In the past few decades, windows, walls, ceilings, and other residential building components have been upgraded to minimize energy loss. Now regulators look to enhancing the efficiency of foundations.
Contractors, designers, and homeowners have wrestled with methods for insulating basements for years. In the past, many builders chose not to provide insulation for basements, leaving the decision of when, and if, the basement should become livable square footage to the homeowner.
The International Residential Code has recommended R-10 basements for some time. A few building jurisdictions have implemented these recommendations but it has not been universally required. Since 2006, the International Residential Code (IRC) has required R-10 foundations for climate zones 4 and above, while the 2009 IRC added R-5 requirements to climate zone 3. More jurisdictions are mandating enforcement of these provisions. Prior to that, climate zone descriptions were much more complex, and insulation requirements for basements ranged from R-5 to R-19 for more than a dozen climate zones nationwide. The question, however, remains: How do you achieve an R-10 foundation?
Insulating a basement
Insulating basements has always been an arduous task when left to a homeowner. At best, the insulation system selected would result in inconsistencies in temperature control. In the 2002 research report “Basement Insulation Systems” from Building Science Corp., Boston, Joseph Lstiburek, PhD, PE, said, “Heat loss from basements accounts for a significant portion of the energy loss from a home.” At the time the trend suggested the leading method for insulating basements was to attach wood studs to the inside of a concrete foundation wall and apply fiberglass batt insulation between the studs with either a drywall finish or a vinyl-faced insulation blanket finish. The report also noted: “Continued use of these approaches by the home building industry will likely lead to a disaster of unprecedented proportions and may result in the construction of energy-efficient homes being set back a generation.”
Fortunately, before these consequences were realized, quality insulation systems for concrete foundations became commonplace. Installing insulation systems for concrete foundations often occurs during the form setup operation. The most common forming systems used in residential foundations include removable, reusable forming systems made of aluminum, wood, or steel (RCFs). These modular easy-to-use and varied installation products can be reused repeatedly. Insulation also can serve as the forms themselves, as in the case of insulating concrete forms (ICFs).
Today, there are multiple proven systems available that attain the desired R-values for the home construction industry. Balancing economy and performance, they offer variability in location, thermal performance, and finishing characteristic.
What all of these systems have in common, however, is the type of insulation. In essence, you need two pieces of 2-inch polystyrene insulation to obtain R-10. Extruded polystyrene (XPS) achieves R-10 by itself and 2 inches of expanded polystyrene (EPS), or beadboard, can reach R-10 when other components in the wall assembly are included.
Polystyrene insulations are best suited for belowgrade applications because of moisture-resistant properties. Where you place the insulation is irrelevant to produce a system R-value that meets the building code. However, if you isolate all or a portion of the concrete mass to the interior of the structure, it will have a greater temperature moderating effect on the living area.
Heavy materials, such as concrete, poorly insulate unless they are exposed to the interior and isolated from the outside by a layer of insulation. Then they store tremendous amounts of energy. As the space cools, the energy is released back into the area, which is commonly referred to as the thermal mass effect. Spaces with a lot of interior isolated mass typically are not subject to wide temperature swings like spaces with lightweight materials and insulation on the interior of the wall.
There are three basic locations for insulation placed into RCF systems for concrete foundations: exterior insulation systems, interior systems, and sandwich wall systems (insulation between two layers of concrete). Each of these offers similar R-values, but the sandwich and exterior systems provide more thermal mass effect. Selection is based on installation costs or on the architectural appeal and building aesthetics desired for the final wall finish.
Exterior insulation systems
Exterior insulation systems can be incorporated during the form setup and casting operation or applied afterward. Including them with the casting operation eliminates a construction step, as well as guarantees a tight fit to the wall and continuity of the thermal envelope. The open matrix nature of most EPS board provides a bond or natural adhesion to the concrete.
The in-form systems vary in how the insulation boards are secured within the forms as well as the attachment points they create for the application of finish and moisture protection systems after stripping the forms. One important characteristic all of these systems share is that they use a break-back tie configuration to eliminate thermal bridging through the insulation layer. The collars providing support for the insulation also conceal notches in the form ties. These notches allow for quick removal of the tie ends after the removal of the form panels.
Exterior insulation systems are designed for cladding systems in a variety of conventional methods per design requirements or owner interest. For abovegrade applications, this protection eliminates damage by ultraviolet radiation and vermin, as well as abuse from lawn mowers and other maintenance activities while providing the structure's finished architectural appearance. Abovegrade coverings include cement-based coatings and siding products, sheet metal or vinyl siding, and a variety of other durable products.
Protection for belowgrade insulation systems includes a waterproofing barrier (that should be installed on all foundations) and provides protection from subterranean vermin and insects. These systems include spray-applied emulsion products, sheet membranes, and drainage systems.
The insulation for these systems also can be placed on the interior side of the wall to provide a durable exterior finish, however, it will greatly reduce the thermal mass effect.
Sandwich insulation systems
The second type of insulation system for an RCF wall is the concrete sandwich wall. A sandwich wall consists of a layer of insulation sandwiched, or positioned, between two layers of concrete. Historically, contractors positioned the insulation against one of the forms with a gap for the concrete between the insulation and the other form. After the initial concrete cure, which typically happened overnight, the form against the insulation would be moved to provide a gap for the second layer of the sandwich wall. This system required special form ties and two separate pouring operations. This system performs well but results in higher labor costs and longer construction schedules, as well as thicker wall sections.
Today, the sandwich wall system typically consists of a 4-inch layer of concrete simultaneously placed into cavities on each side of a layer of insulation. The insulation uses a tie and spacer system specially designed to position the insulation at the proper place in the wall and to provide the structural connection between the two concrete layers through the insulation.
The typical minimum 4-inch-thick concrete layer allows concrete placement around reinforcement and electrical conduit, however, any thickness or combination of thicknesses can be used. An R-10 wall is typically 10 inches thick: 4 inches of concrete on each side of 2-inch XPS insulation. Ideally, the two layers of concrete work together structurally as a single wall unit. This is most effective when the structural connection consists of nonconductive, noncorrosive ties that also eliminate any thermal bridging through the wall that might otherwise lower the designed insulation performance. Insulation thicknesses from 2 to 8 inches have been used with this type of system, depending on insulation requirements.
With this system, the exposed concrete layers provide a durable exterior and interior wall surface. The exterior wall only requires a standard waterproofing system used in any concrete foundation wall application. The interior, depending on the quality control and concrete mix design, typically requires minor finishing or “sacking” of the interior wall surface before the application of paint or other coatings. A coat of skim plaster produces a finish rivaling the best of conventional plaster or drywall applications.
A disadvantage to this system is the casting operation may proceed at a slower pace, particularly with less experienced crews, because you are casting two vertical walls simultaneously. Electrical systems for this type of wall, as well as in an exterior insulation application, can be embedded in conduit placed during the forming operation or they can be post applied in a surface-mounted runway system. The additional time in casting this type of wall is made up with reduced post-treatment of the wall surfaces.
Interior insulation systems
Insulation between wood studs on the interior side of the wall covered with drywall is the simplest form of insulating a foundation wall. This has been the standard procedure of home construction for decades. Properly designed and delivered, it can be effective in meeting the building code requirements. Improperly applied, it can lead to mold, rot, and moisture problems.
The exterior insulation systems can be reversed to produce an interior insulation system. Unlike a standard interior insulation system that uses wood studs, batt insulation, and drywall, the in-form interior insulation system establishes a continuous layer of insulation. This eliminates wood studs that provide both a food source for mold development and thermal zone for moisture development.
Interior insulation systems allow for easy placement of wiring systems that can be installed after removal of the wall forms—a characteristic very similar to an ICF system.
A disadvantage to this system, however, is the barrier created by the continuous insulation isolates the thermal mass of the concrete from the interior living space. Although the thermal mass remains in the wall, it is much harder for the home to benefit from its stabilizing effect on temperature control. The interior of the wall also must be finished with drywall, paneling, or some other rigid material.
Insulating the foundation is not a new requirement; building codes have required it through several code cycles. However, it is a requirement often overlooked during home construction and identified as an owner option. Today, insulating the foundation during construction simply makes sense. The prices of construction and energy are not going to go down. Insulating and finishing foundations as true lower-level-living space is the least expensive square footage you can build. Insulation, combined with window wells, daylight, and walkout configurations turn that extra space into safe, secure, attractive, widely-usable rooms that add value to today's home.
Ed Sauter (firstname.lastname@example.org) is the executive director of the Concrete Foundation Association, Mt. Vernon, Iowa.