  Several system examples of this have been described in this article demonstrating how effective the design and construction decision for an RCF can be at controlling thermal transfer and delivering thermal mass. So, because the insulation is continuous throughout an RCF wall, there are fewer hot and cold spots.

Energy efficiency goes out
the window—air infiltration

Misconception No. 3: The total construction of framed walls is effective at controlling air movement and the affected changes in temperature.

The addition of the surface area for studs certainly reduces the amount of insulation continuity for the envelope of a home. However, the most significant impact of the interfacing of studs, sheathing, and insulation is the increased volume of air that can pass through the wall. Adding further benefit to the RCF home is the wall/deck construction that nearly eliminates air infiltration. When all walls, or walls and decks, are poured together, this is called a monolithic pour. This way of building the shell makes for minimal air infiltration. In comparison to frame wall homes, which are made from hundreds of unique parts nailed together, the air changes per hour (ACH) for RCF walls is far lower. Typical concrete walls measure between 0.25 and 0.33 ACH. This is about half of the rate for walls in new frame houses.

Stud walls never fit together perfectly. As a result, the typical wood frame wall has thousands of tiny gaps. Over time the wood expands and contracts over and over again because of heat and humidity, causing the gaps to increase in size.

Air comes through, especially when there is wind. This air infiltration creates higher conditioning loads on the structure.
RCF walls are poured monolithically, and almost never have significant gaps. When the concrete is poured into the forms, it fills all gaps, sealing the wall. Air can still get through at other points like through the roof or around the doors and windows, but the walls themselves are nearly eliminated as a source of drafts. In cases where there is a concrete deck, then the air infiltration is minimized even more.

Indoor air quality suffers
when energy efficiency fails

Misconception No. 4: The mechanical system for your home can be purchased without regard to the way the home is constructed.

Air quality is tied to many current health issues. Pollution, allergens, and other products in the outside air cause many people to become ill, or develop hypersensitivity to allergens. One of the best ways to limit this is to improve indoor air quality control.

The trend in residential construction since the 1970s has been making airtight buildings to keep out unwanted pollutants and moisture. As mentioned in the above segment, it is much easier to achieve low air infiltration when building with removable concrete forms, which inherently produce a much tighter building envelope. The measuring of air infiltration or ACH tests how long it takes to completely re-circulate the volume of air in the home. If the home is well sealed, with few places for air to seep in or out, it will have a lower ACH.

Even when building with RCFs, special attention has to be paid to the details and connections between the different building parts. The main connection that is a concern is that between the walls and the roof. As mentioned in the earlier section, the exception is a concrete roof deck as part of the monolithic pour, because it seals the building envelope even more.

The result of a well-sealed building is the reduction of pollutants coming into the home. However, if the building is very tightly sealed, changes have to be made in the HVAC system to introduce proper amounts of fresh air and regulate humidity levels. This can be easily accomplished with a commercially available air-to-air heat exchanger. This part of an HVAC system allows a specific amount of fresh air to be introduced into the home. The temperature of this air is controlled as it comes through the exchanger. A minimum amount of energy is used to raise or lower it to indoor temperature because incoming air is cycled past the outgoing exhaust air. The exchanger brings just the right amount of fresh air in to the home, at a controlled temperature, and also controls interior humidity, by dehumidifying air as it comes in from the outside. Condensed water is drained as it enters the home, rather than condensing on walls or making the house feel “muggy.”

Planning is your key to energy efficiency
Though the best performance is gained by having a total concrete shell, the homeowner still enjoys significant benefit with just the walls built of concrete.  If the building’s shell is completely made of concrete, and air infiltration is brought to a minimum, then new mechanical code provisions call for mechanical ventilation in the home. The addition of a good ventilation system helps to filter the air of allergens, and keep the air in the home at a regular humidity.

Optimum energy savings, comfort, and healthfulness come when design and the construction closely consider a building’s shell and fitting it with equipment designed to maximize the benefits of that shell. As costs for operation continue to escalate, we can expect to see greater and greater attention to these details as homeowners shift from desiring a better home to demanding a better home.

The CFA was established in 1974 for the purpose of improving the quality and acceptance of cast-in-place concrete foundations. Evolution of the system has led to the development of a significant above-grade movement with removable forms known as the Concrete Homes Council. Much of the content for this series has come as excerpts from the manual produced by these entities known as Building Homes with Removable Concrete Forms. This and many other promotional materials, educational seminars, opportunities for networking and a telephone network that places members in one-on-one contact with an experienced contractor for assistance in resolving a variety of issues are available through the CFA and CHC. For more information about CFA, see cfawalls.org or call (319) 895-6940. For more information about CHC, see concretehomescouncil.org or call (319) 895-0761.
Jim Baty, jbaty@cfawalls.org, is Technical Director of the Concrete Foundations Association and the Concrete Homes Council.