30 July 2009

Continuous Insulation for Exterior Cavity Walls

There is some debate regarding the best type of insulation to use as the continuous insulation in exterior cavity walls. The most prevalent product used in America is foam plastic insulation (FPI), but we are becoming increasingly aware of its shortcomings. This article will look at some of the issues and then some solutions.

The primary shortcoming of FPI is that it is combustible, and combustible materials are not permitted in concealed spaces in Types I and II construction except under specific conditions. Chapter 7 lists a few exceptions, of which FPI is not one, but cross references to Chapter 6 [717.5 ex. 1] where in regards to FPI, there is a cross reference to Chapter 26 [603.1 no. 3].

In interior or Type V construction, the FPI must have flame spread <= 75 and smoke developed <= 450 [2603.3], and there must be a thermal barrier between FPI and the interior of the building (except in certain specific circumstances not related to typical cavity wall situations) [2603.4]. In exterior walls of all other types of construction, there are more requirements:
  1. If the wall is required to be fire rated, you need to document that ASTM E 119 results remain valid. [2603.5.1]
  2. There must be a thermal barrier between FPI and the interior of the building (except in certain specific circumstances not related to typical cavity wall situations). [2603.4 as reaffirmed by 2603.5.2]
  3. The potential heat of the FPI (NFPA 259) cannot exceed that permitted by NFPA 285. [2603.5.3]
  4. FPI must have flame spread <= 25 and smoke developed <= 450. (ASTM E 84) [2603.3 as modified by 2603.5.4]
  5. The whole wall assembly needs to be tested to NFPA 285 [2603.5.5] (except for certain one story buildings. [2603.4.1.4]).
  6. The FPI needs to be labeled. [2603.5.6]
  7. The FPI needs to be tested to NFPA 268 for ignition. [2603.5.7]
One exception is if the FPI is applied to the interior face of the exterior wall. In such a situation, it may be construed that it is not longer a component of an exterior wall assembly, and therefore only needs to be protected by a 15 minute thermal barrier  complying with 2603.4.

Now that we know the issues, we can look for solutions.

Looking through the ICC Evaluation Service Reports, there are lots of EPS, XPS, and polyiso products that meet the requirements for Type V construction. However, only a few qualify for other Types of construction. For Types III and V construction, there are products by Baysystems North America, Icynene, Inc., SWD Urethane Co., and Thermal Foams. For all construction types, I have only found only the following:
  • Dow Chemical Company: STYROFOAM (XPS), ASTM C 578 Types IV and X (http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-2142.pdf) when used up to 2.5" thick in steel stud or masonry backed cavities with 4" brick veneer. R-value = 5.0/in.
  • Dow also has a Thermax system for which the facing can be 4" clay brick, 3/4" 3-coat stucco, 2" limestone, metal composite material (MCM), 1-1.25" terra cotta, cement board, or metal panel. This system has achieved a 1-hour fire rating.
  • Falcon Foam, A Division of Atlas Roofing Corporation: EPS Insulation Boards, ASTM C 578 Types I, II, VIII, and IX (http://www.icc-es.org/reports/pdf_files/ICC-ES/ESR-1962.pdf). The product is used in approved EIFS assemblies, but I have no information that it has been tested in any cavity wall assemblies.
  • Centria: invelope has passed NFPA 285. Although Centria has not listed with ICC-ES, the other aspects of 2603.5 are easily addressed.
Fiber Board
The scarcity of legal FPI products for non-type V construction is good enticement to check out other products. Fiberglass is not a good choice because it doesn't hold up well to the water exposure that is always present on the exterior side of the weather barrier membrane. Mineral wool, on the other hand, is naturally water resistant and has the added advantage of being fire proof! The following manufacturers manufacture mineral wool products with waterproof binders that are appropriate for cavity installations, R-value = 4.2/in.:
  • Fibrex
  • Roxul
  • Thermafiber
The drawback to fiber insulation is you need 2-1/2" thickness to achieve the same R-10 insulating value as 2" FPI. Fortunately, this additional thickness is still within the range of common masonry anchoring systems.

Foam Glass
Pittsburgh Corning manufactures and markets cellular glass insulation in Europe for exterior wall applications. If you're interested in imports, it is a consideration. See http://www.foamglasinsulation.com/building/.

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