Air Conditioning Considerations

Overview

Air conditioning in hot-humid climates creates a unique set of moisture-related challenges for building envelope assemblies. The cooling of interior surfaces below the dew point of outdoor air can induce inward vapor drive through exterior walls, potentially leading to condensation within or on interior surfaces of the assembly. This reverse vapor drive phenomenon is opposite to the traditional cold-climate concern of outward vapor flow from heated interior spaces.

The fundamental moisture transport mechanism is driven by the vapor pressure differential between the warm, humid exterior environment and the cooled interior space. When interior surfaces are maintained at temperatures below the outdoor dew point, moisture can condense on or within the building assembly if proper moisture control strategies are not implemented.

Vapor Pressure Differential Analysis

The vapor pressure difference across the building envelope drives moisture transport and determines condensation risk.

Vapor Pressure Calculation

The saturation vapor pressure at a given temperature follows the Magnus-Tetens approximation:

$$P_{sat} = 610.78 \cdot e^{\frac{17.27T}{T+237.3}}$$

Where:

• P_sat = saturation vapor pressure (Pa)
• T = temperature (°C)
• e = natural logarithm base

For hot-humid climate conditions:

• Outdoor: 32°C, 75% RH
• Indoor: 24°C, 50% RH

Outdoor vapor pressure: $$P_{v,out} = 0.75 \times 610.78 \cdot e^{\frac{17.27 \times 32}{32+237.3}} = 3,615\ Pa$$

Indoor vapor pressure: $$P_{v,in} = 0.50 \times 610.78 \cdot e^{\frac{17.27 \times 24}{24+237.3}} = 1,490\ Pa$$

Vapor pressure differential: $$\Delta P_v = 3,615 - 1,490 = 2,125\ Pa$$

This substantial inward vapor pressure drives moisture from exterior to interior through the building assembly.

Interior Surface Cooling Effects

Air conditioning systems cool interior surfaces, creating potential condensation sites when surface temperatures drop below the dew point of adjacent air.

Surface Temperature Determination

Interior surface temperature depends on:

1. Conductive Heat Flow Through Assembly

$$T_{si} = T_i - \frac{q \cdot R_{si}}{1}$$

Where:

• T_si = interior surface temperature (°C)
• T_i = indoor air temperature (°C)
• q = heat flux through assembly (W/m²)
• R_si = interior surface resistance (0.12 m²·K/W for vertical surfaces)

2. Radiative Cooling to Cold Surfaces

$$q_{rad} = \varepsilon \sigma (T_s^4 - T_{cold}^4)$$

Where:

• ε = surface emissivity (0.9 for typical interior finishes)
• σ = Stefan-Boltzmann constant (5.67×10⁻⁸ W/m²·K⁴)
• T_s = surface temperature (K)
• T_cold = temperature of cold surface, such as air conditioning diffuser (K)

3. Convective Cooling from Supply Air

Direct impingement of cold supply air on interior surfaces can depress surface temperatures locally by 2-5°C below room air temperature.

Critical Dew Point Considerations

Interior surfaces must remain above the dew point temperature to prevent condensation.

For typical hot-humid climate conditions:

• Outdoor: 32°C, 75% RH (dew point 27.2°C)
• Indoor: 24°C, 50% RH (dew point 13.0°C)

Condensation Risk Scenarios:

Exterior Moisture Drive

Hot-humid climates present persistent inward moisture drive throughout the cooling season.

Vapor Diffusion Through Assemblies

Fick’s law governs steady-state vapor diffusion:

$$G = \frac{\mu \cdot \Delta P_v}{d}$$

Where:

• G = vapor flux (kg/m²·s)
• μ = vapor permeability of material (kg/m·s·Pa)
• ΔP_v = vapor pressure difference (Pa)
• d = material thickness (m)

For a typical hot-humid climate wall assembly with inward vapor drive:

The assembly’s total vapor resistance determines moisture flux:

$$R_{total} = \sum \frac{1}{\text{permeance}_i}$$

Solar-Driven Moisture

Solar radiation on exterior surfaces increases outward moisture drive from wet cladding materials:

$$T_{surface} = T_{ambient} + \frac{\alpha \cdot I}{h_o}$$

Where:

• T_surface = exterior surface temperature (°C)
• T_ambient = outdoor air temperature (°C)
• α = solar absorptance (0.3-0.9 depending on color)
• I = solar irradiance (W/m²)
• h_o = exterior film coefficient (25 W/m²·K)

Dark brick surfaces can reach 50-65°C under direct sun, creating vapor pressures of 12,000-20,000 Pa that drive stored moisture inward.

Condensation Potential Within Assemblies

The temperature profile through the wall assembly determines where condensation may occur.

Temperature Profile Calculation

For steady-state heat transfer through a multi-layer assembly:

$$T_x = T_i - \frac{R_x}{R_{total}} \times (T_i - T_o)$$

Where:

• T_x = temperature at interface x (°C)
• R_x = thermal resistance from interior to point x (m²·K/W)
• R_total = total assembly R-value (m²·K/W)
• T_i = interior air temperature (°C)
• T_o = outdoor air temperature (°C)

Dew Point Profile Analysis

Compare the temperature profile to the dew point profile through the assembly:

$$T_{dp,x} = \frac{237.3 \times \ln(\frac{P_{v,x}}{610.78})}{17.27 - \ln(\frac{P_{v,x}}{610.78})}$$

Where P_v,x is the vapor pressure at location x, calculated from:

$$P_{v,x} = P_{v,i} + \frac{R_{v,x}}{R_{v,total}} \times (P_{v,o} - P_{v,i})$$

Condensation occurs when T_x < T_dp,x at any point in the assembly.

Example: Hot-Humid Climate Wall Analysis

Design conditions:

• Outdoor: 32°C, 75% RH (P_v = 3,615 Pa)
• Indoor: 24°C, 50% RH (P_v = 1,490 Pa)

Assembly from exterior to interior:

1. Brick veneer
2. 25mm air space
3. 12mm plywood sheathing (R = 0.08 m²·K/W)
4. 140mm fiberglass batt (R = 2.29 m²·K/W)
5. 6-mil polyethylene (vapor barrier)
6. 12.7mm gypsum board (R = 0.08 m²·K/W)

Total R-value = 2.45 m²·K/W (not including surface films)

Temperature at interior face of sheathing: $$T_{sheath} = 24 - \frac{0.08}{2.57} \times (24-32) = 24.25°C$$

With a vapor barrier on the interior, vapor pressure at sheathing interior face approaches indoor conditions (1,490 Pa), corresponding to a dew point of 13.0°C.

Since T_sheath (24.25°C) » T_dp (13.0°C), no condensation occurs.

Without interior vapor retarder:

Vapor pressure at sheathing would be approximately: $$P_{v,sheath} = 1490 + \frac{0.027}{0.067} \times (3615-1490) = 2,347\ Pa$$

Corresponding dew point: 19.8°C

Temperature still exceeds dew point, but margin is reduced to only 4.5°C.

Interior Vapor Retarder Requirements

The interior vapor retarder controls inward moisture diffusion during the cooling season.

Vapor Retarder Classes (per ASHRAE 160)

Required Vapor Retarder Permeance

The required vapor retarder permeance depends on:

1. Exterior permeance - Lower exterior permeance requires lower interior permeance to maintain proper ratio
2. Assembly R-value - Higher insulation increases condensation risk
3. Indoor humidity control - Lower indoor humidity reduces required vapor resistance
4. Climate severity - More extreme conditions require stricter control

ASHRAE Guideline: For hot-humid climates (IECC Climate Zones 1-2), interior vapor retarders should be:

• Class III (semi-permeable) or more permeable for standard assemblies
• Class II may be needed with high interior humidity or cool interior temperatures
• Class I (impermeable) generally not recommended unless modeling proves necessary

Vapor Retarder Location and Continuity

Interior vapor retarders must be:

1. Continuous - Penetrations sealed with acoustical sealant or compatible tape
2. On warm side in winter - Interior location suitable for hot-humid climates year-round
3. Detailed at transitions - Floor-to-wall, wall-to-ceiling, wall-to-window/door connections
4. Compatible with air barrier - Often the same element in hot-humid climates

Dehumidification Requirements

Air conditioning systems must provide adequate dehumidification to maintain acceptable indoor humidity levels.

Moisture Removal Load

Total moisture removal required includes:

1. Infiltration moisture load: $$m_{inf} = \rho_{air} \cdot Q_{inf} \cdot (\omega_o - \omega_i)$$

Where:

• m_inf = infiltration moisture load (kg/h)
• ρ_air = air density (1.2 kg/m³)
• Q_inf = infiltration airflow (m³/h)
• ω_o, ω_i = outdoor and indoor humidity ratios (kg_water/kg_air)

2. Ventilation moisture load: $$m_{vent} = \rho_{air} \cdot Q_{vent} \cdot (\omega_o - \omega_i)$$

3. Internal moisture generation:

• Occupants: 40-80 g/h per person
• Cooking: 600-1200 g/h
• Bathing: 200-400 g/h per event
• Plants: 10-50 g/h per m² of foliage

4. Reverse vapor drive through envelope: $$m_{wall} = A_{wall} \cdot \frac{\Delta P_v}{R_{v,total}}$$

Sensible Heat Ratio Requirements

The sensible heat ratio (SHR) of the cooling system must match the building load:

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

For hot-humid climates:

• Building SHR typically 0.65-0.75
• Conventional AC equipment SHR: 0.70-0.80
• Enhanced dehumidification equipment SHR: 0.55-0.70

When building SHR < equipment SHR, supplemental dehumidification is required.

Enhanced Dehumidification Strategies

1. Lower evaporator temperature - Increases latent capacity but reduces efficiency
2. Two-stage cooling - First stage dehumidifies, second stage provides sensible cooling
3. Dedicated outdoor air system (DOAS) - Separate unit handles ventilation and dehumidification
4. Desiccant dehumidification - Chemical moisture removal with regeneration
5. Subcooling and reheat - Cool below needed temperature for dehumidification, then reheat

Target Indoor Humidity Levels

Lower indoor humidity reduces:

• Inward vapor drive through envelope
• Condensation risk on cooled surfaces
• Mold growth potential (< 60% RH prevents most mold)
• Occupant discomfort

System Design Considerations

Oversizing Penalties

Oversized air conditioning systems cycle frequently, reducing dehumidification effectiveness:

• Short runtime prevents coil from reaching steady-state moisture removal
• Reduced latent capacity as proportion of total cooling (SHR increases)
• Increased indoor humidity during off-cycles as moisture re-evaporates from wet coil
• Higher energy consumption from frequent compressor starts

Guideline: Size cooling equipment to actual calculated load, not rules-of-thumb. Maximum oversizing of 15% above design load.

Air Distribution Effects

1. Supply air temperature:

   • Lower supply air (10-13°C) increases dehumidification
   • Higher supply air (13-16°C) reduces overcooling and energy use
   • Must balance dehumidification needs with comfort

2. Supply diffuser location:

   • Avoid cold air impingement on interior surfaces
   • Maintain throw distance to mix supply air before contact
   • Consider low-velocity displacement ventilation

3. Return air placement:

   • Low returns remove cool, dense air
   • High returns in humid bathrooms/kitchens capture moisture at source

Insulation and Surface Temperature Control

Interior insulation maintains interior surface temperatures above dew point:

$$R_{required} = \frac{T_i - T_{dp,outdoor}}{U \cdot (T_i - T_o)} \times 0.12$$

For extreme case of direct contact with outdoor dew point air:

Design conditions: T_o = 32°C, RH = 75%, T_dp,o = 27.2°C, T_i = 24°C

To maintain interior surface above 27°C: $$R_{required} = \frac{24-27.2}{U \cdot (24-32)} \times 0.12$$

This suggests that extremely low R-values (< R-0.5) would allow interior surface temperatures to approach outdoor dew point. Standard insulated walls (R-2.0 to R-3.5) maintain interior surface temperatures close to indoor air temperature.

Thermal bridges create localized cold spots:

• Steel studs reduce effective R-value by 30-50%
• Concrete elements conduct heat readily
• Window frames, particularly aluminum, can be 10-15°C cooler than surrounding wall

Integration with Mechanical Ventilation

Hot-humid climates require careful ventilation design:

1. Energy recovery ventilation (ERV):

   • Recovers both sensible and latent energy
   • Reduces incoming moisture load by 50-70%
   • Efficiency: 60-80% sensible, 50-70% latent

2. Dedicated outdoor air systems (DOAS):

   • Condition ventilation air separately from space loads
   • Deep dehumidification (supply dew point 7-10°C)
   • Neutral or slightly warm supply air temperature

3. Demand-controlled ventilation (DCV):

   • CO2 sensors modulate ventilation rates
   • Reduces moisture load during low occupancy
   • Energy savings of 20-40% in variable occupancy spaces

Code and Standard Requirements

ASHRAE Standard 62.1 (Ventilation)

Minimum ventilation requirements introduce outdoor moisture:

• Breathing zone outdoor airflow: 2.5 L/s per person + 0.3 L/s per m² floor area (offices)
• Residential: 0.15 L/s per m² plus 3.5 L/s per bedroom (ASHRAE 62.2)

ASHRAE Standard 55 (Thermal Comfort)

Acceptable humidity ranges:

• Upper limit: 65% RH (mold growth prevention)
• Lower limit: 20% RH (avoid dry air discomfort)
• Comfort zone: 30-60% RH at 20-26°C

ASHRAE Standard 160 (Moisture Control)

Envelope assemblies must not support mold growth:

• 30-day running average surface RH < 80%
• 7-day running average surface RH < 98%

These criteria inform required interior vapor retarder class and insulation R-values.

International Energy Conservation Code (IECC)

Climate Zone 1-2 requirements:

• Continuous insulation or cavity insulation required
• Air barrier required (tested to 0.2 L/s·m² at 75 Pa for residential)
• Vapor retarder not mandatory but recommended in high-humidity applications

Design Best Practices Summary

1. Maintain low interior humidity (40-50% RH) through adequate dehumidification capacity
2. Size equipment accurately to avoid short-cycling and poor humidity control
3. Install interior vapor retarder (Class II or III) to control inward moisture drive
4. Insulate adequately (minimum R-13 walls, R-30 ceilings) to keep interior surfaces warm
5. Detail thermal bridges with insulating shims, thermal breaks, or exterior insulation
6. Design air distribution to avoid cold air impingement on interior surfaces
7. Use energy recovery ventilation to reduce incoming moisture loads
8. Consider dedicated dehumidification when building SHR is low (< 0.70)
9. Model critical assemblies using WUFI, MOISTURE-EXPERT, or similar hygrothermal analysis
10. Verify performance through post-occupancy monitoring of temperature and humidity

Conclusion

Air conditioning in hot-humid climates creates unique moisture management challenges due to inward vapor drive and potential for interior surface condensation. Successful building envelope design requires:

• Understanding vapor pressure differentials and temperature profiles through assemblies
• Proper interior vapor retarder selection and installation
• Adequate dehumidification capacity matched to building moisture loads
• Careful air distribution design to maintain interior surface temperatures above dew points
• Integration of mechanical systems with envelope moisture control strategies

The interaction between air conditioning systems and building envelope moisture behavior must be considered holistically during design to ensure durable, moisture-safe assemblies.