Rigid Board Insulation
Rigid insulation is made from fibrous materials or cellular plastic foams and is pressed, extruded, or molded into boardlike forms. Board products can provide both thermal and acoustical insulation, possess modest strength properties with low weight, and provide adequate coverage with few heat-loss paths when installed properly. Rigid insulation boards also may be manufactured with various facers to enhance or protect certain physical properties.
Even though rigid board products are typically more expensive than other types of insulating materials, they are used commonly in buildings where there are space limitations, where rigidity is critical to the application, or where higher R-values are necessary. Rigid board insulation R-values can range from 4 to 8 per inch of thickness depending on the composition and the method of aging.
Board insulation products can be applied in a variety of locations. Exterior sheathings applied over wall framing members (especially with steel stud framing systems), within wall cavities, in masonry veneer wall cavities, or behind an interior finish material are but a few examples.
A number of board insulation products are also used with low – slope roofing systems. Although predominantly found in commercial construction, the most common types include cellular glass, glass fiber, mineral fiber, perlite, polyisocyanurate, polystyrene (expanded and extruded), wood fiberboard, and composites. (Polyisocyanurate is the most common roof insulation according to 1999 statistics.)
Rigid insulation is also being used in innovative ways in new construction to create more energy-efficient homes. Stressed-skin walls can replace traditional stick framing with engineered panels consisting of a foam core with structural sheathing adhered to both sides. Known as structural insulated panels (SIPs), they usually incorporate expanded polystyrene (EPS) or polyisocyanurate foam as the core material. Insulating concrete formwork is another foam-based product that is used in lieu of traditional construction (see Chap. 15 for SIPs and ICF).
Many variables affect the installed thermal performance of rigid board insulation. These include the density of the foam, the blowing agent used to create the foam, the method of aging, the cellular structure, the durability of the material, the presence of dents and chips, the thickness and type of facer (if any) that is used, the thickness of the board, and the conditions in which the foam is installed. Owing to the quantity and variety of products, interested readers will require more information than what can be presented within the scope of this book. The long-term thermal performance of any insulation product always should be evaluated prior to its selection or application. Manufacturer’s literature is a place to start, but any information should be complemented with independently sponsored research.
Rigid insulation can be used as an air infiltration retarder when installed properly. With respect to moisture movement, special attention must be paid to the use of rigid board products on the exterior of a wall when a vapor retarder is already in place on the interior of the wall. Permeance ratings must be verified to ensure that moisture vapor will not become trapped within the wall assembly.
Chlorofluorocarbons (CFCs) are no longer used as a blowing agent for plastic foams. Hydrochlorofluorocarbons (HCFCs), originally a U. S. Environmental Protection Agency (EPA)-approved blowing substitute for (the discontinued) CFCs, are currently found in extruded polystyrene foam boards and polyisocyanurate foam boards. As discussed in detail in Chap. 10, the scheduled phase-out of HCFCs also has played a role in the research, development, and modification of various rigid board insulation products.
Burrowing insects can reduce the thermal performance and structural integrity of the insulation. Although rigid board insulation offers no food value to insects, it provides the potential for insects to easily tunnel from the ground to more desirable materials. The foam is also an attractive nesting environment. For these reasons, some manufacturers treat their foam products with an insecticide, usually a borate compound. Additional precautions such as treating the earth around the building with insecticides, using bait station treatment methods, keeping an inspection area bare of insulation board, removing all wood debris, maintaining specific above-grade clearances, or installing the foam board over the interior of the basement walls rather than the exterior also will help minimize the risk of insect infestation.
All organic cellular plastics, whether or not they contain fire retardants, should be considered combustible and handled accordingly. Terms such as fire-retardant and flame-resistant are sometimes used to describe the combustibility characteristics of foams. While they are valid measures of the performance of these materials under small fire exposure, they are not intended to reflect hazards under exposure to large-scale fire conditions.1 The combustion characteristics of foam insulation products vary with the combustion temperatures, chemical formulation (which will determine thermoplastic or thermoset behavior), and available air. Plastic foam insulation may appear relatively difficult to burn, but when ignited, it burns readily and emits a dense black smoke containing toxic gases (as do all organic combustibles commonly used in construction). Foam insulation used in construction requires a fire – protective covering such as V2"-thick gypsum wallboard or similar 15-minute code-approved thermal barrier. Building codes contain many exceptions regarding the use of thermal barriers, so always verify requirements with the local building code or fire officials and insurers.
Geofoam is another popular buzzword of late. Even though the term has been used since 1992, there is still some confusion as to its definition. Quite simply, geofoam is the generic name for any foam material, usually expanded polystyrene, used in a geotechnical (above-grade or in-ground) application. Ground stabilization, embankment, or other ground-fill applications where a lightweight fill material is required can use geofoam to reduce stresses on underlying soils. Geofoam is now recognized worldwide as a geosynthetic product category in the same sense as geotextiles, geomembranes, geogrids, etc.2 (Fig. 11.1).