Why Humidity & Mold Happen After Spray Foam

A spray-foam attic becomes a “conditioned space,” meaning the attic should maintain stable indoor-like temperatures and humidity. When installers overlook crucial steps, the attic becomes a moisture trap.

1. Missed Foam Coverage (Gaps and Voids)
Even small uncovered sections can disrupt the entire temperature balance of the attic. When hot or cold air from outside meets the conditioned air inside the attic, condensation forms on the roof deck, framing, or even within the foam itself. This moisture collects over time, encouraging mold growth, wood rot, and a musty odor that often spreads through the home. A single gap may seem insignificant, but in a sealed attic environment it creates a chain reaction that affects the entire structure.

2. Incorrect Foam Thickness
Both open-cell and closed-cell foam must be applied at precise depths to perform as intended. Foam that is too thin fails to create an air seal, allowing outside humidity to creep in and settle on surfaces. Foam applied too thick can cure improperly, leading to spongy texture, chemical odors, or soft spots that trap moisture. Proper thickness is not optional; it is essential for moisture control, temperature stability, and long-term durability.

3. No Mechanical Ventilation or Dehumidification
After spray foam is installed, the attic no longer breathes the way it did before. Traditional passive ventilation is shut off, and the space becomes completely sealed. If the installer did not integrate mechanical moisture control such as a dedicated dehumidifier, makeup air supply, or a return air connection to the HVAC system, humidity levels can climb rapidly. Once humidity rises above a safe threshold, condensation begins to appear on the roof deck and rafters, eventually leading to visible mold growth. A sealed attic without mechanical moisture management is almost guaranteed to develop long-term problems.

4. HVAC Imbalance
The HVAC system plays a major role in regulating humidity. A spray-foam attic that is sealed but not included in the home’s air circulation plan often becomes stagnant. Without adequate airflow, moisture has no pathway to leave the space. This imbalance causes the attic to retain humidity faster than the HVAC system can remove it. Over time, homeowners may notice sweating ducts, rust around HVAC components, or elevated humidity readings even when the indoor air feels normal.

5. Improper Curing or Low-Grade Product
If spray foam is mixed at the wrong temperature, applied too quickly, or sourced from low-quality materials, the foam may not cure correctly. Poorly cured foam can absorb moisture like a sponge, giving mold an ideal place to grow. It may also release strong chemical odors, shrink, crack, or separate from the roof deck. In some cases, the foam continues to off-gas long after installation, signaling that the material never properly stabilized. These curing failures often require targeted removal and replacement to restore a safe attic environment.