Article No: 196
Tech Talk - Energy performance of RCF concrete homes Part II
By: Text by Ed Sauter, Director, Concrete Foundations Association
The previous article in this series explained the effect of thermal mass on comfort, heating and cooling in RCF (Removable Concrete Form) concrete homes. Position of mass and insulation was discussed. In this article, we will cover air infiltration, control and introduction of fresh air, and the sizing of mechanical systems for concrete homes.
The largest single energy-loss component in older homes is often air infiltration, air that leaks in and out of the house through the siding, around windows and doors, and through electrical outlets and similar penetrations in the exterior shell. Lack of fresh air in older homes has never been an issue. The typical Victorian-era home, with loose double-hung windows, clapboard siding, and loose-fitting wood doors, has air-exchange rates of one to two air changes per hour or more. Needless to say, increasing the insulation levels is secondary to stopping air flow with air exchange rates of this magnitude. Air enters the home at the outside air temperature and mixes with the conditioned (heated or cooled) air in the home. If you mix minus 10 F air with 70 F air, the result is still cold. The cold feeling is accentuated because of the drafts created when the cold air currents move throughout the space.
Concrete homes, in particular those with integral concrete walls and decks (previously discussed in Part I of this article), are cast in one continuous operation. The only possible source of air leakage is around doors and windows. This can be controlled easily with quality window selection and proper caulk or sealant application. Stopping the air leakage with a monolithic house shell and properly constructed openings is the primary step to energy efficient homes. Selection of the proper ventilation and space conditioning equipment further adds to energy savings and solves the myriad of common ailments in today’s home.
Indoor air quality and heat recovery
Most people believe that when they move indoors they escape air pollutants. In fact, the Environmental Protection Agency states that air pollution inside the home can be two to five times higher than outdoor levels. The EPA had identified 14 pollutants as sources of problems with indoor air quality. The pollutants can cause sickness and aggravate existing health problems.
Some of these pollutants are the result of the construction materials used in our homes, but others are problematic because we are building our homes tighter, trapping air indoors instead of letting it leak to the outside. Building tighter homes is prudent because it saves energy, but we must also take steps to provide clean air for the inhabitants.
The natural air infiltration rate in the typical concrete home is around 0.15 air changes per hour. This compares with 0.35 to 0.50 for the average frame home constructed today. Since many health experts recommend a minimum of 0.25 air changes per hour, additional fresh air can be provided by an air-to-air heat exchanger in concrete and other tight homes. The great thing about a heat exchanger is that air, which is normally exhausted directly outdoors, is used to preheat the fresh air being infused into the distribution system by the heat exchanger. This further adds to the efficiency of the heating system.
A variety of controls, connections, and accessories including humidistats, thermostats, defrosters, filters, and booster fans are available to assure that a constant supply of fresh air is being delivered to the home. Several well-respected manufacturers of home heating and space conditioning equipment produce air-to-air heat exchangers.
Moisture control and mold
Mold is widely considered the “modern-day” asbestos in terms of a potential threat to comfortable and safe living. Three conditions are required for mold growth: mold spores, a food source, and moisture. Mold spores are everywhere—it’s almost impossible to rid your home of them. The second requirement is food. While drywall, wood, and other organic products provide this vital component for mold growth, concrete is not a food source. The more components of the home constructed with concrete, the less food available. The third ingredient, moisture, is present in all homes to varying degrees. The key to controlling moisture in a home is elimination at the source. Kitchens, bathrooms, and laundry areas are the primary contributors of moisture in homes, so a good ventilation system, which collects the warm, moist air and vents it to the outside, will help control the moisture. These sources may be able to be vented through the heat-exchange system, capturing more energy for heating incoming air.
Concrete needs moisture to cure properly. Depending on the environmental conditions of the house when constructed and length of time from when the concrete was cast, concrete homes can experience high humidity levels for several months. A humidistat, interfaced with the mechanical or air-exchange system, can provide additional control of moisture throughout the life of the structure.
Another cost-saving feature of concrete homes is the ability to reduce the size of heating and air conditioning equipment. The insulation combined with the thermal mass will result in substantially reduced peak demand for the mechanical systems. A mechanical system is correctly sized when it operates continuously during peak design conditions (hottest day of the summer or coldest day of the winter). Actually, most systems are designed to operate at capacity slightly below the most extreme conditions. Over-sizing of heating and air conditioning equipment means that it will cycle on and off more frequently. This cycling will reduce efficiency of the system and produce wider temperature swings, resulting in wasted energy and a less comfortable space.
Mechanical systems for commercial buildings are typically sized by an engineer using the latest in software. Residential structures, on the other hand, are often selected using rules of thumb based on square footage of the structure. This type of selection does not account for the contribution of thermal mass. A software tool developed as part of a HUD-sponsored project and marketed by the Portland Cement Association will aid equipment suppliers and designers in equipment sizing. This software calculates system capacities for concrete homes based on house dimensions, construction materials, location (United States, Canada, and Mexico), and thermostat set points. It is based on U.S. DOE 2.1 calculation engine and various ASHRAE standards. Don’t be taken in by someone who figures bigger is better. Bigger will cost you money and make the home less comfortable.
The Portland Cement Association study calculated that reductions in mechanical equipment for heating and air conditioning ranged from 7 percent to 17 percent for RCF housing depending on the location of the insulation relative to the mass. These calculations did not take into account additional thermal storage from systems incorporating concrete decks.
If you are looking for a solid reason to select concrete as your choice for your building material take a close look at concrete. You’ll find that Removable Concrete Form construction will meet all of your building requirements—and more.