This article was originally published in the July/August 1998 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online July/August 1998
Foam Forms Bring Concrete Results
by Pieter VanderWerf
Concrete may be cold and hard, but some energy-efficient builders have been warming to insulating concrete forms (ICFs). As ICFs become more common, home performance professionals need to know how they're built and how well they work.
The field study results are good news for those concerned with energy-efficient housing. But the ICFs put new demands on the energy and HVAC professionals. Sizing heating and cooling equipment and specifying ventilation, in particular, can be challenging. Manual J, the usual technique for sizing heating and cooling systems, needs to be tweaked to work with ICFs. And, ICF homes almost always need supplemental ventilation. HVAC contractors using the same techniques they use on conventional construction will tend to oversize the equipment and provide too little ventilation.Introduction to ICFs ICFs are hollow panels or blocks made of foam plastic. They are stacked into the shape of a building's exterior walls; rebar is installed; and concrete is poured into the wall cavity. The planes of plastic foam are about 2 inches thick and held 4-12 inches apart by crosspieces known as ties or webs. The foam stays in place to provide insulation and a backing for finishes. The result is an exterior envelope that, compared with standard low-rise construction, is strong, energy-efficient, and sound-dampening.
Currently about 30 brands of ICF are available in the United States and Canada. All of them fall into three general types: panel, plank and block. Panels are typically 4 ft x 8 ft sheets with flat edges that are fastened or glued to one another. Planks are usually 8 inches wide x 8 ft long and about 2 inches thick. Steel or plastic ties snap into slots in the foam to separate the planes and to connect adjacent planks to one another. Blocks are usually the smallest (anywhere from 8 inches x 16 inches to 16 inches x 4 ft). They connect to one another with teeth or tongue-and-groove joints along their edges like Lego blocks.
Systems also create differently shaped internal cavities, so they have different ratios of concrete to foam. Panels and planks produce a flat cavity of constant thickness, like a conventional cast-in-place wall. Grid systems have wavy interiors that mold the concrete into a shape resembling a waffle. Post-and-beam systems leave cavities for posts to be poured vertically every 4 feet, and for horizontal beams to be cast every 8 vertical feet.
Most ICFs are made of pure foam plastic--either expanded polystyrene (EPS), extruded polystyrene (XPS), or polyurethane. Two systems use a composite of cement (about 20% by volume) and EPS beads. These are heavier and more expensive, but manufacturers claim such blocks are more durable.
Construction begins with a particularly level slab or footer. The crews generally stack the units on this foundation up to the height of the first level (the basement or first story, as the case may be). Rebar is installed as necessary. As with conventional cast concrete, the possibility of high stresses like ground freezing, wind, earthquake, or multiple stories, require more frequent reinforcement. Crews then pour the concrete, using pumps if the top of the wall is well above grade.
After the concrete cures, they construct a floor deck. This is usually done by anchoring ledger boards, or the joists themselves, into the concrete. Construction of the next level follows, and so on until the top of the wall reaches roof height. The crew then attaches the top plates and frames the roof and interior walls in more or less conventional fashion. Finishes attach to the foam walls by a variety of simple means. Crews can frequently attach sidings and wall board directly to the ties; wall board can be glued on; and stucco and plaster adhere directly to most of the foams.ICFs Satisfy Owners, Builders In a study I conducted last year for the Portland Cement Association, over 98% of the homeowners surveyed liked their ICF homes. The reasons most often cited were comfort, lower transmission of noise from the outside, energy efficiency, and greater structural strength, in that order.
It is because of the energy efficiency that the homeowners were comfortable. For example, regarding comfort, occupants often mention the absence of drafts in ICF houses--a result of reduced air infiltration through the walls. They mention the absence of cold spots and the even temperature along the walls; these are attributable to the virtually unbroken layer of insulation. Homeowners also note that the interior temperature swing is usually small and more gradual over the heating-cooling cycle; this is probably due to the high thermal mass of the concrete.
The study compared the energy consumption of a sample of pure-foam ICF houses with that of frame houses of similar sizes and climates. We identified 29 matched house pairs for which complete utility bill data were available. All houses were 6 years old or less, and had been occupied for more than a year. To compare apples to apples, we adjusted the energy consumption to correct for important differences in the houses' physical characteristics and the life-style of the occupants. These included such things as the size of the house, the number of stories, window area, the efficiency of the HVAC equipment used, and the number of occupants.
We found that the ICF houses, after all corrections, consumed an average of 44% less energy for heating and 32% less for cooling (see Table 1). Total dollar savings are higher in cold climates because the ICFs save more energy during the heating season. This appears to be because a significant proportion of the cooling load results from the solar gain of the windows, a factor that the walls cannot influence.Sources of Energy Savings We believe that the energy savings of ICF homes result from three factors: a higher R-value, which reduces heat conduction through the wall; a lower air permeability, which reduces air infiltration; and a greater thermal mass, which partially buffers the interior from outdoor extremes in temperature.
The calculated R-values of ICF walls range from about 17 to 23. Recent studies at Oak Ridge National Laboratory confirm the high R-value of ICFs (see Wall R-Values that Tell It Like It Is, HE Mar/Apr '97, p. 15). In contrast, the tests showed that typical finished 2 x 4 frame walls with R-11 insulation were effectively about R-9. This is mainly because the studs act as a thermal bridge, compromising the insulation. Since ICFs have about twice the R-value of frame walls, we can expect conduction losses to be about half as much. When the home is being heated, wall conduction losses run around 25%, so we can expect ICFs to save about 12%-13% of heating energy use.
Blower door tests on ICFs from a variety of sources have yielded estimated air changes per hour (ACH) that range from about 0.11 to 0.5. Large-sample surveys of new frame houses yield an average of about 0.5. Thus ICF walls appear likely to cut losses from air infiltration by almost half as well. Since infiltration losses account for 20%-40% of total thermal losses, the reduction in infiltration could reduce energy consumption by almost another 10%-20%.
Thermal mass in a wall absorbs large amounts of heat without the temperature of the wall changing rapidly. This buffers the interior somewhat from sharp swings, leveling out the highs and lows of the day. That reduces energy consumed, especially when the average daily outdoor temperature is around 70°F. In such circumstances thermal mass prevents exterior temperature extremes from being noticed inside the house. Engineering simulations of thermal mass effects suggest that buildings with well insulated high-mass walls (like ICF walls) will tend to consume 4%-8% less conditioning energy than well insulated low-mass walls, the exact amount depending on the local climate.Costs, or Investments? Currently ICF walls add an estimated 1%-5% to the total cost of a house (or about $0.75-$4.00 per ft2 of wall area) in both the United States and Canada. Conservative payback calculations estimate that the higher cost of ICF walls is offset by savings from lower fuel costs in about 10 years; but this calculation is of limited relevance to most people's decision making. In the first place, most people do not pay the extra cost of an ICF house because of the energy savings. Greater comfort and sound reduction are more often cited as reasons for paying the premium. In light of this, we might view the fuel savings as a nice side benefit.
The more dramatic cost savings come from correctly sizing the HVAC equipment (see Efficient Cooling: Making It Happen, HE Mar/Apr '98, p. 35). Sizing it to match the home's lower energy consumption can save enough money to offset much of the higher cost of the walls.
One can probably install about one-third to one-half as much heating capacity in an ICF home as one would install in the same stick frame house, and about one-third as much cooling capacity. Heating energy consumption is only about 44% less, but HVAC is supposed to be sized for the peak load, not the total load. Simulations show that walls of high thermal mass, because of their damping effect, tend to have lower peak loads than their lightweight counterparts.
Thus it is not a stretch to assume that heating capacity can safely be cut by half. A few ICF builders in the North say that they routinely install half as large a furnace as they would in a similarly sized frame house, without problems.
The same reasoning seems to justify downsizing the cooling system by at least one-third. For an average-size house these equipment reductions could translate into $2000-$3000 in up-front savings, which can be used to help pay for the ICFs.
However, most HVAC contractors are loathe to downsize equipment, let alone to downsize it so much. They fear that the lower load projected will not be accurate, the house will fail to maintain set temperature, and they will be blamed. They point to the uncertainty of the exact load in any given house.
In fact, even in the study sample, there were a few cases in which ICF houses and frame houses had similar HVAC loads. Presumably, this was because of unmeasured differences among houses such as the tightness of the roof construction, extent of roof insulation, solar incidence, routing of HVAC ductwork, and the energy efficiency of windows and doors. Such variation inclines HVAC contractors to be conservative.
Proponents of ICFs argue that equipment on frame houses is usually oversized already, and that leaving the sizing the same when switching to houses as energy-efficient as ICFs can not only overcharge the customer for the initial equipment cost, but can also create other problems. Oversized equipment will tend to hit the house abruptly with a blast of hot or cold air and quickly shut off. The equipment will not have time to cycle efficiently, leading to such problems as unnecessarily high fuel consumption and failure to dehumidify when air conditioning.
Regardless, without hard numbers HVAC contractors tend to keep the equipment large. Those few who use load simulation packages, such as BLAST and HOT2000, calculate lower loads and install smaller equipment. But most contractors remain conservative.
The issue of ventilation to the outdoors arises because ICF homes tend to have much lower natural air infiltration. In practice, the builders of about half of all ICF homes install no supplementary ventilation, and say they've seen no problems. However, many experts recommend designing in some air exchange when the unassisted air change rate would be below 0.35 ACH. And according to some studies, most ICF homes are below that.
Some builders who install an air exchange use a simple intake pipe. This adds only a couple of hundred dollars to the initial cost, but it makes the house less energy-efficient. Others use a complete air-to-air heat exchanger. This has much less of a detrimental effect on energy efficiency, but it costs $1,000-$2,000.A Future of Foam The Portland Cement Association is currently preparing sizing guidelines for HVAC contractors. These guidelines will show how to simply and accurately modify standard equipment sizing procedures laid out in Manual J for buildings with ICF walls.
Proper ventilation is more problematic. There are few standards for any form of construction. In both of these areas, further studies will provide more information. At that point, it will be possible to specify the HVAC sizing and ventilation procedures that maintain a comfortable, healthful environment yet take advantage of the savings permitted by ICF construction. In the meantime, the home performance professional can best serve the customer by basing decisions on a more detailed knowledge of ICF houses and how they work.
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