Energy Performance & R-Values of Insulated Concrete Forms
Fact or Fiction: "The R-value tells me how much energy my house will use, right?" R-value measures the resistance a material has to heat transfer, this much is true. R-value alone, however, does not fully describe the energy performance of a building. Everyone in the ICF community knows that ICF buildings far outperform framed buildings with comparable stated R-values in terms of energy efficiency and comfort level, but why is that? Let's look at the main factors affecting actual energy performance of a building: Thermal Conduction, Convection, Radiation & Mass.
Thermal Conduction is the heat transfer through a material by contact of one molecule to the next. This is the only factor an R-value measures.
- We have all heard builders claim to build "R-13" or "R-21" walls with wood frame construction. The problem is that only the highest rated component in the wall - the insulation itself - performs at these stated R-values. A wood frame wall is made up of several components, not all of which have the same R-value. For instance, a 2x4 or 2x6 stud has an R-value of about R-5 or R-7. Every 16 inches or so, one of these components breaks the insulation layer and forms a 'thermal bridge', conducting heat through the walls at high rates. Adding up the area of studs, plates, and headers, 12% to 16% of the total wall area is an R-5 or R-7 thermal bridge, all detracting from the stated R-value. In addition, batt insulation tends to sag over time and leave spaces without any insulation! How can those builders claim only the highest component R-value? From a 'whole-wall' perspective, framed walls operate at far lower R-values.
- Most ICF walls consist of two layers of EPS, with a center cavity to contain concrete. The EPS remains in place to provide two largely uninterrupted layers of insulation rated at roughly R-20 and higher. From a 'whole-wall' perspective, an ICF wall actually lives up to the stated R-values because thermal bridging is minimal.
Thermal conduction is not the only mode of energy loss in a building. In fact, conduction contributes less to energy losses in wood frame buildings than convection, which is not even measured by R-values.
Thermal Convection is heat transfer by movement of currents within fluids (or gases). When considering energy performance of buildings, it's the air moving between the inside and outside or 'air infiltration'. A common measurement is 'Air Changes per Hour' at a blower-door induced pressure differential of 50 Pascal (ACH50). US Energy Star standards for new homes require less than 4-7 ACH50. By comparison, British standards are 3-5 ACH50, Canadian R-2000 standards are 1.5 ACH50, and Swedish standards are 0.5 ACH50 or less.
- In wood frame buildings convection can be felt as 'drafts' and is usually the biggest source of energy loss. Air infiltration accounts for up to 40% of the energy losses of a wood framed structure. Heat is carried by air leaking through thousands of cracks, openings, and joints between all the pieces of the building shell. Major culprits include framing connections, wall, floor & roof intersections, shrinkage of wood and caulking, and poor installation of components and sealants. A typical new wood frame home has between 1.75 and 3 air changes per hour (ACH50) and after some years it's often between 5 and 10 ACH50 as the wood shrinks and sealants deteriorate. Old wood frame homes commonly have 10 to 20 ACH50.
- ICF walls & roofs are an effective air barrier because the concrete is poured in semi-liquid form, forcing air out of the cavity and filling every void after consolidation. A chemical reaction turns the concrete into a solid without passages for air to leak, thus eliminating a major percentage of air infiltration. ICF homes consistently show results of 0.5 to 2.5 ACH50 and less, largely depending on the installed roof type and sealing. Most air infiltration in an ICF home is through a conventional roof and around windows & doors, so pay attention to these areas, e.g. use Quad-Deck for the top of the building. Building consultants recommend heat-recovery ventilators (HRV) for further energy savings and adequate air exchange in very airtight buildings.
Thermal Radiation transfers heat via electromagnetic waves, which for buildings are mostly the sun's rays. Depending on factors like site & location of the building and the prevailing climate, Passive Solar Building Design helps optimize a building's absorption and reflection of solar radiation through solar orientation, placement of windows and shading elements, choice of finishes, and incorporation of thermal mass.
Thermal Mass refers to a material's capacity to store heat. Concrete and (Adobe) bricks have high thermal mass, which can act like a battery for heat. The classical use of thermal mass is in desert climates, where outside temperatures swing above inside temperatures during the day and below at night. High mass building shells can store the heat from the outside during the day and release that heat to the inside at night - keeping the inside comfortable using almost no additional energy. In temperate climates, thermal mass is best used in combination with the principles of passive solar design, e.g. let the sun heat high mass floors through windows.
- Wood frame buildings have almost no thermal mass - unless finished with brick.
- High mass construction built into ICF walls & floors can significantly reduce the requirements for active heating and cooling systems in many climates - which translate into ongoing energy savings and savings from smaller sized HVAC equipment. Most current residential HVAC sizing software does not factor in the effects of thermal mass. In response, the Portland Cement Association (PCA) developed HVAC Sizing software that uses Dept. of Energy 2.1E calculations to estimate the required heating and cooling system capacity for single-family concrete homes based on a user-defined thermostat set point, house dimensions, construction materials, and location.
Getting back to the original questions, the answers are...
'No - the R-value of one component alone does not tell you how your building will perform'. Remember, the Building Code is only a MINIMUM standard, and there are many factors that influence energy performance.
'Yes - ICF buildings far outperform framed buildings despite similar stated R-values'. The secret lies in the combination of reduced conduction & convection, and high thermal mass. The result is a building with a lower appetite for energy and more consistent and comfortable temperatures inside the building.
Every building must be considered from a 'whole system' perspective. Besides walls, roof and slab, windows also have a significant impact on performance since they usually make up 10% to 20% of the total wall area and range widely in energy performance. Building envelope consultants now offer modeling services that will paint a much more accurate picture of how your building will actually perform. A small investment in a computer analysis helps in formulating the most energy efficient design to save tens of thousands - even hundreds of thousands - in energy costs over a building's lifetime.
Further Reading (Research): Thermal Metrics for High Performance Enclosure Walls: The Limitations of R-Value
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* Source: Portland Cement Association Technology Briefs