# Exterior Building Envelope

## Envelope Principles

### Four Control Layers
```
From exterior to interior:
1. Water control (rain, bulk water)
2. Air control (air barrier)
3. Vapor control (condensation)
4. Thermal control (insulation)

Rule: Keep them continuous and connected.
```

### Layer Placement (Climate Dependent)
| Climate | Vapor Barrier Location |
|---------|------------------------|
| Hot-humid (Florida) | Exterior side of insulation |
| Cold | Interior side of insulation |
| Mixed | Analyze carefully or avoid |

---

## Water Management

### Rain Screen Principle
```
Exterior cladding → Air gap → Drainage plane → Sheathing → Structure

Components:
1. Screen (cladding deflects most rain)
2. Gap (allows drainage and drying)
3. Barrier (blocks any penetrating water)
4. Flashing (directs water out)
```

### Weather Barrier Types
| Type | Permeance | Application |
|------|-----------|-------------|
| Building paper (felt) | 5-60 perms | Traditional, shingles |
| Housewrap (Tyvek type) | 50+ perms | Wood/steel frame |
| Self-adhered membrane | 0.05-1 perm | Critical areas, below grade |
| Fluid-applied WRB | Varies | Commercial, complex shapes |

### Permeance Guide
| Permeance | Classification | Example |
|-----------|----------------|---------|
| < 0.1 perm | Vapor impermeable | Polyethylene, foil |
| 0.1-1.0 perm | Vapor semi-impermeable | Kraft paper, latex paint |
| 1.0-10 perm | Vapor semi-permeable | Plywood, OSB |
| > 10 perm | Vapor permeable | Housewrap, fiberglass |

---

## Air Barrier Systems

### Requirements
- Continuous around building envelope
- Structural integrity (withstand wind pressure)
- Durable for building life
- Impermeable to air (< 0.004 CFM/SF @ 1.57 PSF)

### Common Air Barrier Assemblies
| System | Application |
|--------|-------------|
| Plywood + taped seams | Residential |
| Self-adhered membrane | Commercial |
| Fluid-applied membrane | Complex geometry |
| Spray foam (closed-cell) | Air + vapor + thermal |
| Rigid insulation (taped) | Over sheathing |

### Critical Connections
- Foundation to wall
- Wall to window/door
- Wall to roof
- Penetrations (pipes, ducts, electrical)
- Corners and transitions

---

## Insulation Systems

### Exterior Insulation Types
| Type | R-value/inch | Vapor Perm | Fire Rating |
|------|--------------|------------|-------------|
| EPS | R-3.8-4.2 | 2-5 perm | Must be covered |
| XPS | R-5.0 | 0.5-1 perm | Must be covered |
| Polyiso | R-5.6-6.0 | 0.5-1 perm | Some foil-faced |
| Mineral wool | R-4.0 | Very high | Non-combustible |

### Continuous Insulation Benefits
- Reduces thermal bridging (studs = R-1)
- Keeps structure warm (reduces condensation)
- Improves energy performance
- Required by code in most climate zones

### Thermal Bridge Locations
| Location | Solution |
|----------|----------|
| Studs | Continuous insulation |
| Floor/roof edges | Extend insulation over edge |
| Window/door frames | Insulated frames |
| Parapets | Continuous insulation wrap |
| Balconies | Thermal break connectors |

---

## Cladding Attachment

### Through Exterior Insulation
| Insulation Thickness | Attachment Method |
|----------------------|-------------------|
| Up to 1-1/2" | Long screws to studs |
| 1-1/2" to 4" | Clips/rails or long screws |
| Over 4" | Subframe required |

### Clip/Rail Systems
```
Cladding → Vertical rail → Horizontal clip → Structure

Benefits:
- Thermal separation from structure
- Adjustability for plumb/level
- Drainage cavity automatic
```

### Fastener Considerations
- Corrosion resistance (stainless or coated)
- Thermal conductivity (can be thermal bridge)
- Load capacity (weight + wind)
- Movement accommodation

---

## Flashing Design

### Critical Flashing Locations
| Location | Flashing Type |
|----------|---------------|
| Window/door head | Drip flashing above |
| Window/door sill | Pan flashing with end dams |
| Wall base | Through-wall with weeps |
| Roof-to-wall | Step + counter flashing |
| Deck ledger | Multiple layers |
| Penetrations | Pipe boots, sheet metal |

### Through-Wall Flashing
```
Elements:
- Sheet metal or membrane
- Extends past face of wall
- End dams at terminations
- Weep holes above (24" O.C. max)
- Sealant at back edge
```

### Kick-Out Flashing
Required at:
- Roof terminating at wall
- Prevents water behind siding
- Extends flashing into gutter

---

## Joints and Sealants

### Joint Types
| Joint | Application |
|-------|-------------|
| Working joint | Accommodates movement |
| Non-working joint | Minimal movement |
| Butt joint | Between panels |
| Lap joint | Overlapping materials |

### Sealant Selection
| Sealant Type | Movement | UV | Substrate |
|--------------|----------|-----|-----------|
| Silicone | ±50% | Excellent | Non-porous |
| Polyurethane | ±25% | Good | Most |
| Modified silicone | ±35% | Excellent | Most |
| Acrylic latex | ±7.5% | Fair | Paint-over |

### Joint Design
```
Width-to-depth ratio:
- Working joints: 2:1 (width:depth)
- Typical depth: 3/8" to 1/2"
- Backer rod: 25% larger than joint

Joint width calculation:
Width = Movement + 1/4" min
Movement = Length × thermal expansion × temp difference
```

---

## Wall Assembly Examples

### Florida CMU Exterior
```
Exterior to interior:
1. Stucco or paint (elastomeric)
2. CMU wall (8")
3. Furring + continuous insulation (R-5 to R-8)
4. Gypsum board
```

### Commercial Metal Stud
```
Exterior to interior:
1. Metal panel or EIFS
2. Air gap (if rain screen)
3. Fluid-applied WRB/AB
4. Exterior sheathing
5. Metal studs with batt
6. Interior gypsum board
```

### Residential Wood Frame
```
Exterior to interior:
1. Siding (lap, panel, stucco)
2. Air gap (if rain screen)
3. Housewrap
4. Plywood/OSB sheathing
5. Wood studs with cavity insulation
6. Vapor retarder (climate-dependent)
7. Gypsum board
```

---

## Moisture Analysis

### Hygrothermal Modeling
When required:
- Unusual assemblies
- High interior humidity
- Historic renovation
- Warranty claims

Tools:
- WUFI
- THERM
- Energy Plus

### Dew Point Calculation
```
If interior surface temp < dew point → condensation risk

Prevention:
1. Warm surfaces (exterior insulation)
2. Reduce interior humidity
3. Vapor control layer correctly placed
```

---

## Quality Control

### Field Testing
| Test | Purpose |
|------|---------|
| ASTM E1186 | Air leakage of building |
| ASTM E1105 | Water penetration of fenestration |
| AAMA 501.2 | Water penetration of curtain wall |
| Infrared scan | Thermal bridging, moisture |

### Mock-up Requirements (Large Projects)
- Representative wall section
- Include window/door
- Full-size, exterior exposed
- Test before proceeding

### Inspection Checklist
- [ ] Air barrier continuous
- [ ] All penetrations sealed
- [ ] Flashing properly lapped
- [ ] Weeps clear and functional
- [ ] Sealant properly applied
- [ ] Cladding secured correctly
- [ ] Drainage plane unobstructed

---

## Common Failures

### Water Intrusion Causes
1. Missing or discontinuous flashing
2. Sealant failure
3. Improper window installation
4. No drainage behind cladding
5. Kick-out flashing missing

### Air Leakage Causes
1. Penetrations not sealed
2. Top/bottom of walls unsealed
3. Electrical boxes not air-sealed
4. Window/door perimeter gaps
5. Service penetrations

### Condensation Causes
1. Vapor barrier wrong location
2. Thermal bridging
3. Insufficient insulation
4. High interior humidity
5. Poor ventilation
