Moisture Control in Portable Classrooms

Moisture Challenges in Portable Construction

Portable classrooms present unique moisture control challenges due to their lightweight construction, minimal thermal mass, rapid temperature fluctuations, and frequent envelope penetrations. Unlike permanent structures, these modular buildings have thin wall assemblies (typically 3.5-5.5 inches total), limited insulation, and numerous seams that create pathways for air leakage and moisture intrusion. The high occupancy density—often 25-30 students in 800-960 square feet—generates substantial internal moisture loads that must be managed to prevent condensation, mold growth, and indoor air quality deterioration.

Moisture Generation and Load Calculations

The primary moisture source in occupied classrooms is human metabolic activity. Each occupant generates moisture through respiration and perspiration at rates dependent on activity level and ambient conditions.

Moisture generation per occupant:

$$ \dot{m}_{\text{person}} = 0.10 \text{ to } 0.20 \text{ lb/h} $$

For a typical portable classroom:

$$ \dot{m}{\text{total}} = N{\text{occ}} \times \dot{m}_{\text{person}} $$

where $N_{\text{occ}}$ represents the number of occupants. For 28 students plus one teacher at 0.15 lb/h each:

$$ \dot{m}_{\text{total}} = 29 \times 0.15 = 4.35 \text{ lb/h} $$

This moisture must be removed through ventilation to maintain acceptable indoor humidity levels. The required ventilation rate for moisture control is:

$$ \dot{V}{\text{moisture}} = \frac{\dot{m}{\text{total}}}{\rho_{\text{air}} \times (W_{\text{in}} - W_{\text{out}})} $$

where $W$ represents humidity ratio (lb water/lb dry air) and $\rho_{\text{air}} \approx 0.075$ lb/ft³.

Dewpoint Analysis and Condensation Risk

Condensation occurs when interior surface temperatures fall below the dewpoint temperature of indoor air. In portable classrooms with minimal insulation, windows and exterior wall surfaces are particularly vulnerable.

Dewpoint temperature approximation:

$$ T_{\text{dp}} \approx T_{\text{db}} - \frac{100 - RH}{5} $$

where $T_{\text{db}}$ is dry-bulb temperature (°F) and $RH$ is relative humidity (%). For indoor conditions of 70°F and 50% RH:

$$ T_{\text{dp}} = 70 - \frac{100-50}{5} = 70 - 10 = 50°F $$

Interior surfaces must remain above 50°F to prevent condensation. The interior surface temperature depends on thermal resistance:

$$ T_{\text{si}} = T_{\text{in}} - \frac{(T_{\text{in}} - T_{\text{out}}) \times R_{\text{si}}}{R_{\text{total}}} $$

where $R_{\text{si}}$ is the interior surface film resistance (typically 0.68 h·ft²·°F/BTU for vertical surfaces) and $R_{\text{total}}$ is the total wall assembly R-value.

Vapor Barrier Placement and Design

Proper vapor barrier placement is critical in portable classroom construction. The vapor retarder must be located on the warm side of the insulation to prevent moisture migration into the wall cavity where it can condense on cold surfaces.

Climate-specific placement:

• Heating climates: Vapor barrier on interior (warm) side
• Cooling climates: Consider vapor-open interior finishes
• Mixed climates: Class II or III vapor retarders (0.1-10 perms)

The vapor diffusion rate through building assemblies follows:

$$ \dot{m}_{\text{diff}} = \frac{A \times \Delta p}{M} $$

where $A$ is surface area (ft²), $\Delta p$ is vapor pressure difference (in Hg), and $M$ is vapor resistance (perm-inch). Wall assemblies with $M > 1.0$ perm provide adequate vapor control for most portable classroom applications.

Building Envelope Coordination

The building envelope must function as a comprehensive moisture control system with four critical control layers:

1. Water control layer: Exterior cladding with proper flashing and drainage
2. Air control layer: Continuous air barrier at all joints and penetrations
3. Vapor control layer: Appropriate vapor retarder based on climate
4. Thermal control layer: Continuous insulation without compression

Air leakage is typically the dominant moisture transport mechanism, carrying 50-100 times more moisture than vapor diffusion. Portable classrooms should achieve air tightness of less than 0.30 cfm/ft² at 75 Pa pressure differential.

HVAC System Integration

The HVAC system must coordinate with the building envelope to maintain appropriate moisture levels. Key requirements include:

Ventilation control: Minimum outdoor air of 15 cfm per occupant per ASHRAE 62.1, with provision to increase ventilation during high-moisture activities.

Humidity control: Maintain indoor relative humidity between 30-50% during heating season and below 60% during cooling season. For mild climates:

$$ RH_{\text{target}} = 40% \pm 10% $$

Dehumidification capacity: In humid climates, dedicated dehumidification may be required when sensible heat ratio exceeds 0.75:

$$ \text{SHR} = \frac{Q_{\text{sensible}}}{Q_{\text{sensible}} + Q_{\text{latent}}} $$

Continuous operation: Maintain air circulation during unoccupied periods to prevent moisture accumulation. Setback mode should maintain at least 2-3 air changes per hour to prevent stagnant conditions.

Mold Prevention Strategies

Mold growth occurs when relative humidity at building surfaces exceeds 80% for extended periods (typically 24-48 hours) with temperatures between 40-90°F. Prevention requires:

• Maintain interior surfaces above dewpoint temperature
• Control indoor humidity through adequate ventilation
• Eliminate bulk water intrusion through proper drainage and flashing
• Provide rapid drying capability when moisture events occur
• Inspect and maintain roof-to-wall interfaces quarterly

Portable classrooms in humid climates should incorporate vapor-impermeable exterior insulation to raise exterior sheathing temperature above dewpoint, preventing interstitial condensation within wall cavities.

Monitoring and Maintenance

Continuous monitoring of temperature and humidity conditions enables early detection of moisture problems. Install hygrometers to track:

• Indoor relative humidity (target 30-50%)
• Supply air dewpoint during cooling
• Surface temperatures at vulnerable locations (windows, corners)

Regular inspection should identify water stains, musty odors, or visible mold within 24 hours of detection, with immediate remediation to prevent amplification.