Hot-Humid Climate Assemblies

Hot-Humid Climate Assemblies

Hot-humid climate building assemblies (IECC Climate Zones 1-2A, < 2000 HDD65°F) experience sustained inward vapor drive during summer cooling periods. Exterior moisture-laden air migrates toward cooled interior surfaces, creating condensation risk on interior faces of assemblies. Proper hot-humid climate design avoids interior vapor retarders, controls inward vapor drive through exterior vapor control layers, and ensures interior surfaces remain above dewpoint temperature.

Climate Characteristics

Hot-humid climate zones exhibit:

Cooling Degree Days: > 5000 CDD50°F (Zone 1), > 3500 CDD50°F (Zone 2A)

Summer Vapor Drive: Strong inward vapor drive from hot-humid exterior toward cool interior

Representative Locations:

• Zone 1: Miami, Key West, Honolulu
• Zone 2A: Houston, New Orleans, Orlando, coastal South Carolina/Georgia

Moisture Risk Period: May through September when outdoor dewpoints exceed 65°F and interior surfaces are cooled

Critical Psychrometric Conditions:

• Outdoor: 90°F, 70% RH → dewpoint = 79°F, pv = 1.39 in. Hg
• Indoor: 75°F, 50% RH → dewpoint = 55°F, pv = 0.37 in. Hg
• Inward vapor drive: Δpv = 1.02 in. Hg

Fundamental Design Principle

Hot-humid climate moisture control follows critical principles opposite to cold climate design:

1. Avoid interior vapor retarders - Interior surfaces must dry inward

2. Control inward vapor drive - Limit exterior moisture entry through vapor control layers

3. Maintain interior surface temperature above dewpoint - Prevent surface condensation

4. Provide air conditioning with adequate dehumidification - Control interior humidity

Vapor Control Strategy

Exterior Vapor Control

Purpose: Limit inward vapor drive without preventing outward drying

Effective Materials:

• Foil-faced polyisocyanurate: μ = 0.05 perm, provides vapor control plus insulation
• XPS rigid foam: μ = 1 perm/in., 2 in. minimum (μ = 0.5 perm)
• Vinyl wallcovering (exterior): μ = 0.5-1.0 perm
• Low-perm exterior finishes: stucco over foam, EIFS

Installation: Continuous layer on exterior side of framing, ahead of structural sheathing or as replacement for traditional sheathing

Interior Vapor Permeability

Requirement: High permeability to allow inward drying

Suitable Materials:

Critical Avoidances:

• No polyethylene vapor retarders on interior
• No impermeable interior finishes (vinyl wallpaper, oil-based paints)
• No foil-faced interior insulation
• Minimal vapor-impermeable interior layers

Wall Assembly Configurations

Configuration 1: Mass Wall with Exterior Insulation

Assembly (interior to exterior):

1. Gypsum board: 1/2 in., latex paint (μ = 10 perm)
2. Concrete block or poured concrete: 8 in., R-1 to R-2
3. Foil-faced polyisocyanurate: 2 in., R-13, μ = 0.05 perm
4. Drainage mat or furring strips
5. Cladding: stucco, fiber cement, metal panels

Total R-value: R-14 to R-15

Vapor Control: Exterior foil facing limits inward vapor drive

Performance Analysis:

Summer conditions (To = 90°F, 70% RH; Ti = 75°F, 50% RH):

Interior surface temperature:

Q = (To - Ti)/Rtotal = (90 - 75)/15 = 1.0 Btu/hr·ft²

Tsi = Ti + Q × Rsi
Tsi = 75 + 1.0 × 0.68 = 75.68°F

Interior dewpoint at 75°F, 50% RH = 55°F Surface temperature (75.68°F) » dewpoint (55°F) → no condensation

Advantages:

• Thermal mass moderates temperature swings
• Exterior insulation keeps mass cool
• Vapor control limits inward moisture drive
• Simple, robust construction

Configuration 2: Frame Wall with Exterior Foam Sheathing

Assembly (interior to exterior):

1. Gypsum board: 1/2 in., latex paint
2. Fiberglass batt insulation: 5-1/2 in., R-21 (unfaced)
3. Foil-faced polyiso sheathing: 1 in., R-6.5, μ = 0.05 perm
4. Furring strips for ventilation/drainage
5. Cladding

Total R-value: R-27.5

Vapor Control: Foil-faced foam provides both thermal and vapor control

Critical Detail: Cavity insulation must be unfaced (no kraft facing, no foil facing)

Sheathing Temperature (at foam interior face):

Rinterior = 0.68 + 0.45 + 21 = 22.13
Rtotal = 22.13 + 6.5 + 0.17 = 28.8

Tfoam,int = Ti + (To - Ti) × (Rinterior/Rtotal)
Tfoam,int = 75 + 15 × (22.13/28.8) = 86.5°F

Foam interior face is warm (86.5°F), well above interior dewpoint (55°F) → no condensation at interface

Exterior Foam Thickness Requirements:

For hot-humid climates, exterior insulation should provide R-5 to R-10 to significantly reduce inward heat gain and vapor drive:

Configuration 3: Ventilated Rainscreen Wall

Assembly (interior to exterior):

1. Gypsum board: 1/2 in., latex paint
2. Fiberglass batt insulation: 5-1/2 in., R-21 (unfaced)
3. Plywood or OSB sheathing: 1/2 in.
4. Building wrap: μ = 50 perm
5. Ventilated air space: 3/4 in. minimum
6. Cladding: fiber cement, metal panels, wood siding

Total R-value: R-22 (nominal)

Vapor Control: Ventilated rainscreen removes moisture through air movement

Performance:

• Air space ventilation removes moisture-laden air before it reaches sheathing
• Building wrap drains liquid water, passes vapor
• Sheathing can dry to exterior through ventilation
• Interior can dry inward through permeable gypsum

Ventilation Requirements:

• Openings at top and bottom: minimum 1/16 in. continuous or equivalent
• Air space depth: 3/4 in. minimum, 1 in. preferred
• Unobstructed vertical airflow path

Configuration 4: Advanced Wall with Interior Continuous Insulation

Assembly (interior to exterior):

1. Gypsum board: 1/2 in.
2. Rigid foam insulation: 1 in., R-5 to R-6 (unfaced, moderate permeance)
3. Stud cavity (2×6): air space or minimal insulation
4. OSB sheathing: 1/2 in.
5. Building wrap
6. Ventilated cladding

Total R-value: R-5 to R-11

Purpose: Interior insulation warms interior surface, reducing condensation risk

Surface Temperature Analysis:

Rsi + Rfoam = 0.68 + 5 = 5.68
Rtotal = 11

Tsi = Ti + (To - Ti) × (Rsi/Rtotal)
Tsi = 75 + 15 × (0.68/11) = 75.93°F

Interior surface remains warm (75.93°F) well above dewpoint (55°F)

Critical Requirements:

• Interior foam must have moderate permeance (> 1 perm) to allow inward drying
• Avoid foil-faced or very low-perm interior insulation
• Recommended materials: unfaced EPS, mineral wool board, semi-rigid fiberglass

Roof/Ceiling Assembly Configurations

Vented Attic in Hot-Humid Climates

Assembly (interior to exterior):

1. Gypsum board: 1/2 in., painted
2. No vapor retarder (critical difference from cold climates)
3. Attic insulation: R-30 to R-38 (blown fiberglass or cellulose)
4. Attic ventilation: NFA = 1/150 minimum
5. Radiant barrier (optional): on underside of roof deck
6. Roof deck: plywood or OSB
7. Underlayment
8. Light-colored roofing (reduces solar gain)

Vapor Retarder: None required, would trap moisture

Radiant Barrier Benefits:

• Reduces radiant heat gain to attic floor insulation
• Lowers attic temperature by 20-30°F
• Reduces cooling load
• Does not impede moisture flow (perforated or naturally permeable)

Ventilation:

NFA = Ceiling area / 150

Preferred: 50% intake (soffit), 50% exhaust (ridge vent)

Unvented Attic (Sealed Attic)

Assembly (interior to exterior):

1. Interior finish (optional)
2. Spray foam insulation on underside of roof deck: R-30 minimum
   • Open-cell foam: R-3.7/in., 8 in. minimum, μ = high (requires separate vapor control)
   • Closed-cell foam: R-6.5/in., 5 in. minimum, μ = low (self-vapor-retarding)
3. Roof deck: plywood or OSB
4. Underlayment: vapor-permeable (allows deck to dry upward)
5. Ventilated roof cladding OR light-colored roofing

Advantages:

• Brings ducts and air handler into conditioned space (major efficiency gain in hot climates)
• Eliminates attic heat gain to ceiling
• Simplifies air sealing

Code Requirements (IRC for unvented attics in hot-humid climates):

• Air-impermeable insulation applied directly to underside of deck
• Conditioned air supplied to attic space, OR
• Attic within building thermal envelope

Closed-Cell vs. Open-Cell Selection:

Interior Surface Temperature Management

Preventing surface condensation requires maintaining interior surfaces above dewpoint temperature.

Critical Surface Temperature Calculation

Required interior surface temperature:

Tsi,min = Td,interior + safety margin

where safety margin = 5-10°F

For typical hot-humid interior (75°F, 50% RH, Td = 55°F):

Tsi,min = 55 + 5 = 60°F

Any interior surface below 60°F risks condensation.

Thermal Bridge Management

Thermal bridges in hot-humid climates create cold interior surfaces:

Common Thermal Bridges:

• Steel studs or joists
• Concrete structural elements
• Uninsulated slab edges
• Window/door frames
• HVAC ducts in unconditioned spaces

Temperature at Thermal Bridge:

Steel stud (R-1) in R-20 wall:

Tsteel,int = Ti + (To - Ti) × (Rsi/Rsteel)
Tsteel,int = 75 + 15 × (0.68/1.68) = 81.1°F

Uninsulated concrete beam (R-0.5):

Tbeam,int = 75 + 15 × (0.68/1.18) = 83.6°F

Both surfaces remain well above dewpoint (55°F), but localized cold spots can occur with aggressive interior cooling (< 70°F setpoint) or high humidity (> 60% RH).

Thermal Bridge Solutions:

• Continuous exterior insulation
• Thermal breaks at structural penetrations
• Insulate interior surfaces of concrete elements
• Increase interior surface temperature through local insulation

Dehumidification Requirements

Hot-humid climates require mechanical dehumidification integrated with air conditioning.

Latent Load Dominance

Typical hot-humid cooling loads:

• Sensible: 60-70%
• Latent: 30-40%

High latent fraction requires dedicated dehumidification capacity.

Interior Humidity Control

Target Interior Conditions:

• Temperature: 75-78°F
• Relative humidity: 45-55% RH maximum
• Dewpoint: 55-60°F maximum

Dehumidification Strategies:

1. Proper AC sizing: Avoid oversizing, which short-cycles and fails to dehumidify
2. Variable-speed equipment: Longer runtime at lower capacity improves dehumidification
3. Dedicated dehumidifiers: ERV/HRV with energy recovery
4. Subcooling and reheat: Cool below setpoint to condense moisture, then reheat
5. Desiccant dehumidification: For very high latent loads (pools, spas)

Ventilation Air Dehumidification

ASHRAE 62.2 ventilation requirements introduce humid outdoor air requiring treatment:

Ventilation load for 100 CFM outdoor air (90°F, 70% RH → 75°F, 50% RH):

Latent load = 1.08 × CFM × (Wo - Wi) × hfg
Latent load = 1.08 × 100 × (0.0165 - 0.0093) × 1060
Latent load = 830 Btu/hr (0.28 tons latent)

Ventilation air must be dehumidified before entering conditioned space.

Solutions:

• Energy recovery ventilator (ERV) with enthalpy wheel
• Dedicated outdoor air system (DOAS)
• Ventilating dehumidifier

Common Hot-Humid Climate Moisture Failures

Failure Mode 1: Interior Vapor Retarder

Symptoms: Mold on interior surfaces, wet insulation, musty odors

Cause: Polyethylene or foil vapor retarder on interior traps inward-diffusing moisture

Solution: Remove interior vapor retarder, use latex paint on gypsum only

Failure Mode 2: Vinyl Wallcovering

Symptoms: Mold behind vinyl wallcovering, peeling wallpaper

Cause: Vinyl acts as Class II vapor retarder, preventing inward drying

Solution: Use vapor-permeable wall finishes, or apply vapor-permeable primer before vinyl

Failure Mode 3: Cold Interior Surfaces

Symptoms: Condensation on walls, windows, or ducts

Cause: Over-cooling (< 72°F setpoint) or inadequate dehumidification (> 60% RH)

Solution: Raise thermostat setpoint, add dehumidification, insulate cold surfaces

Failure Mode 4: Uninsulated Ducts in Attics

Symptoms: Condensation on supply ducts, wet attic insulation

Cause: Cold duct surfaces below attic dewpoint temperature

Solution: Bring ducts into conditioned space (sealed attic), or increase duct insulation (R-8 minimum)

Material Selection Guidelines

Sheathing Selection

Insulation Selection

Quality Control and Verification

Design Phase:

• Avoid interior vapor retarders in specifications
• Specify exterior vapor control layers
• Calculate interior surface temperatures
• Verify adequate dehumidification capacity

Construction Phase:

• Inspect for unintended vapor retarders (kraft facing, foil facings)
• Verify continuous exterior insulation/vapor control
• Check duct insulation (R-8 minimum in unconditioned spaces)
• Air seal building envelope (target < 5 ACH50)

Post-Construction:

• Commission dehumidification systems
• Monitor interior RH (maintain 45-55% RH)
• Infrared thermography to identify cold surfaces
• Verify adequate ventilation air treatment

Related Topics

• Exterior Vapor Control - Low-perm exterior layer design
• Drying to Interior - Inward drying mechanisms
• Air Conditioning Considerations - Dehumidification and interior humidity control
• Hot-Humid Wall Systems - Specific assembly configurations

Hot-humid climate assemblies require exterior vapor control with highly permeable interior finishes to allow inward drying while limiting summer moisture intrusion.

Sections