Continuous exterior insulation (ci) is a core strategy for improving building envelope performance. This article compares two common ci materials—mineral wool and rigid foam board—for energy performance, moisture resistance, fire rating, and constructability.
Material Properties Comparison
- Mineral Wool: Non-combustible, vapor-permeable, high fire resistance
- Rigid Foam: Higher R-value per inch, moisture-resistant, various types (XPS, EPS, polyiso)
- Mineral wool offers sound attenuation; foam offers lightweight installation
Thermal Performance
- Polyiso R-6 to R-6.5 per inch, XPS R-5, EPS R-3.6 to R-4.2
- Mineral wool is typically R-4.2 to R-4.4 per inch
- Use foam for thinner wall assemblies; mineral wool when vapor drying is prioritized
Installation Considerations
- Mineral wool requires rigid furring or screw-through supports
- Foam boards must be taped and sealed at all joints for air barrier performance
- Both can be installed over sheathing, behind cladding, and integrated with WRBs
- Use longer fasteners to penetrate framing through insulation
Code Compliance and Fire Ratings
- Mineral wool meets NFPA 285 requirements without additional fire barriers
- Polyiso and XPS may require thermal barriers depending on wall type and height
- Verify foam board flame spread and smoke-developed ratings per ASTM E84
- Use fire-tested assemblies when foam insulation is used above height thresholds
Best Practices and QA
- Mockups should include fastener layout, WRB integration, and cladding clips
- Use long screws with washers for insulation holding power
- Ensure consistent contact with substrate and avoid insulation gaps
- Document all transitions and penetrations in QA reports
Choice of material should be driven by climate zone, fire resistance needs, and budget constraints.
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