Moisture is the hidden enemy of home performance. It causes mold, rot, structural damage, and poor indoor air quality. Understanding how moisture interacts with insulation is critical for a healthy, efficient home.
How Moisture Gets Into Walls
- Air leakage: Warm, humid indoor air leaking into wall and ceiling cavities — this is the #1 moisture transport mechanism, carrying 50-100x more moisture than vapor diffusion
- Vapor diffusion: Slow movement of moisture through materials from high humidity to low humidity
- Bulk water: Rain, groundwater, and plumbing leaks
- Condensation: When warm, moist air contacts a cold surface (like the inside of exterior sheathing in winter)
Vapor Barriers: Where and When
Vapor barriers are often misunderstood and misapplied:
- Crawl spaces: Always — a heavy vapor barrier on the ground is essential
- Exterior walls: In Indiana (Climate Zone 5), a vapor retarder (kraft-faced batts or latex paint) on the warm-in-winter side is recommended
- Attics: Generally no — attics need to breathe. An air barrier (sealed drywall) is more important than a vapor barrier
- Closed-cell spray foam: Acts as its own vapor barrier — no additional barrier needed
The Role of Ventilation
- Attic ventilation: Removes moisture that migrates through the ceiling and prevents ice dams. Balanced intake (soffit) and exhaust (ridge) is ideal.
- Bathroom and kitchen exhaust: Must vent to the exterior, never into the attic. Attic-vented exhaust fans are a leading cause of moisture problems.
- Whole-house ventilation: Tight, well-insulated homes need mechanical ventilation (ERV or HRV) to maintain indoor air quality.
Getting It Right
The key principle: insulate and seal to prevent moisture from entering assemblies, and provide a drying path so any moisture that does get in can escape. This balance of control and forgiveness is what separates good insulation work from problems waiting to happen.
At Homeward Insulation, we understand building science. Contact us for insulation that performs — without moisture problems.