When we talk about energy efficiency, we first have to understand some basic definitions Energy efficiency of building materials is determined by how it handles heat, how heat transfers through materials and how well materials hold or store heat. Remember, heat always moves from warm to cold, so during the summer, if the outside temperature is warmer than the inside temperature of a home, heat transfers through the walls from the outside in. Conversely during winter, if the inside air temperature is warmer than the outside, heat transfers from inside the home out.
<b>Clear Wall R-Values </b>
Clear-wall R-Value calculations do not take into consideration the effects of additional construction like windows, doors, exterior wall corners and how the roof joins the walls. When factored in, R-Values typically drop from those stated as Clear-wall R-Values. Clear-wall R-Values and their thermal performance are determined by testing a solid wall (complete with its insulation system), usually a section 8 ft. x 8 ft., with no openings for doors and windows.
<b>Whole Wall R-Values </b>
Clearly more accurate is the Whole-wall R-Value rating system. In this system, not only is the thermal performance of the wall tested, so is the typical envelope interface details. These include wall-to-wall corners, wall-to-roof, wall-to-floor, wall-to-doors and wall-to-window connections. Why include these details? Because energy efficiency is lost at these points of construction and including these elements provides a more accurate R-Value rating system. (Although taking the time to tape these joints during the construction helps reduce some of hte heat loss significantly.)
To add to the confusion we have to consider thermal mass, perhaps the most confusing energy issue facing engineers, contractors and homebuyers today. Thermal mass ratings are determined by measuring the building materials/wall unit energy efficiency in conjunction with other layers of materials attached to the wall, i.e.; particle boards, drywall, stucco. The measurement of the energy performance of a "stated R-value" material, combined with the layering of other materials to enhance the energy value, is often referred to as "mass-enhanced R-value." There are a number of combinations of materials used in construction and their thermal mass and R-value efficiencies vary depending on how they are used in the region of the country (climate considerations). How efficient a system is and how much energy consumption is reduced depends on how fast heat transfers through materials, how well materials hold the heat and the fluctuation of outside temperature. Attention to details like the windows you select, like low e-thermal, dual pane windows that are tinted, is just as important as the R-value in the walls. In fact, much heat loss or gain, up to 48%, is through windows, not walls!
Additionally, according to the US Department of Energy, for energy efficiency, your home should be properly insulated from the roof down to its foundation. This includes the following areas:
Attic access doors to unfinished attics
Knee walls in finished attics
Ducts in unconditioned spaces
Floors above unheated garages
<b>Let us look at some of the current choices in construction of today ... </b>
<b>Block Masonry and Poured-Concrete for residential construction</b> There are numerous insulation methods and new insulated block systems on the market with varying R-values. Standard 8" hollow block's R-value is 1.75, which by itself, is not enough R-value to achieve energy efficiency. The addition of outside insulation, integral insulation or interior insulation can be added to standard block to make it very energy efficient. There are several ways to incorporate foam insulationsuch as polystyrene, polyisocyanurate or polyiso, and polyurethaneinto concrete blocks. The hollow cores of concrete blocks can be filled by pouring and/or injecting loose foam beads or liquid foam. Some manufacturers make concrete blocks that accommodate rigid foam inserts.
<b>Stud Wall Construction using wood or metal studs ... </b> The exterior wall constitutes a large part of the total insulated package. In exterior walls, a 2x6 wall offers upto 5.5" of insulation. With the use of 2x6 studs spaced a 24" c/c , not only is the depth of the wall cavity increased but the width of the cavity is also increased. Studs conduct more heat than the insulation since it extends through the wall cavity and has a smaller R-value. So, in 2x6 construction there are fewer studs in the exterior wall and less heat is conducted through the wall. The following website has a good table describing R-Values of various stud-construction methods http://www.homeenergy.org/archive/hem.dis.anl.gov/eehem/99/991110.html
<b>Insulating Concrete Form (ICF) Construction </b> ICF uses reinforced concrete as structure and where insulation does double duty also as permanent stay-in-place formwork for concrete during placement is a very attractive choice for walls of energy efficient buildings. The insulating material, typically expanded polystyrene (EPS) affords a variety of state-of-the-art exterior finishes, such as siding, brick, cultured stone, stucco, and so on. Constructed from expanded polystyrene and stacked like building blocks to form the exterior walls of a home, the forms are reinforced with steel and filled with concrete. The forms interlock and fasten one to the other to provide seamless "foundation to rafter" fully insulated, reinforced concrete walls. Window and door openings of any size are possible. Because of the triple insulative nature of these forms, ICFs are extremely energy efficient. The R-value of the insulation, coupled with the thermal mass of the concrete, and the elimination of air leakage, makes for an R-40 energy rating or higher.
<b>Structural Insulated Panels (SIP)</b> SIPsconsist of two outer skins and an inner insulation core. Most structural panels today use either plywood or oriented strand board (OSB) as skins. OSB is most commoly used because it is available in large size upto 12 ft by 36 ft. Expanded polystyrene (EPS) is the most commonly used insulation core material. Extruded polystyrene (XPS) commonly known as styrofoam, polyurethane foam and isocyanurate are also used as the inner core but predominantly EPS is the used as the inner core. As you can see in the table below, EPS has an R-value of 5.00 per inch. Many SIP manufacturers allow you to select the thickness of the panels for your home ranging from 4" to 12". SIPs are becoming increasingly common in modular home building as they have shown extreme energy efficiency, are much stronger than wood-framed homes and the outer envelope can often be constructed in under one week! If you are a contractor or engineer who wants more information about SIP's see http://www.sipweb.com/monitor/bc_main.asp. Some manufacturers choose to use polyurethane and isocyanurate as the insulating material. Aged polyurethane and isocyanurate SIPs have a nominal R-value of around R-6 to R-7 per inch (2.5 cm) of thickness. Polyurethane/isocyanurate panels, although more expensive, are more fire and water vapor-diffusion resistant than EPS, and insulates some 30% to 40% better than EPS or XPS, per given thickness.
<b>Precast Concrete Wall Construction </b>... Precast concrete walls use use high-strength concrete, typically 5,000 psi with reinforcing steel and fibers. Priced in the 55 - 65 range per linear foot of wall, precast systems are competitive with other foundation walls, particularly when costs are examined as an assembly that includes footings and sub-slab drainage. The precast concrete walls have full-length insulation built in to increase energy efficiency. They are smart, with convenient features like built-in accesses for wiring and treated wood nailers for drywall. The walls have a built in footing to be placed on a base of stone. The average R-Rating I found doing a search was about 12.5.
Overall, there are a lot of choices you have to make...Do you want to spend more money at the front end to make an energy efficient home, or foot the increasing power bills. Many of the new construction methods dramatically improve the speed at which homes can be built. Discuss these options with your contractor and or engineer to find out what is best for your specific purpose and area where you live.
At the following website, you will find a table of the typical R-Values for construction materials. http://www.coloradoenergy.org/procorner/stuff/r-values.htm