by Elaina Adams | October 1, 2012 11:23 am
By Gary Brown and Steven H. Miller, CDT
Insulating concrete forms (ICFs) offered an energy-efficient mode of construction long before sustainability was widely pursued, or even understood, in the overall building industry. In the intervening years, competing building methods have seen improvements in thermal energy efficiency, but the properties of ICF have remained virtually constant, until recently.
There has been (more or less) one standard thickness of insulation available, and the most widely used insulating form material—expanded polystyrene (EPS)—has remained unchanged. In the past few years, however, manufacturers have stepped up their insulating capabilities. To stay ahead of the sustainability curve, they have pursued different strategies for getting a higher effective R-value from their wall systems. The resulting products, which can be thought of as ‘enhanced-performance ICFs,’ deliver the advantages of traditional forms, along with a significant increase in energy efficiency.
Sources of energy efficiency
As its name suggests, an ICF is a method of making structural concrete walls (and other structural elements, though this is beyond the article’s scope) using formwork with insulating properties. Designed to be left in place to serve as insulation, the formwork itself has certain permanent structural properties allowing the attachment of interior and exterior finishes. (The properties and details of ICF construction have been previously covered extensively in Construction Canada. Readers can find greater detail on the system in Gary Brown’s July 2010 article, “Building With Insulating Concrete Forms,” or “ICFs and the New Green Standards,” by Paul Nutcher in the January 2012 issue. For more, visit www.constructioncanada.net and select “Archives.”) Essentially, the completed wall is a concrete sandwich with EPS for bread slices.
The sandwich derives energy efficiency from both concrete and EPS, the latter of which contributes its insulating value. Its thin internal cellular structures provide resistance to thermal conduction—the primary property measured in R-value tests. Its light colour reflects radiant heat, reducing the second mode of heat transfer. (Since this concerns radiant heat reflection, the issue of actual exposure to the elements is irrelevant. Radiant energy entering through either the cladding (from the exterior) or the drywall (on the interior) can then transfer by conduction, convection, and radiation. Every hot object gives off some radiant energy—this is why infrared photography works, and why the tip of one’s nose looks darker on an infrared photo than one’s cheeks, even though they reflect the same color in the visible spectrum. The light colour of the EPS reflects not only visible spectrum, but also infrared—radiant energy. It reflects radiant energy even in total darkness). Its closed surface resists convection of hot air through the insulating material.
White EPS is generally given an R-value of about 4.1. Since R-value testing primarily measures resistance to conduction, it should be noted two materials with similar R-values may have differing performance due to radiant energy absorption or reflection. A dark-coloured material with little reflectance absorbs more radiant energy at its surface than a light-coloured one. Although both materials may offer similar resistance to conducting heat from the surface through the material’s thickness, the dark-coloured material has more heat to resist, and therefore more heat transfers.
Since the introduction of EPS ICFs, most available systems have featured pairs of 65-mm (2.5-in.) thick panels, for a total insulation thickness of 130 mm (5 in.). Adding in the (low) insulating value of the concrete wall, plus additional value from the interior and exterior finishes, a total system resistance of about R-22 has been common for ICF walls.
The concrete provides thermal mass, which is the ability to absorb and retain heat energy (see “Understanding Thermal Mass,” page 38). When heat is applied to one side, it must ‘fill’ the concrete before it can be transmitted to the cool side. This can slow interior heat gain when the sun is shining. When the concrete has stored heat and the surrounding temperature drops, the concrete gives back its own heat before transmitting it from one side to the other. This can help maintain more evenness in the interior temperature when the exterior temperature drops at night.
Thickness of concrete in an ICF assembly—and therefore available thermal mass—is usually determined by the wall’s structural requirements, not its insulating requirements. ICF blocks are available in standard sizes ranging from 100-mm (4-in.) concrete thickness up to 300 mm (12 in.), meaning available thermal mass may vary by up to 300 per cent between projects. It would, of course, be possible to use a thicker wall than structurally necessary to boost thermal mass, but it is not usually done. (For various reasons, walls are usually sized for structural needs, rather than thermal mass performance.)
Thermal mass increases the system’s effective energy efficiency, but the amount of that boost depends on climate conditions. Calculating a specific system’s performance is complex, and must factor in not only the wall materials, but also the prevalent exterior temperatures and day/night temperature swings.
Over the past 15 years, ICF block sizes have remained unchanged largely due to the high cost manufacturers would incur to create new block moulds. This has limited the insulating value of ICF structures.
In response to the recent upswing in insulation demand, both from customers and building codes, manufacturers have introduced improved, enhanced-performance ICFs. One type is made with a specialized form of expanded polystyrene with different properties than standard white EPS. Graphite-impregnated EPS ICFs typically have a grey or silvery appearance, giving rise to several brand names that include a reference to ‘metal.’ The EPS beads are coated before expansion to yield a widely spaced distribution of graphite
(i.e. black carbon particles) in the light-coloured EPS.
The manufacturer of a graphite-impregnated EPS product (used by various ICF manufacturers to mould blocks) says the material’s effective R-value can be 20 per cent higher than white EPS. Despite the temptation to think the silvery colour is more ‘mirror-like,’ this claim is not based solely on increased reflectance, but also heat absorption. In other words, graphite is giving the EPS higher thermal mass. This suggests the actual boost in effective R-value may vary with the conditions affecting thermal mass performance.
A representative of one ICF manufacturer that uses graphite-impregnated EPS has told the authors the R-value increase is actually closer to 15 per cent in his experience. In a conventional-thickness ICF, this means the total effective R-value increase would thus be limited to 3.15 for a system total of R-24.15. Assuming the manufacturer’s 20 per cent figure, a total system value of about R-26.5 would be achieved.
A possible problem with graphite-impregnated EPS has been mentioned by a contractor in the southwestern United States. On two projects, portions of graphite-impregnated EPS blocks melted during the construction process.
The first incident was during construction of a fire station in Scottsdale, Ariz., in the summer of 2010—blocks that had been outside for three or four days had their inside corners melted. Daytime temperatures had been about 38 C (100 F)—not normally high enough to melt EPS. In this instance, concrete had already been placed and was curing before the meltdown, so damaged insulation could not readily be replaced. Since the structural integrity of the wall was not affected in any way, the missing insulation was simply drywalled over. (One of the advantages of ICF construction is because there is insulation on the interior and exterior, any defects like this can be covered over with minimal loss of R-value. The complete system insulation, thermal mass, and airtightness is very forgiving; in this case, the practice of covering the defect would not have had a major loss of performance as it would have with traditional stick-frame and fibreglass construction).
The second incident occurred the following summer while constructing a custom home in Cave Creek, Ariz. Several pieces melted in inside corners, and also along one straight run of wall. Daytime temperatures were once again about 38 C. In this case, concrete had not yet been placed, and the damaged blocks were trashed and replaced before concreting.
When the manufacturer was consulted, the contractor was offered several possible explanations, such as a mirror-like effect, reflecting sunlight onto the blocks. None of the explanations seemed to apply to the project circumstances. It was deemed possible sun reflection from the concrete slab, or from the opposite wall of blocks, may have increased the radiant heat, and the energy absorption of the graphite surface caused the blocks to heat to the melting point.
Granted, one might assume what may be problematic in Arizona would not be an issue in Canada. However, it is important to note ambient temperature does not seem to have been the issue—the air was not hot enough to melt the EPS. The conjecture is radiant heat (i.e. the intensity of the sunshine) reflecting off these dark-coloured blocks led to the heightened temperatures at the corner. Essentially, this ‘melting’ problem could theoretically happen anywhere on the continent.
The other route toward higher performance is to increase insulation thickness. As previously stated, the price of new manufacturing tooling has long been a barrier to this approach.
One solution is to use standard ICF blocks and add sheets of additional EPS to the interior of the blocks as they are being placed. If the structure called for 150-mm (6-in.) concrete walls, 200-mm
(8-in.) ICF blocks would be used, and 25-mm (1-in.) EPS sheets would be added to each side of the block’s interior.
This type of product, however, has been reported to cause problems with concrete consolidation; the added sheets can be prone to being displaced or damaged while using a vibrator . (The normal process for an ICF form is to add rebar inside the block as it is being stacked. By adding additional EPS sheets or panels inside the block, it further compromises the opening available to allow a vibrator inside the block as concrete is being poured. Further, without visual capability to ensure the additional EPS panels have not moved, there is potential for air pockets inside the block that could affect structural properties. The most secure way of fixing additional panels would be to use a foam-to-foam adhesive when installing to ensure there is no separation. However this adds to the labour and time component).
They also require increased labour because of field assembly, raising construction costs.
Another method simply extends the thickness of an existing ICF design. The panels have the appearance of a standard-thickness ICF that has had a flat slab of additional EPS added to the exterior side, although in fact they are moulded as a single unit. The engineering of the block remains unchanged from the standard blocks, with top interlock and mating bottom indentations along the inside 62.5 mm (2.5 in.) of the panel thickness, and webs embedded to the standard depth from the inside surface. Due to the thickness added to the outside surface, extra-long screws must be used for attaching interior and exterior finishes to the wall, with the manufacturer supplying a table of appropriate screw lengths.
The added-thickness panels are available in sizes ranging up to 200 mm (8 in.), resulting in a theoretical maximum R-value for the complete wall of about R-66. Panels can be paired in any combination of thickness to make blocks. However, this versatility comes at a cost—they are only available as unassembled panels (i.e. knock-down [KD] panels) that must then be assembled into blocks. This reduces shipping costs because more material can be transported in the same space, but significantly adds to labour and construction time because the webs must be inserted onsite to assemble complete blocks before construction can begin.
Another option involves ICF panels that have been fully redesigned to be thicker. In this version, the mating structures on the top and bottom cover the panels’ full thickness, and properly mate without field-cutting. Panels that are 82.5 mm (3.25 in.) thick add about 38 mm (1.5 in.) of insulation to the system, increasing insulation R-value to R-30, in factory-assembled blocks. Webs are spaced 200 mm on-centre (oc), and are embedded in almost 20 mm (0.75 in.) more EPS (measured from the inside surface) than in standard ICF panels, increasing the panel’s strength against web pullout. Web anchors are at the same embedment depth from the outside surface as in conventional ICFs (i.e. 16 mm [0.6 in.]), allowing attachment of finishes using standard-size screws and other familiar techniques.
One of the first projects built with these thicker ICF blocks is a 930-m2 (10,000-sf) two-storey office building in Collingwood, Ont. It is the first structure the designer/builder, Royalton Homes, has ever constructed using ICFs. Company vice-president Sam Chaaya and his partners selected the material primarily for its high R-value and the energy efficiency it would provide the building. Temperatures in Collingwood went below –22 C (–8 F) two of the past three winters, and rose to more than 35 C (95 F) this summer. Royalton was also attracted by the durability of a concrete structure.
The foundation was built with conventional 62.5-mm (2.5-in.) ICFs, switching to the thicker forms for the two above-grade floors. Chaaya’s assessment was the new, wider blocks are easier to build with than the conventional ones, and it sped up construction on the upper floors. According to Chaaya, mating between block-tops and bottoms is more precise with the 82.5-mm (3.25-in.) ICFs due to the wider mating surface, so it is easier to keep wall alignment straight, resulting in faster construction.
He also observed the enhanced-performance ICF blocks seem stronger than conventional ones due to the increased EPS thickness. For the Collingwood project, the crew almost suffered a blow-out in the conventional block on the foundation level, where they were using 62.5-mm (2.5-in.) ICFs. They had cut a section out of the interior side of a block to allow for a column, and the missing web weakened the block’s integrity on the exterior side. (It is a common practice to cut the ICF forms to make columns and other structural elements. These areas should be strapped with extra bracing to prevent blowouts). At this point, they should have braced the remaining panel to make up for the strength lost because of the missing web, but this routine precaution was omitted (possibly because it was their first experience with ICF construction). The exterior EPS panel bulged outward, but it was braced from the outside before it actually ruptured or leaked concrete.
By contrast, Chaaya has seen no such problems with the newer, thicker ICF blocks. He also noted the corners of the new system seemed stronger than in conventional blocks, and said the fact the legs of the angle are longer saves a small amount of time.
Royalton initially expected construction cost to be about 30 per cent higher than wood-frame construction, but felt the energy savings would be worth the investment. Once construction got underway, the company found the actual differential in expense would be considerably less, closer to 10 per cent.
The most common problems with ICF projects occur during concrete placement, and are largely avoidable when the formwork is designed with a consciousness of the construction process.
Concrete is as heavy in its liquid state as it is when hardened. The block must be able to bear the pressure of a column of wet concrete. It depends on the integrity of the EPS panels, the strength and quantity of plastic webs connecting them, and the strength of the EPS slot where the webs are embedded.
The number of webs per block also contributes—all other factors being equal, webs located every 150 mm (6 in.) oc are stronger than webs every 200 mm (8 in.). Further, the deeper the web’s anchor is embedded in EPS, the stronger the block. The web can come within 16 mm (5/8 in.) of the block’s outside surface for maximum inside embedment and high pull-out strength.
The EPS must also be tough enough to withstand the pressure waves generated by a pencil vibrator or similar consolidation tool. If the vibrator weakens the panel, it can blow out and allow wet concrete to leak.
To obtain appropriately strong blocks, specifiers should seek products that have tested to 41.4 kPa (865 psf) using the Construction Material Centre (CCMC) Technical Guide 03131, Forming Capacity Strength Test.
Gary Brown is vice-president of marketing for Toronto-based Amvic Building System, a manufacturer of insulating concrete forms (ICFs) and rigid foam insulation. Including his seven years at Amvic, he is a 22-year veteran of the energy-efficient, sustainable building industry. Brown can be reached at email@example.com.
Steven H. Miller, CDT, is an award-winning writer and photographer, and a marketing consultant, specializing in issues of the construction industry. He can be contacted via e-mail at firstname.lastname@example.org.
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