Building envelopes, climate and how materials handle water

In plain English

The envelope (roof, walls, foundation, windows and doors) keeps out rain and ground water and controls heat and moisture flow. R520 identifies poorly flashed windows, doors and penetrations as a frequent entry point and illustrates a correct flashing sequence. Climate matters too. In hot, humid climates moisture tends to drive from outside in and can be trapped in walls, so interior vinyl wallcovering or plastic vapor retarders can stop a wall from drying. More insulation also reduces a wall's ability to dry.

R520 reviews how common materials behave when wet. Concrete dries slowly and unevenly. Paper-faced drywall is treated as porous and supports mold if it stays wet. Wood is more prone to mold above about 16% moisture content. OSB can be hard to dry once soaked. Laminate flooring usually has to come up if water gets under it, and vinyl flooring often fails at the adhesive.

For insulation, wet loose-fill fiberglass or rock wool that has compacted should be replaced. Closed-cell foam generally keeps its R-value, while open-cell foam can hold water. Wet or contaminated internal duct lining should be replaced under the NADCA standard. R520's material notes refer to the IICRC S500 water-damage standard for the categories of water contamination.

Key points

• Bad flashing at windows, doors and penetrations is a frequent leak source.
• In humid climates, interior vinyl or plastic layers can trap moisture in walls.
• Wood is more mold-prone above about 16% moisture content.
• Wet drywall is treated as porous; compacted wet loose-fill insulation is replaced.
• Wet or moldy internal duct lining is replaced, not cleaned.

Official text

This topic is covered in our own words because the standard behind it is copyrighted; we cite it below rather than reproduce it.

Citations

• IICRC S520 Reference Guide, IICRC S520 Standard and Reference Guide for Professional Mold Remediation — Reference Guide — Ch. 4, pp. 36–48. Best practice

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