Rigid Insulation: Best Practices
Extra-thick layers of exterior insulation can greatly improve the energy performance of homes, but installation requires some special techniques.
INCORPORATING EXTERIOR INSULATION on wall assemblies addresses many common building enclosure energy and durability issues. Exterior insulation can increase the overall thermal resistance of the assembly; it also provides increased condensation resistance in cold climates.
When the joints of the exterior insulation are taped and sealed, it can act as the water control layer.
Building Science Corporation has produced a Measure Guideline that covers the use of rigid exterior insulation boards such as expanded polystyrene (EPS), extruded polystyrene (XPS), polyisocyanurate (PIC) and rigid mineral fiber (MF) as exterior insulation on wall assemblies. It also addresses the use of layers of insulation thicker than 1-1/2”. These extra-thick layers exceed the practical limit of directly attaching siding through the insulation back to the structure, and so require a secondary cladding attachment location.
Placement of Control Layers
Wall assemblies include four “control layers” that protect the building and increase its energy performance and comfort: (1)water control layer; (2)air control layer; (3)vapor control layer; (4)thermal control layer.
Air control. This can occur from the inside of the building or at the sheathing; the rigid insulation can also serve as the air control layer.
Vapor control. When rigid insulation is added to the exterior of the structural sheathing, the interior surface temperature of the structural sheathing increases. When the temperature of the condensing surface of interest is raised sufficiently, interior water vapor migrating into the wall assembly does not condense. This allows assemblies to be constructed in cold climates without interior vapor control layers. The amount of exterior rigid insulation needed is a function of the climate and the amount of insulation added to the interior wall cavities.
Thermal control. Exterior rigid insulation can significantly improve the wall assembly’s thermal performance because the continuous layer diminishes thermal bridges. The amount of exterior insulation added to the assembly will depend on the climate zone and design goals. Minimum levels should be based on the minimum requirements for vapor control and minimum building and energy code requirements. More insulation can be added to create high R-value wall assemblies; these typically add 4” or more of exterior rigid insulation.
If using furring strips to provide cladding attachment, additional furring is needed to accommodate the jamb trim.
Water Control. The water control layer can be installed in front of or behind exterior rigid insulation. When it is located in front of the insulation, two approaches can be used:
- Install building paper or housewrap over the top of the rigid insulation. This option should only be used when the rigid insulation in 1-1/2” thick or less.
- Tape or seal the joints of the rigid insulation to act as the water control layer.
When the water control layer is located behind the rigid insulation, typical products include building paper and housewrap. Other options include fully adhered membranes, liquid-applied membranes and the taped-and-sealed joints of faced structural panels. These products should be installed according to industry standard practice and are independent of installing the rigid insulation. All flashing and other water control interfaces must connect to the wall water control layer.
No fundamental changes are required to the building’s structural design to accommodate exterior rigid insulation. It can be installed in either a single layer or in multiple layers, but the joints should be offset in multi-layer applications.
Mechanical and electrical penetrations should be sealed at the location of the water control layer. The sealing details are similar to those used with a sheet WRB, so the top edge of any membrane flashing should be adhered with construction tape.
For thinner insulation, the cladding can often be attached directly through the rigid insulation back to the structure. The practical limit of this approach is around 1-½” of rigid insulation, due to the limited fastener lengths for many pneumatic nail guns.
Extra-thick insulation. For thicker insulation, vertical wood furring strips can provide a cladding attachment location. This approach also provides a drainage and ventilation gap behind the cladding, which helps manage water. To adequately support the cladding, the furring strips should be attached back to the structure using #10 or larger wood screws that are sized to maintain a 1-¼” minimum embedment into the structural framing. For example, a 6” wood screw can be used to attach up to 4” of insulation (4” insulation + ¾” of furring + 1-¼” of embedment = 6” total).
Stucco and stone veneer. Some claddings, such as stucco and adhered stone veneer, may require additional support between the structural furring strips to facilitate the installation. In these cases, additional structural furring or nonstructural spacer strips can be added.
Shingles. Wood shingles require a continuous nail base, which is best provided by installing another layer of structural sheathing directly over the insulation in place of wood furring strips. the new sheathing becomes analogous to the wood structural sheathing of the wall assembly, so water management details can now be managed exactly per standard construction.
Openings. At openings such as windows and doors, the details for trim attachment can differ slightly from standard construction practice. At the head and sill, the furring strips that are already in the field of the wall are generally sufficient to attach those elements; however, additional furring strips at the jambs are generally required to attach the jamb trim and support the abutting cladding.
If the sill profile is similar to a traditional sill, attaching it through the front face of the trim is often impractical. In this case, small segments of furring strips can be preattached to the back of the sill and the furring can be attached back to the structure to attach the trim. The head flashing above the head trim does not need to be installed all the way back to the water control layer. In reality, that flashing principally maintains the shingle lapping of the siding with the trim, and can thus be installed over the top of the furring.
Additional details, as well as instructions for integrating exterior rigid insulation with windows, roofs, balconies and decks are included in the Measure Guideline.