Why You Should Use Air and Moisture Barrier with Foam Sheathing
There are a number of reasons why you need an air and moisture barrier over foam sheathing. Airtight exterior wall systems are critical to maximizing energy efficiency and gaining LEED points. However, certain characteristics of foam sheathing conspire to compromise airtight construction. The solution? Use foam sheathing together with a high-performing, moisture-resistive air barrier.
As building codes increasingly require effective, continuous insulation, foam sheathing products have become more popular. Some feel that foam sheathing eliminates the need for an air and moisture barrier. However, there are two key reasons why it is best to combine a high-performing air-moisture barrier with foam sheathing: dimensional instability and reverse shingling. Foam structural panels expand and contract, and reverse shingling is unavoidable when taping edges of thick sheathing, because overlapping is not possible.
There are three kinds of foam sheathing currently in use: extruded polystyrene (XPS), expanded polystyrene (EPS), and polyisocyanurate (Polyiso). Dimensional instability varies to a degree, because EPS and XPS are thermoplastics, while Polyiso is a thermoset foam.
Dimensional instability is a key issue that can compromise R-value and allow unwanted moisture intrusion.
The problem is that extruded polystyrene or polyurethane foam can expand and contract. For example, XPS foam board has a maximum shrinkage rate of two percent. Although this may not sound like much, it is a full 1.9 inches on a 96-inch board. Stress on tape joints is such that tape failure becomes a possibility. Even with polyurethane foam, the coefficient of expansion and contraction is approximately 1/4 inch on a 96-inch board.
As a result, tape may come loose as the foam sheathing expands and contracts. Even when the tape retains its integrity, the necessary gap compromises the thermal insulating value of the sheathing.
Because of the dimensional instability inherent of these products, improper installation can quickly exacerbate problems of air and moisture intrusion.
Another issue with foam sheathing is the fact that it cannot be lapped, so it does not always shed water properly. It is possible for water to enter along the top of a taped seam and migrate into the wall system. Once excess moisture is inside the wall, it can lead to decreased R-values, mold or rotting.
Ideally, foam sheathing adds important insulation value to a wall system. The R-value of EPS is usually around four per inch, while XPS forms often have an R-value of about five per inch. Since thermal drift is more of an issue with polyiso, so the long-term thermal resistance (LTTR) R-value is important. Most Polyiso products have an LTTR-value of 6.5 per inch. However, moisture intrusion must be managed to maintain the full insulating capability of the product.
Other potential issues may be helped by the addition of a high-performing air barrier when using foam sheathing. Unfaced foam boards are more susceptible to UV degradation. EPS boards subjected to excessive handling may grow rounded along the edges, decreasing the thermal value at the joints and enhancing the potential for taping problems. Maintaining continuity at difficult detail areas where there are changes in plane (e.g. soffit areas) or at interfaces with other materials such as at sills can lead to further vulnerability in air and moisture protection.
Thermoplastics have other properties that are potentially problematic in building construction: softening at elevated temperatures and reacting to organic solvents. Since these foams may degrade when they come in contact with certain adhesives, paints and fuels, it is advantageous to add a layer of protection via a high-performing air barrier.
The use of a high-performing air and moisture barrier becomes a critical line of defense when used in tandem with foam sheathing. The possibility of dimensional instability, reverse shingling and other problems with foam sheathing are best controlled by using such sheathing with DELTA®-VENT SA , a high performance vapor permeable, moisture-resistive air barrier that can maximize the performance of foam sheathing.
When a builder combines DELTA®-VENT SA and foam sheathing, an airtight structure with high R-value exterior walls is possible. Energy savings, reduced carbon emissions and the comfort of occupants are better addressed.
DELTA®-VENT SA is a so-called “sticky wrap” that results in more airtight and watertight structures. Its self-adhering design eliminates mechanical fasteners, which further reduces the potential for leaks.
DELTA®-VENT SA is a more substantial WRB that resists damage during construction. The material is highly vapor permeable (50 perms), and its design promotes quick draining and drying. It is the only self-adhering and vapor-permeable WRB both evaluated by the Air Barrier Association of America and International Code Council (ESR 2932).
Dörken delivers innovative, high-performance air and moisture barriers for commercial and residential construction sold under the DELTA® brand name. A North American manufacturer based out of Beamsville, Ontario, Dörken Products, Inc. is a subsidiary of Ewald Dörken AG, a leading European developer and manufacturer of waterproofing and drainage products sold worldwide. Dörken is known for delivering premium products while providing educational programs and full technical support. For more information, call 1-888-4DELTA4 (433-5824) or visit www.dorken.com.