by Herbert Slone, RA, and Art Fox
A masonry cavity wall system must successfully perform multiple functions throughout the life of the building. A proper wall is expected to manage moisture, air, and heat, contain fire, and hold up the structure itself. For a wall to perform all these functions, specifications should include all the products necessary for the components to work together.
For the contractor, building a masonry cavity wall is just as challenging as specifying it is for the architect. Contractors rely on the architect for highly precise drawings and specifications so they can produce an accurate bid. They want to be able to build with familiar, proven methods and materials that are compatible and readily available through distribution.
For these reasons, specifying a complete wall system with all the components tested and warrantied together can offer many advantages to the design professional, such as helping support risk management. The design professional’s ability to thrive depends on his or her ability to provide timely documentation for the building’s performance.
Components of a masonry veneer wall system
The structural components forming the basis of the substrate may be steel or wood studs or concrete masonry units (CMUs). On the outside is the weather-resistant component—the cladding or masonry veneer. Between those are three functional component categories that complete the wall system and make the wall perform: moisture/air, thermal, and structural management.
Moisture/air management relies on:
- an air- or water-resistive barrier (WRB);
- a vapor barrier;
- through-wall flashing (including mortar dropping collection and weep vents); and
- water- and air-sealing washers on fasteners.
Thermal management involves:
- insulation between the stud framing;
- continuous insulation (CI) outside and over the framing; and
- fire safing insulation.
Structural management depends on:
- masonry anchors;
- wall ties; and
- water- and air-sealing washers on fasteners.
Systemization
Having all of the right components in the wall is not enough. A true wall system must have passed extensive testing proving the components, as a system, meet the code-mandated performance criteria and are physically and chemically compatible. Further, the system must pass industry-standard tests, such as:
- National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Nonloadbearing Wall Assemblies Containing Combustible Components;
- ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials;
- ASTM E2307, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using Intermediate-scale, Multistory Test Apparatus
(used only for joint firestopping); - ASTM E331, Standard Test Method for Water Penetration of Exterior Windows, Skylights, Doors, and Curtain Walls by Uniform Static Air Pressure Difference; and
- ASTM E2357, Standard Test Method for Determining Air Leakage of Air Barrier Assemblies.
Individual product components of the system can also provide the protection of a warranty that covers them against defects. In the event there is a problem, unified and cooperative solutions are best rather than multiple companies acting separately.
I am providing a few clarifications to points about polyisocyanurate (polyiso) insulation which could be misconstrued in the article:
1. Closed-Cell: Polyiso is a closed-cell rigid foam insulation for use in roofs and walls. Polyiso wall insulation materials meet the ASTM C1289 and are an excellent choice for a continuous insulation system.
2. Moisture resistance: PIMA polyiso manufacturers test products for water absorption. For example, foil-faced polyiso insulation is water repellent due to the low impermeability of the facers and the closed cell nature of the foam core. These properties provide long-term moisture resistance. In fact, the Brick Industry Association’s Technical 28B cited by the author recognizes ASTM C1289-compliant materials as options for brick veneer/steel stud walls. The author’s use of the phrase “truly small and closed cell” in reference to XPS insulation is not recognized by a test standard and may mislead specifiers.
3. High R-Value: With polyiso you can meet the energy code requirements with less product due to the high R-value per inch of polyiso wall insulation.
Thanks for your comments, Justin. We’d like to address your points as follows:
1. Closed-Cell: The term “closed cell” is relative and has more meaning when placed in comparative context. The photos in the article show the difference in XPS versus polyiso cell structure.
2. Moisture resistance: The test standards you cite are different and thus difficult to compare. Polyiso uses a much less rigorous water absorption standard compared to extruded polystyrene. Test standard C 209 uses 2 hours of immersion with 10 minute drain time, compared to test standard C 272 which uses 24 hours of immersion with no drain time. Even still, polyiso has a higher water absorption level in C 209 versus XPS in C 272. The use of different standards, one much less rigorous than the other, is not widely realized among specifiers and leads to confusion when comparing polyiso to XPS. The information in this article sheds light on the differences. Or for more information, read this FAQ, link below:
http://www.owenscorning.com/NetworkShare/EIS/10019950-FOAMULAR-XPS-vs-Poly-FAQ.pdf
3. High R-Value: Polyiso R-values vary by manufacturer. Depending on which manufacturer’s claim is used, the difference in product thickness between polyiso and XPS to achieve an R-10 value may only be 1.75” of material versus 2”.
Further, the use of the LTTR method by polyiso manufacturers to estimate long-term thermal performance a few years ago introduced “actual” R-value uncertainty. LTTR was demonstrated to yield a positive bias in estimating long-term performance. The rigid insulation industry experienced significant debate regarding the validity and reliability of aging estimates for R-value via LTTR.
More recently, the cold weather performance of polyiso has been measured and shown to decline at cold temperatures, per the link below:
http://www.greenbuildingadvisor.com/articles/dept/musings/cold-weather-performance-polyisocyanurate
So, like water absorption, R-value claims are also subject to the need for interpretation.