Article No: 30
Maximizing ICF performance
By: CAROLE MCMICHAEL
Of the many advantages builders can list for using insulating concrete forms (ICFs), energy efficiency is near the top. Unfortunately, there is more to achieving this efficiency than just using ICFs. If the builders or mechanical contractors don't factor in other design and material choices, they are likely to be dealing with serious consequences for which ICFs are often wrongly blamed.
"People don't realize that when building an ICF house properly, you are building an ultra energy efficient house," says Richard Rue, president of Mansfield, Texas-based EnergyWise Structures. "Seventy percent of all the leakage in a house is through the walls. ICFs eliminate that leakage because they provide a monolithic slab of EPS foam, then concrete and then foam again."
ICF walls are only one of several ways to reduce air filtration, Rue points out. Other techniques include putting cellulose or foam insulation in the attic in lieu of fiberglass, although the latter touts high R-value.
"R-values can be misleading," Rue says. "They are just part of the equation. There are a number of ASTM tests (C-518, C-177, C-236 and C-976) to determine R-values. All of these tests are variations of a guarded hot box test. On one side you have a hot plate, and on the other, a cold plate in which a sample of material is placed. Then the material in the chamber is allowed to reach 'steady state' at 75 degrees Fahrenheit, which means the material is totally saturated. All insulation materials (except for urethane foam) are going to test out at between R-3 and R-4 per inch.
"What these tests don't tell you is the amount of time that it takes for the different materials to reach steady state," Rue continues. "Materials with higher air infiltration-control properties and the ability to trap air inside their fibers (such as EPS foam and spray-on cellulose) take much longer to reach steady state because they have higher thermal inertia. When these are combined with concrete, you have a complete monolithic seal."
Featured in the August/September 2001 issue of Concrete Homes, the Home on the Hill model home (see story) in Cedar Hill, Texas, was designed for energy efficiency, with ICFs below grade and structural insulated panels and energy efficient windows above grade. Energy bills for the new home have been relatively low — only twice as high as the bills for the previous facility, which was five times smaller. However, an analysis showed that the home's HVAC units were 10 tons oversized. With proper HVAC sizing, the home's energy bills can be reduced by at least 60 percent, says Richard Rue of EnergyWise Structures.
Any design element that puts a hole in the thermal envelope must also be evaluated. A can light without an airtight trim ring creates one square foot of hole space in your ceiling, which may leak as much as three quarters of a ton of air per hour that the unit is running, according to blow door testing. Poorly designed skylights, as well as outside sockets, can likewise create holes. Builders should only use thermally broken windows with low-E or tinted glass to retard heat loss in cold weather and heat gain in hot weather. Atrium doors are preferred to sliding glass doors, which often have sealing problems.
"To tighten up a house, get the ductwork out of the attic," Rue says. "If air can go through the insulation system, you have defeated the insulation properties of the system. In a single story house, ductwork can easily be put in the floor — there are PVC duct systems. Or it can be put in false ceilings or plenums. Much smaller ductwork is also available now. It is made about 8 inches around with feeder lines only 2 inches around that can run under the insulation. It makes no sense to strap the ductwork up in the middle of the attic where it's like basting a turkey."
ICF builders are likely to have trouble finding HVAC contractors who understand ultra-efficient ICF structures. According to Rue, most contractors don't run load calculation before determining the size of heating and cooling systems.
"The rule of thumb in Sun Belt states is one ton of air per 500 square feet," Rue says. "In a 2,000 square-foot home, the bid would be 4 tons and raised to 1 ton for every 300 square feet if there is a lot of glass. An ICF-built house may actually need only 1 ton of air per 750 to 1,250 square feet."
Rue advises builders and homeowners to expect HVAC contractors to ask questions about the direction the house faces, what kind of windows and doors are used, what kind of mechanical system is being considered, and what type of attic insulation is used. If a contractor just asks for square footage and number of stories, and uses the rule of thumb to calculate the size of the system, you need to find a different contractor, Rue says.
"I'll give you an example of what can happen when using HVAC contractors that are not experienced with ICF energy efficiency," Rue says. "A 4,000 square-foot house in Aransas Pass, Texas, was fitted with an 8-ton system. After a month, the home's ceiling turned green with mold.
"The contractor blamed it on the ICF blocks. When the builder requested help, we told him to do a sling slyclometer test to get indoor and outdoor humidity readings, wet and dry bulb — after all, an air conditioner is just a big dehumidifier. The builder also needed to test with a stopwatch to determine how often the system came on and stayed on and how long it stayed off. This unit came on and stayed on between one and three minutes and then stayed off between 10 and 15 minutes. This means it was short cycling four to five times an hour."
In a worst/best scenario based on more than 400 blower door tests on different structures, the average stick-built, fiberglass constructed building will normally average between 3.5 to seven air changes per hour when tested at 40 Pascals, Rue says. An ICF structure combined with EnergyWise specifications and EPS or spray-on cellulose R-38 insulation, tested at 40 Pascals, will average between one and 1.5 changes per hour, he says.
"What many contractors and home owners don't realize is that a unit uses more energy in the first three to five minutes ramping up compressors and coils than in the next 25 minutes of continuous running," Rue adds. "The cost of that short cycling is the equivalent of 2.5 hours of continuous run time. Still, it is not running long enough to get the humidity out of the air. After we analyzed the house plans for this house, we determined the builder needed a 3.5-ton unit. The contractor refused to believe it was enough, but put in the unit. In a month, the problem was gone."
In another example, Rue's advice on the correct size for a home's air unit was ignored. The house had wood-framed windows and wood floors. Within a year's time, warping damage from too much humidity required the owner to replace the windows and flooring.
Rue, whose mechanical engineering company specializes in providing energy efficiency analysis and designing HVAC systems sized for a structure's thermal envelope, tries to show customers in black and white how each choice they make will contribute to their utility bills. The energy analysis starts with a sheet of questions that must be filled out and submission of the house plans. EnergyWise runs a computerized program on every house twice, comparing area utility rates for comparable sized stick-built structures with the new home. The program uses a combination of seven different manuals, including Manual M and Manual J, which were written by Dr. William Poucher, head of the department of Computer Science and Engineering at Baylor University. Manual M gives a breakdown on each room's load based on true direction and glass to assure air balancing, while Manual J tests for whole house loads to determine appropriate tonnage.
Comparisons include energy consumption for different systems such as geothermal heat pumps, air-to-air heat pumps, gas heat systems and resistance heat systems. Customers get a 1-inch vinyl binder including engineering reports and a monthly average cost guarantee if the builder follows the EnergyWise plan.
"The secret to true energy efficiency is the proper selection and installation of materials, air infiltration control and HVAC sizing," Rue says. "If an ICF home is built properly, once [the temperature] stabilizes, it takes little energy to stay there. ICF builders have the opportunity to be the leader in the home construction industry. There is nothing you can build that is any stronger or can be any more efficient than an ICF house."
Rue's company has worked with ICF manufacturers Polysteel, Reward Walls, Reddi-Form, Eco-Block and Arxx. Builders he's worked with have been quite receptive. They know clients will be happy to buy a less expensive HVAC unit and pay much less in utility bills. Builders also want to avoid the potential liability of someone getting sick with toxic mold, Rue says. And beyond that, they want to avoid a bad job. One bad job (even if it is not the fault of the ICF) may lose the builder 100 jobs.
"Unfortunately, most of the ICF industry is selling wall systems when they need to be selling holistic systems," Rue says. "Manufacturers and distributors need to let builders know what to do to make the ICFs perform up the their ultimate specifications. The true measure in energy efficiency is how air-tight can you make the structure and still keep indoor air quality pure. To do that, you have to get the mechanics right."