Insulating Masonry Walls
by Scott Wehrmeyer, Plymouth Foam, Inc.
When using concrete masonry wall construction, the architect has several options for insulating the walls. Cavity wall construction, which provides a convenient space between the backup wall and the veneer, is a popular choice for many reasons. However, if there is no veneer, the insulation must be placed inside the wall or on the interior face. Pouring insulation in the block cores is sometimes used in utilitarian structures, but it offers limited insulating value; it is also difficult to provide an effective vapor retarder for this system.
This discussion will focus on insulation applied to the interior face of the wall. Due to the location, these systems are also used for cast-in-place concrete and precast concrete structures. Most interior systems use wood or metal furring fastened against the inside face of the wall. The furring provides support for interior finishes, and the spaces between members is filled with glass fiber or foam insulation.
wood stud framing
Wood is a good choice for wall framing if metal is not required. It is a familiar material, easy to use, and has useful insulating properties. In a typical wall, wood framing members account for approximately 10 percent of the surface area. At 3-1/2 inches thick, a wood stud has an R-value of 4.35. When wood framing members are factored into total thermal performance, walls with R-11 and R-13 fiberglass batt insulation have effective R values of R-10.3 and R-12.1, respectively.
Because batt insulation is not always uniformly installed, air movement may occur in the insulation space. If there are any voids, warm air will rise on the inner side of the void, then move to the cold side, cool off, and fall back down. This creates an air current within the wall that diminishes the insulation's effective performance.
steel stud framing
In commercial construction architects usually use metal framing instead of wood. It serves the same functions, but has a significant weakness: it has essentially no insulating value.
When the average R value of a system that uses metal framing is calculated, we see the same effect that we did with wood, but it is much more pronounced. Walls with R-11 and R-13 batt insulation have effective R values of R-6.6 and R-7.2, respectively, a loss of 40 to 45 percent! A related problem is condensation. Because the steel brings cold to the inside of the wall, we see problems such as water staining, shadow lines, and mold on interior walls.
The same effects occur when metal z-furring is used with rigid foam insulation. An additional concern with furring is the potential for "creep" to occur between z-furring strips. Drywall contractors have stated that, unless great care is taken during the installation of the furring, incremental increases in the spacing between the strips result in the seams of the gypsum board failing to properly align with z-furring strips, requiring the drywall installer to cut panels to fit.
Self-furring insulation consists of rigid insulation with factory-installed furring. Because the furring is separated from substrates by the rigid insulation, thermal bridging is eliminated. Because the furring does not come in contact with the masonry wall, the thermal performance of these products is superior to conventional wood furring, and moisture-related problems are no longer a concern. Also, because it comes in large sheets, installation is much faster than if the furring and insulation are installed separately.
Where wood is acceptable, the architect has been able to specify products that consists of rigid foam insulation with integral wood furring. Until recently, the wood-and-foam systems have been the only readily available self-furring insulation systems. One or two systems with special metal furring and insulation were available in the past, but were not successful.
Earlier this year, Plymouth Foam, Inc., introduced Gold-Wall, a composite insulation consisting of a 4 X 8 expanded polystyrene sheet with factory-installed steel furring strips. The panel also includes a 2 mil plastic film which adds durability and provides vapor barrier properties. Gold-Wall is attached with threaded or shot/pin mechanical fasteners installed through the furring strips. Gold-Wall is available with furring at either 16 or 24 inches on center, and in a variety of thicknesses.
When selecting insulation, the architect must weigh the benefits of each type. Speed of installation, effective R-value, and moisture problems must all be considered. It is apparent that self-furring systems offer performance superior to conventional framing or furring, and the savings in installation costs will go a long way in offsetting the higher material costs.
Scott is the Sales and Marketing Coordinator for Plymouth Foam, Inc., 800-669-1176.
© 2000 Scott Wehrmeyer, firstname.lastname@example.org