Mold thrives when three factors are present inside wall cavities: air circulation, moisture and a nutrient base. All three factors are typically found in fiberglass-insulated exterior walls.

Of the three, moisture is by far the most significant factor leading to mold. That’s why Building Codes require the installation of a vapor barrier. A vapor barrier is any material that stops passage of moisture through wall, ceiling and floor assemblies of buildings. It prevents interior moisture (generated from bathing, cooking, washing and even breathing) from penetrating into and condensing in unheated attics, basements, crawlspaces, and wall cavities.

Unfortunately, the vapor barriers commonly used today just don’t do the job properly. Materials such as 6 mil poly, at best, act merely as a vapor “retarder”, slowing moisture down, but never stopping it. Let’s examine why.

The following points are illustrated in the diagram below:

1. In fiberglass house construction, a 6 mil poly plastic sheet is typically installed as a vapor barrier
   
   
2. During the construction process, the 6 mil poly vapor barrier gets stapled, punctured by drywall screws and torn for electrical receptacles. The integrity of the barrier is compromised, and it now becomes a vapor retarder.
   
   
3. Moisture in air passes through the 6 mil poly, condenses and becomes trapped in the wall cavity.
   
   
4. During winter conditions, as fiberglass insulation’s real-time R-value decreases, the moisture is trapped as water crystals.
   
   
5. In warmer conditions the crystals thaw to water particles and settle on the bottom lumber plate – the nutrient base for mold growth.

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The following points are illustrated in the diagram below:

A. In SIP construction, the OSB and EPS insulation is the vapor barrier.

B. Drywall screws and electrical receptacles do not compromise this vapor barrier.

C. The EPS’s true R-value also prevents temperature fluctuations, thus preventing condensation from occuring.

Did you know that most home insurance policies have ‘mold exclusion’ clauses? Insurance companies, racked by a slew of mold-related claims several years ago, now explicitly refuse to cover mold-related damages. An example of a standard clause in a property damage policy is as follows: