Simple Solution: Exterior Continuous Insulation with Drainage

Originally published by: The Building EnclosureJanuary 23, 2014

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Adoption of the 2012 International Energy Code will require exterior continuous wall insulation for climate zones 6, 7, and 8.  When used in combination with cavity insulation, the minimum requirements include 20+5 or 13+10.  The first value pertains to cavity insulation whereas the second is continuous exterior insulation (CI).   Although comparable insulation strategies are also outlined by the 2012 code, envelope trade-off options will be limited and equipment trade-off options will be prohibited.  When considering typical 2×6 construction, most builders will likely default to a minimum R-5 on the exterior side and batt-filled cavities to achieve the ‘20+5’ option.

The 2012 requirements complicate matters considerably.  First, we are faced with the problem of moving the drainage plane to the exterior face of the CI as opposed to the sheathing and weather-resistive barrier.  This requires perfect seals at all insulation joints, terminations, and wall penetrations in order to prevent moisture entrapment between the sheathing and CI.  We are also asking builders to change the way we integrate this plane with window rough openings – a tall order for current construction practice, which, after 30 years of mayhem, has finally begun to resolve the window flashing problem.  Failed buildings constructed according to the 2009 CI options suggest that quality practices simply lack the necessary stringency to implement perfect sealing.  Some degree of moisture penetration beyond the CI is unavoidable.

Also new to 2012 is the explicit requirement for sealing interior vapor retarders.  In Minnesota, the 2009 energy code had similar requirements for non-residential and certain multi-family residential.  As with continuous insulation, documented failures showed that perfect seals are difficult to achieve and almost never achieved in today’s ‘value engineered’ mindset. The two conditions – that is, continuous insulation and sealed vapor retarders, are very much related.   Adding R-5 CI does not thermally isolate wall sheathing, which stays sufficiently cold and vulnerable to moisture accumulation.  Moisture sorption is exacerbated by low-permeability outbound insulation.   Cavity insulation makes the matter even worse by thermally buffering the sheathing from the conditioned indoor environment.

So we are faced with a two-fold challenge.  Be perfect in sealing the continuous outbound insulation; and be equally perfect in sealing interior vapor retarders.  In reality, one failure is just as likely as the other.

Gold cannot be pure, and people cannot be perfect.
Chinese Proverb

Hygrothermal Performance of a 20+5 Wall
Consider typical 2×6 construction utilizing a ‘20+5’ configuration:  brick; 1” air space; 1” extruded polystyrene insulation; spun-bond polyolefine WRB; OSB sheathing ; 5-1/2” low density fiberglass batt; polyethylene vapor retarder, and 1/2″ interior gypsum board.  Variations to this assembly are described as wall types A, B, C, and D.

Hygrothermal performance of OSB sheathing is illustrated below.  The unforgiving nature of a typical ‘20+5’ configuration is apparent.  Either failure results in moisture levels that exceed minimum thresholds for mold (16% MC).  Combining the two failures yields even higher moisture levels, leaving no doubt as to the fate of this assembly.   In contrast, Wall D, which relies entirely on outbound continuous insulation, performs extremely well even with an increased moisture deposition behind the continuous insulation.

Wall A is the most troubling.  Here, the interior vapor retarder is assumed to be perfect; therefore, high moisture burdens are due entirely to the 0.25% wind-driven rain component.   We chose a 0.25% load based on documented building failures showing fungal growth and wood swelling on similarly configured ‘20+5’ walls.  Such conditions are indicative of moisture levels in excess of 16%, which is also predicted by the hygrothermal simulations.  It should be noted that the above scenarios assumed well-conditioned interior climates with low interior humidity (i.e. 69.8°F ± 1.8°F; 45% ± 15%).  Failures are significantly more pronounced under situations of medium and high interior moisture.

The industry is smitten with insulation.  According to current dogma, more is better and no cavity shall go unfilled.  Clearly, this creed is inordinately risky and will have us picking up the pieces for decades to come.  The solution is simple:  1) rely on exterior continuous insulation with drainage channels abutting the exterior face of the WRB/sheathing, 2) remove cavity insulation; and 3) remove the interior vapor retarder.   Sound familiar?  It should to those familiar with Exterior Insulation and Finish Systems (EIFS).   In 2006, a study completed by the Department of Energy demonstrated vastly improved performance for assemblies employing the above strategy.  Sheathing stays sufficiently warm while enabling inward drying and safe moisture storage.  Hygric nirvana is achieved by anticipating imperfection rather than demanding the perfect build.

It is a colossal understatement to say that we are an industry entrenched in failed practices.  Where betterment demands perfection, we are destined to achieve neither.

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