Thermally efficient, airtight building envelopes are crucial components of today’s sustainable buildings. They are needed to meet the increasingly stringent requirements of green building standards and codes, such as LEED Version 4 and the International Energy Conservation Code
(IECC). They provide more comfortable, healthier interiors. However, in the current economy, it’s more important than ever to make specifications that deliver clients the maximum amount of performance for their money. One excellent, contemporary strategy for creating high-performance building envelopes within client budgets is to incorporate a hybrid insulation system into the design of the exterior wall assembly.
Hybrid insulation systems combine two or more insulation products, utilizing the products’ best attributes to provide a cost-effective airtight seal around buildings, as well as high thermal and moisture resistance. In this article, we’ll look at the components of a hybrid insulation system strategy CertainTeed Corp., Valley Forge, Pa., recommends–from exterior to interior–and what each one contributes to the ultimate goal of a more sustainable building envelope.
Weather-Resistive Barrier
The first component of a high-performance system from exterior to interior is a weather-resistive barrier (WRB), also known as building wrap. The WRB is positioned directly under the exterior cladding, over the exterior sheathing and acts as a full wall flashing to mitigate the infiltration of bulk moisture from the exterior. As an added advantage, it also allows interior water vapor to escape from the building’s wall cavity to help promote drying.
Continuous Insulation
Often placed between the sheathing and exterior cladding, continuous insulation covers the entire exterior wall, including structural framing members. Depending on the installation detail, this positioning can create a thermal break in the wall to impede air leakage, which reduces energy loss. Though the most common choice for continuous insulation is a foam board-usually expanded polystyrene (EPS), extruded polystyrene (XPS) or polyisocyanurate (PIR)-there are other materials that can be used. One lesser-known, but effective, alternative is high-density, rigid fiberglass board.
Rigid fiberglass, proven in countless commercial applications, has the same advantages as polyfoam, such as easy installation and high thermal performance. It has the superior fire resistance of the two materials, but tends to be more solid and slightly less expensive. In regards to thermal resistance, the R-value per inch of rigid fiberglass board is comparable to that of standard polyfoam boards-fiberglass rates slightly above R-4 per inch, with EPS at R-4 per inch and polyisocyanurate at about R-6 per inch. The placement of the boards should be over the exterior sheathing with the faced side looking toward the interior, with all seams taped.
Closed-Cel SPF and Blown-in Fiberglass Insulation
Made from polyurethane, closed-cell SPF insulation offers a thermal resistance of up to R-6.4 per inch of installed thickness-one of the highest insulating values available on the market today. It has a higher density than open-cell SPF and is therefore more durable, has a higher R-value and a stronger resistance to moisture and vapor. These attributes make it the preferred SPF material choice for high-performance hybrid systems. Closed-cell SPF is sprayed as a liquid into the wall cavity and expands via chemical reaction to up to 30 times its initial volume, filling crevices, gaps and other hard-to-reach spaces. This creates a very effective air barrier and vapor retarder. In addition to being a high-performance, multi-functional insulation material, SPF applied to exterior walls and attic ceilings provides additional structural support and racking strength.
The combination of SPF and fiberglass insulation in the wall cavity as part of a hybrid insulation system provides an air seal similar to a full cavity of SPF at a significantly lower installed cost. This strategy consists of a 1- to 2-inch flash coat of closed-cell SPF against the interior surface of the exterior sheathing, with blown-in, loose-fill fiberglass insulation filling the remainder of the cavity. Fiberglass batts can also be used in these strategies, but the installer will need to work slowly and carefully to ensure the same perfect fit that one gets with blown-in loose-fill fiberglass insulation.
Smart Vapor Retarder/Air Barrier
The final step of CertainTeed’s hybrid insulation system strategy is applying a polyamide-film “smart” vapor retarder, such as CertainTeed’s MemBrain. This vapor retarder is known for its ability to change permeability with ambient humidity conditions. With a high resistance to water vapor permeance at low relative humidity levels, MemBrain protects the hybrid insulation system as much as traditional poly or kraft vapor retarders. But, unlike traditional solutions, it reacts to high relative humidity by increasing its pore size, allowing water vapor to pass through it. The product is also able to form an interior air barrier system when combined with recommended tapes and sealants.
MemBrain is typically positioned on the interior side of the insulated wall, over the fiberglass insulation, for maximum moisture protection. It is important to consult state building codes first, however, as vapor retarder placement recommendations can vary between climate zones.
Conclusion
Hybrid insulation systems like the one referenced above, offer clients ample performance value for their money. They present a cost-effective strategy for meeting or exceeding the thermal, air and moisture resistance requirements of the IECC and LEED Version 4. And, most importantly, the client will get a building with lower utility bills and a more comfortable interior with higher indoor air quality. The benefits of these systems are substantial and are excellent selling points to the building owner who wants sustainability for the best price possible.
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Lucas Hamilton is manager, building science applications for CertainTeed Corp., Valley Forge, Pa. To learn more, visit www.certainteed.com.