Building Envelope

Building Envelope

Section 1: Envelope Performance Requirements

The building envelope is the critical interface between interior and exterior environments. It must simultaneously perform four functions:

1. Structural support: Transfer wind loads, dead loads (self-weight + cladding), and seismic forces to the primary structure. Resist impact loads (wind-borne debris in hurricane zones).
2. Weather protection: Exclude rain, snow, wind, and UV radiation. Manage moisture in all three phases (liquid water, water vapor, ice).
3. Thermal and energy performance: Control heat flow (conduction, convection, radiation), solar gain, air leakage, and condensation. The envelope accounts for 25–50% of total building energy consumption.
4. Aesthetics and identity: The facade is the public face of the building. Material, proportion, texture, color, and transparency define architectural character.

1.1 Performance Metrics

1.2 Climate-Specific Performance Targets

Hot-arid climate (ASHRAE CZ 1B–3B, e.g., Riyadh, Phoenix):

• Wall U-value: ≤0.35 W/m²K
• Roof U-value: ≤0.20 W/m²K
• Window U-value: ≤2.4 W/m²K (double glazed, solar control)
• SHGC: ≤0.25 (critical — solar rejection is primary concern)
• Shading: external shading devices essential; shading coefficient 0.3–0.5
• Thermal mass: beneficial for diurnal temperature swing damping (>300 kg/m² desirable)
• Air permeability: ≤3.0 m³/h/m² at 50 Pa (sand/dust exclusion)
• Key strategy: reject solar radiation, provide thermal mass, shade all glazing, minimize WWR to 25–35%

Hot-humid climate (ASHRAE CZ 1A–2A, e.g., Singapore, Miami):

• Wall U-value: ≤0.45 W/m²K
• Roof U-value: ≤0.25 W/m²K
• Window U-value: ≤2.4 W/m²K
• SHGC: ≤0.25
• Ventilation: critical — cross-ventilation design, operable windows where security permits
• Vapor control: vapor barrier on exterior side of insulation (vapor drive inward)
• Air permeability: ≤5.0 m³/h/m² at 50 Pa
• Key strategy: reject solar gain, manage humidity, enable natural ventilation, prevent condensation on cold AC surfaces

Temperate climate (ASHRAE CZ 4A–5A, e.g., London, New York):

• Wall U-value: ≤0.18 W/m²K (UK Building Regs Part L 2021: 0.18)
• Roof U-value: ≤0.13 W/m²K (Part L: 0.11)
• Window U-value: ≤1.2 W/m²K (Part L: 1.2)
• SHGC: 0.25–0.40 (balance winter gain with summer overheating)
• Air permeability: ≤5.0 m³/h/m² at 50 Pa (Part L: 8.0 max, 5.0 recommended)
• Key strategy: balance heat loss prevention with solar gain utilization; avoid summer overheating; continuous insulation with airtight layer

Cold climate (ASHRAE CZ 6A–8, e.g., Stockholm, Montreal):

• Wall U-value: ≤0.15 W/m²K
• Roof U-value: ≤0.10 W/m²K
• Window U-value: ≤0.80 W/m²K (triple glazing essential)
• SHGC: 0.40–0.60 (maximize passive solar gain on south facades)
• Air permeability: ≤1.0 m³/h/m² at 50 Pa
• Key strategy: maximize insulation, eliminate thermal bridges, airtight construction, triple glazing, maximize south-facing glazing, MVHR essential

Passive House standard (all climates):

• Wall U-value: ≤0.15 W/m²K
• Roof U-value: ≤0.10 W/m²K
• Floor U-value: ≤0.15 W/m²K
• Window U-value: ≤0.80 W/m²K (installed, including frame)
• SHGC: ≥0.50 for south-facing (maximize passive solar)
• Air permeability: ≤0.6 ach at 50 Pa (entire building, blower door test)
• Thermal bridges: ψ ≤0.01 W/mK at all junctions ("thermal bridge free")
• Heating demand: ≤15 kWh/m²/year
• Primary energy demand: ≤60 kWh/m²/year (PER: 120 kWh/m²/year)

Section 2: Wall Systems

2.1 Masonry Cavity Wall (Brick–Insulation–Block)

Build-up (outside to inside): 102.5 mm facing brick → 50 mm clear cavity → 100–150 mm mineral wool / PIR insulation → 140 mm concrete block → 12.5 mm plaster

Performance:

• Total thickness: 405–455 mm
• U-value: 0.18–0.25 W/m²K (depending on insulation thickness)
• Fire rating: 120+ minutes (non-combustible throughout)
• Acoustic: Rw 50–55 dB
• Embodied carbon: 80–120 kgCO2e/m²
• Cost range: $150–250/m² (materials + labor)
• Best-fit: Residential, low-to-mid-rise commercial, institutional (UK/Northern Europe tradition)
• Advantages: Durable (100+ year lifespan), low maintenance, good thermal mass, familiar to trades
• Disadvantages: Slow to construct (wet trade), heavy (self-weight ~350 kg/m²), limited height without support (typically max 15 m without lateral restraint intervals at 9 m)

2.2 Insulated Concrete Form (ICF)

Build-up: 50–100 mm EPS formwork (external) → 150–300 mm reinforced concrete core → 50–100 mm EPS formwork (internal) → plasterboard finish

Performance:

• Total thickness: 300–500 mm
• U-value: 0.11–0.20 W/m²K
• Fire rating: 120–240 minutes (concrete core)
• Acoustic: Rw 50–58 dB
• Embodied carbon: 120–180 kgCO2e/m² (high due to concrete)
• Cost range: $180–300/m²
• Best-fit: Residential, schools, swimming pools, buildings requiring high thermal mass and security
• Advantages: Fast construction (formwork stays in place), excellent airtightness, good thermal mass, no cold bridging through structure
• Disadvantages: Requires skilled contractors, EPS is combustible (requires protection), heavy, difficult to modify post-construction

2.3 Timber Frame (Platform Frame)

Build-up: Cladding (variable) → 25 mm ventilated cavity → breather membrane → 9 mm OSB sheathing → 140 mm timber studs with mineral wool between → VCL → 12.5 mm plasterboard

Performance:

• Total thickness: 250–350 mm (plus external cladding)
• U-value: 0.18–0.25 W/m²K (with 140 mm studs); 0.12–0.15 with additional external insulation
• Fire rating: 30–60 minutes (with plasterboard protection; 2 layers = 60 min)
• Acoustic: Rw 40–48 dB (improved with resilient bars + additional board)
• Embodied carbon: 25–50 kgCO2e/m² (carbon sequestration in timber offsets)
• Cost range: $120–200/m²
• Best-fit: Residential (up to 7 storeys with fire engineering), low-rise commercial, schools
• Advantages: Lightweight, fast erection (prefab panels), low embodied carbon, good insulation between studs, dry construction
• Disadvantages: Moisture-sensitive (requires careful detailing), limited height without CLT/glulam, acoustic performance requires careful design, thermal bridging through studs (0.15 W/mK vs 0.04 for insulation)

2.4 Steel Frame with Infill

Build-up: Cladding → 50 mm cavity → breather membrane → 100–150 mm insulation (between steel studs) → vapor barrier → 12.5–15 mm plasterboard

Performance:

• Total thickness: 200–350 mm
• U-value: 0.20–0.35 W/m²K (severe thermal bridging through steel studs without thermal break)
• Fire rating: 30–120 minutes (with fire-rated board linings)
• Acoustic: Rw 42–50 dB
• Embodied carbon: 60–100 kgCO2e/m²
• Cost range: $130–220/m²
• Best-fit: Commercial, industrial, healthcare, fast-track projects, high-rise infill panels
• Advantages: Lightweight, non-combustible, fast erection, spans large openings
• Disadvantages: High thermal bridging (steel studs conduct 50x more than timber); requires thermal break clips or continuous external insulation; corrosion risk

2.5 Structural Insulated Panels (SIPs)

Build-up: 11 mm OSB → 117–217 mm EPS/PUR core → 11 mm OSB. External cladding and internal finishing applied on site.

Performance:

• Total thickness: 139–239 mm (panel only) + cladding + internal finish
• U-value: 0.12–0.20 W/m²K
• Fire rating: 30 minutes (OSB faces); requires additional lining for 60 min
• Acoustic: Rw 32–38 dB (lightweight — requires additional mass)
• Embodied carbon: 35–65 kgCO2e/m²
• Cost range: $140–230/m²
• Best-fit: Residential, modular buildings, self-build, fast-track low-rise construction
• Advantages: Excellent airtightness (factory-sealed joints), fast erection (panels pre-cut), no thermal bridging (continuous insulation), good structural performance (stressed-skin)
• Disadvantages: EPS/PUR core is combustible, OSB is moisture-sensitive, limited to low-rise (typically 3–4 storeys), difficult to modify, acoustic performance requires supplementation

2.6 Cross-Laminated Timber (CLT) with External Insulation

Build-up: Cladding → ventilated cavity → breather membrane → 100–200 mm mineral wool / wood fiber → 100–160 mm CLT panel → internal finish (exposed CLT or plasterboard)

Performance:

• Total thickness: 280–450 mm
• U-value: 0.12–0.18 W/m²K
• Fire rating: 60–120 minutes (CLT chars at ~0.65 mm/min; 100 mm panel = ~90 min structural fire resistance)
• Acoustic: Rw 38–44 dB (CLT alone); 55+ dB with resilient mount + plasterboard
• Embodied carbon: -10 to +30 kgCO2e/m² (carbon sequestration in timber can achieve net negative)
• Cost range: $200–350/m²
• Best-fit: Residential (up to 18 storeys demonstrated), offices, schools, mid-rise institutional
• Advantages: Carbon-negative potential, fast erection (panel installation), structural wall and insulation combined, exposed timber interior aesthetic, good airtightness
• Disadvantages: Moisture management critical during construction, cost premium over concrete frame, acoustic flanking through solid panels, fire engineering required for tall buildings

2.7 Curtain Wall (Stick System and Unitized)

Build-up: External glass/panel → aluminum mullion/transom frame with thermal break → IGU (double/triple glazing) or opaque spandrel panel with insulation

Performance (glazed zone):

• Total thickness: 100–200 mm (frame depth)
• U-value (center-of-glass): 1.0–1.6 W/m²K (double); 0.5–0.8 (triple)
• U-value (whole curtain wall, including frame): 1.4–2.2 W/m²K (double); 0.8–1.2 (triple)
• Fire rating: E30/EW30 typical (glass holds 30 min with fire-rated interlayer); spandrel panels 60–120 min with fire backing
• Acoustic: Rw 32–42 dB (dependent on glass thickness and lamination)
• Embodied carbon: 80–150 kgCO2e/m² (aluminum-intensive)
• Cost range: $400–1200/m² (stick system lower; unitized higher; structural glazing highest)

Stick system: Mullions and transoms assembled on site from aluminum extrusions; glass/panels installed piece by piece. Suitable for low-to-mid-rise, irregular facades.

Unitized system: Factory-assembled panels (typically 1.5 m wide x floor-to-floor height) installed as complete units from inside the building. Suitable for high-rise (faster, weather-independent installation).

• Best-fit: Commercial offices, institutional, high-rise, landmark buildings
• Advantages: Maximum transparency, architectural flexibility, lightweight, high-quality factory finish
• Disadvantages: High cost, high embodied carbon (aluminum), thermal performance limited by frame, acoustic performance lower than masonry, condensation risk at thermal bridges

2.8 Rainscreen Cladding (Ventilated Facade)

Build-up: Cladding panel (stone, metal, fiber cement, terracotta, HPL) → 50 mm ventilated cavity → support brackets/rails → breather membrane → continuous insulation (100–200 mm mineral wool / PIR) → structural wall (concrete, masonry, CLT, steel frame)

Performance:

• Total thickness: 250–450 mm (cladding + cavity + insulation + structure)
• U-value: 0.12–0.20 W/m²K (governed by continuous insulation)
• Fire rating: dependent on backing wall + insulation; cladding panel must be non-combustible for buildings >18 m (UK post-Grenfell requirement; BS 8414 / BR 135)
• Acoustic: Rw 45–60 dB (dependent on backing wall mass)
• Embodied carbon: 50–200 kgCO2e/m² (varies enormously by cladding material)
• Cost range: $250–600/m² (material-dependent)

Ventilation principle: The cavity behind the cladding is open at top and bottom (50 mm min gap). Air circulates by stack effect, removing moisture from insulation and reducing solar-driven inward vapor. Pressure equalization reduces wind-driven rain penetration.

• Best-fit: Commercial, institutional, residential (mid-to-high-rise), renovation/overcladding
• Advantages: Continuous insulation (no thermal bridging through cladding), moisture management, design flexibility (wide cladding options), easy panel replacement
• Disadvantages: Cavity fire risk (requires fire barriers at every floor and around openings per BS 9414 / NFPA 285), support bracket thermal bridging (use thermal break brackets), cost

2.9 Precast Concrete Panels

Build-up: 75 mm precast concrete face → 100–150 mm insulation → 100 mm precast concrete inner leaf (sandwich panel). Or: single-leaf precast with external insulation and cladding.

Performance:

• Total thickness: 275–375 mm (sandwich panel)
• U-value: 0.15–0.25 W/m²K
• Fire rating: 120–240 minutes (non-combustible)
• Acoustic: Rw 50–58 dB
• Embodied carbon: 100–160 kgCO2e/m²
• Cost range: $200–400/m²
• Best-fit: Commercial, parking structures, industrial, high-rise residential, modular/prefab construction
• Advantages: Factory quality, fast erection (crane-placed), durable, fire-resistant, good acoustic mass, consistent finish
• Disadvantages: Heavy (200–500 kg/m²), requires crane access, large panel transport logistics, joint detailing critical (sealant maintenance), limited design flexibility post-manufacture

2.10 Mass Timber with Internal Insulation

Build-up: CLT or glulam structural wall (100–200 mm) → service void (50 mm) with insulation → VCL → plasterboard. External face: exposed timber with weather-protective finish (oil, stain, charring) or rainscreen cladding.

Performance:

• Total thickness: 200–350 mm
• U-value: 0.15–0.25 W/m²K (with internal insulation + timber thermal resistance)
• Fire rating: 60–90 minutes (100 mm CLT + plasterboard)
• Acoustic: Rw 40–50 dB
• Embodied carbon: -20 to +20 kgCO2e/m² (net carbon benefit)
• Cost range: $220–380/m²
• Best-fit: Low-to-mid-rise residential, cultural buildings, mountain/rural architecture, eco-buildings
• Advantages: Carbon-negative potential, exposed timber aesthetic (external), unique character, structural + envelope combined
• Disadvantages: External timber requires maintenance (re-oiling every 3–5 years or charring treatment), weathering unevenness, fire engineering for exposed timber facade, limited to lower heights without additional protection

Section 3: Glazing Systems

3.1 Glass Types

3.2 Coatings

Low-e (low emissivity) coatings:

• Reduce radiative heat transfer across cavity
• Hard coat (pyrolytic): applied during manufacture; durable; emissivity ~0.15–0.20
• Soft coat (sputtered): applied post-manufacture; lower emissivity (~0.02–0.05); more delicate, must face cavity
• Position: surface 3 in double IGU (inner face of outer pane) for solar control; surface 2 (outer face of inner pane) for cold climates (retain heat)

Solar control coatings:

• Reduce SHGC to 0.15–0.35 while maintaining VLT 40–70%
• Selective coatings: transmit visible light, reflect near-infrared
• Tinted solar: body-tinted glass (grey, bronze, green) — reduce VLT proportionally
• Products: Guardian SunGuard (SHGC 0.19, VLT 50%), AGC iplus (SHGC 0.22, VLT 62%), Pilkington Suncool (SHGC 0.25, VLT 55%)

Self-cleaning coatings:

• Pilkington Activ, Saint-Gobain Bioclean
• TiO2 photocatalytic coating: breaks down organic dirt with UV light
• Hydrophilic surface: rain sheets off carrying loosened dirt

3.3 Gas Fills and Spacer Bars

Gas fills:

Optimal cavity width: 16 mm for argon, 12 mm for krypton (beyond this, convection currents reduce benefit).

Spacer bars:

• Aluminum spacer: traditional, high conductivity = thermal bridge at edge of glass. Psi-value ~0.08 W/mK
• Warm-edge spacer (TGI/Thermix/Super Spacer): stainless steel, hybrid, or foam. Psi-value 0.03–0.04 W/mK
• Passive House certified spacers: psi ≤0.032 W/mK (e.g., Swisspacer Ultimate: 0.028)

3.4 Window U-Values

3.5 Frame Types

3.6 Framing Systems for Facades

Punched windows: Individual window units set into a solid wall. Clear visual separation between wall and window. Easiest to insulate and detail for thermal bridges. Typical residential and traditional architecture.

Ribbon windows: Continuous horizontal bands of glazing, usually separated by floor-level spandrel panels. Le Corbusier's "fenetre en longueur." Good daylight, emphasizes horizontality.

Curtain wall: Continuous facade system — mullions and transoms span floor-to-floor, glazing and opaque panels infill. See Section 2.7 for system types.

Structural glazing: Glass bonded to frame with structural silicone sealant — no visible external framing. Clean, flush appearance. Requires factory-applied sealant for warranty.

3.7 Window-to-Wall Ratio (WWR) Guidance

ASHRAE 90.1 baseline: 40% WWR. Higher WWR requires compensating measures (better U-value, lower SHGC, external shading).

Section 4: Roof Systems

4.1 Flat Roof — Warm Deck

Build-up (top to bottom): Waterproof membrane (single-ply or built-up) → insulation (PIR/EPS/mineral wool) → VCL → structural deck (concrete/metal/timber)

• Insulation thickness: 120–250 mm (U-value 0.10–0.18 W/m²K)
• Falls: min 1:60 (preferred 1:40) formed in insulation (tapered) or structure
• Drainage: internal rainwater outlets at 1 per 100–200 m² or perimeter gutters
• Advantages: VCL warm (low condensation risk), simple construction, insulation continuous
• Membrane options: single-ply (EPDM, TPO, PVC) — 1.2–2.0 mm; built-up felt (3-layer) — 12–15 mm; liquid-applied — 2–3 mm
• Lifespan: single-ply 25–35 years; built-up 20–25 years; liquid 15–25 years

4.2 Flat Roof — Inverted (Upside-Down)

Build-up: Ballast (gravel/paving) → filter fleece → insulation (XPS only — must resist water absorption) → waterproof membrane → structural deck

• Insulation thickness: 100–200 mm XPS (U-value 0.15–0.25 W/m²K)
• Advantages: Membrane protected from UV/thermal cycling/mechanical damage, longer membrane life, accessible roof surface
• Disadvantages: Rainwater cooling factor (water runs under insulation, reducing thermal performance by ~5–10%); XPS only (higher embodied carbon than mineral wool)
• Falls: formed in structure or screed below membrane

4.3 Flat Roof — Green Roof

Build-up: Vegetation → growing medium (80–300 mm) → filter fleece → drainage layer (25–60 mm) → root barrier → waterproof membrane → insulation → VCL → structure

Extensive green roof: Sedum/moss, 80–150 mm growing medium, 60–180 kg/m² saturated, low maintenance Intensive green roof: Shrubs/trees, 300–1500 mm growing medium, 300–1500 kg/m² saturated, irrigation required

• Additional structural load: 1.0–2.0 kN/m² (extensive); 5.0–15.0 kN/m² (intensive)
• Benefits: stormwater retention (50–90% annual), urban heat island reduction (surface temp 30°C lower than dark membrane), biodiversity, extended membrane life (2x), acoustic insulation (+8–10 dB), thermal performance improvement (~10% effective U-value reduction)
• Standards: FLL Guidelines (Germany), GRO Code (UK), ASTM E2397/E2400 (USA)

4.4 Pitched Roof — Ventilated (Cold Roof)

Build-up: Tiles/slates → battens → counter-battens → breathable underlay → ventilated cavity (50 mm min) → insulation between rafters/at ceiling level → VCL → plasterboard

• Ventilation: 10,000 mm² per metre at eaves, 5,000 mm² per metre at ridge (UK Building Regs)
• Insulation: between rafters 100–150 mm + at ceiling level 200–400 mm; or all between/above rafters for vaulted ceilings
• Minimum pitch: 15° for interlocking tiles; 20° for plain tiles; 25° for natural slates; 35° for thatch
• U-value: 0.10–0.16 W/m²K achievable with 300+ mm total insulation

4.5 Pitched Roof — Warm Roof

Build-up: Tiles/slates → battens → counter-battens → breathable underlay → continuous insulation over rafters (rigid board) → air barrier/VCL → rafters → plasterboard

• Advantages: no ventilation required; insulation continuous (no thermal bridging through rafters); rafter depth available for services
• Disadvantages: thicker build-up above rafters raises ridge height; heavier

4.6 Membrane Structures and Barrel Vaults

• ETFE cushions: U-value 1.8–3.5 W/m²K (3-layer); lightweight (0.35 kg/m²); 95% VLT; up to 100 m span. Examples: Eden Project, Allianz Arena, Beijing Aquatics Center
• PTFE-coated fiberglass: tensile membrane; translucent; U-value poor (single skin ~6.0); best for semi-outdoor/shading applications
• Standing seam metal roofs: zinc, copper, aluminum, steel; pitch min 3°; lifespan 40–100 years (zinc/copper)

Section 5: Moisture Management

5.1 The "Perfect Wall" Principle

From exterior to interior, the control layers should be in this order:

1. Rain screen / rain control: The outermost layer deflects bulk water (>99% of moisture load). Drained and ventilated cavity behind cladding.
2. Air barrier: Prevents air-transported moisture from moving through the assembly. Must be continuous, sealed at all joints, and able to resist wind pressure. Air barrier is the single most important moisture control layer.
3. Thermal insulation: Controls heat flow and determines temperature profile through the wall. Position governs condensation risk.
4. Vapor control layer (VCL): Controls vapor diffusion. Position depends on climate:
   • Cold/temperate: VCL on warm (interior) side of insulation
   • Hot-humid: VCL on warm (exterior) side of insulation (or use smart VCL)
   • Mixed climate: smart/variable VCL that adjusts permeability with humidity
5. Structure: Loadbearing element.

5.2 Vapor Drive Analysis

Smart vapor control layers: Materials like Intello Plus (Pro Clima) or DB+ (SIGA) have variable vapor resistance: sd-value 0.25 m in summer (allows drying inward) and sd-value 10 m+ in winter (blocks outward vapor diffusion). Essential for mixed climates and timber construction.

5.3 Condensation Risk — Interstitial Analysis (Glaser Method)

The Glaser method (BS EN ISO 13788) calculates the risk of condensation within the wall assembly:

1. Determine temperature gradient through wall (linear, based on thermal resistance of each layer)
2. Determine dewpoint temperature at each interface (from vapor pressure gradient based on vapor resistance of each layer)
3. If temperature at any interface drops below dewpoint, condensation occurs there
4. Calculate cumulative condensation over the heating season
5. Check that condensation evaporates during the drying season

Acceptable limits (BS EN ISO 13788):

• Condensation shall not drip, stain, or damage materials
• Maximum accumulated moisture: 200 g/m² for non-absorbent layers; 500 g/m² for absorbent (mineral wool)
• All condensation must evaporate within the drying season (net annual balance must be zero or negative)

Limitations of Glaser: Steady-state only; does not account for moisture storage, capillary action, air movement, solar-driven vapor, or real weather variability. For accurate analysis, use dynamic simulation: WUFI, DELPHIN, or HYGROTHERMAL tools.

5.4 Detailing at Junctions

Wall-to-roof junction:

• Continuous air barrier from wall to roof (typically membrane lapped and sealed)
• Insulation continuity: roof insulation overlaps wall insulation at parapet/eaves
• Parapet: internal gutter preferred (warm roof under coping); external gutter if parapet is cold (condensation risk)
• Eaves overhang: min 300 mm to protect wall below from rain; 600 mm in exposed locations

Wall-to-window junction:

• Insulation return into window reveal (min 30 mm, preferred 50 mm, to window frame)
• Air barrier sealed to window frame with tape or gasket (e.g., SIGA Fentrim, Tescon Profil)
• Internal VCL lapped and sealed to window frame
• Sill flashing: turned up behind window frame, drip edge projecting 30 mm min beyond wall face
• Cavity tray above window head, weep holes at 450 mm centers

Wall-to-ground junction:

• Below-grade waterproofing: tanking membrane (Type A, BS 8102), or structural waterproof concrete (Type B), or drained cavity (Type C)
• Insulation extends below grade: XPS or foam glass (moisture resistant)
• Thermal bridge at foundation: insulate under slab edge or use insulated foundation system
• DPC (damp-proof course): min 150 mm above finished ground level (UK Building Regs)

Section 6: Thermal Bridge Prevention

6.1 What Is a Thermal Bridge?

A thermal bridge is a localized area of the building envelope where the heat flow is significantly higher than through the adjacent general envelope area. Thermal bridges:

• Increase total heat loss by 10–30% in conventional construction
• Reduce internal surface temperature, increasing condensation and mold risk
• Are the primary barrier to achieving Passive House performance

6.2 Psi-Values (ψ) for Common Junctions

The psi-value (linear thermal transmittance) quantifies the additional heat loss per metre length of junction, in W/mK.

6.3 Strategies for Thermal Bridge Prevention

Continuous insulation: The single most effective strategy. Insulation wraps continuously around the entire building envelope without interruption. External insulation (ETICS/rainscreen) achieves this more easily than cavity or internal insulation.

Thermal break connectors:

• Schock Isokorb: structural thermal break for concrete-to-concrete balcony connections. Reduces ψ from ~0.70 to ~0.15 W/mK. Available for moment, shear, and combined loads.
• Halfen HIT: similar structural thermal break system
• Armatherm: FRP (fiber-reinforced polymer) thermal break pads for steel-to-steel and steel-to-concrete connections
• Typical thermal break thickness: 80–120 mm of insulation within the structural connection

Proprietary brackets for rainscreen cladding:

• Standard aluminum bracket: ψ ≈ 0.04–0.08 W/mK per bracket
• Thermal break bracket (e.g., Leviat Halfen, Fischer Thermax): ψ ≈ 0.01–0.02 per bracket
• Number of brackets: typically 4–6 per m² of facade
• Total bracket thermal bridge on a facade: can add 0.02–0.05 W/m²K to effective wall U-value if not thermally broken

6.4 Common Thermal Bridge Locations

Window heads, sills, and jambs:

• Install window in the insulation plane (not at the back of the reveal)
• Insulation return into reveal: min 30 mm over frame
• Use insulated window sub-frames (Purenit, Compacfoam) for Passive House detailing
• Passive House install zone: window positioned where insulation layer crosses (typically 1/3 from exterior face)

Floor edges (intermediate floors):

• Concrete slab edge exposed at facade: major thermal bridge (ψ = 0.07–0.15)
• Solution: wrap insulation around slab edge (perimeter insulation strip, 30–50 mm)
• Or: use insulated curtain wall spandrel panel covering slab edge

Balcony connections:

• Uninsulated concrete balcony penetrating insulation layer is the worst common thermal bridge
• Solutions: structural thermal break (Isokorb type); hung balconies on independent structure; cantilevered steel brackets with thermal breaks; prefabricated balcony units with thermal separation

Parapets:

• Concrete or masonry parapet extending above insulated roof is a thermal bridge and condensation risk
• Solution: insulate parapet on all three sides (inner face, top, outer face) to maintain warm temperatures
• Or: eliminate parapet with roof edge detail and external gutter

Foundations:

• Concrete foundation wall extending below insulated wall: thermal bridge to ground
• Solution: insulate foundation externally to depth of 600 mm minimum (frost depth); use insulated foundation systems (Passive House foundations with XPS/foam glass sub-slab and perimeter insulation)
• Passive House approach: Foamglas Perinsul blocks at base of wall to break thermal bridge at DPC level

6.5 Thermal Bridge Calculation Methods

• 2D analysis: THERM (free, LBNL), Flixo, HTflux, Psi-Therm. Calculate ψ-values for linear junctions per BS EN ISO 10211.
• 3D analysis: Required for point thermal bridges (brackets, anchors, balcony connections). HEAT3, AnTherm, Comsol.
• Conventions: BRE IP 1/06 (UK); PHI Protocol (Passive House). Interior dimensions method vs. exterior dimensions method affects ψ-values — always state convention used.
• SAP/SBEM (UK): Uses tabulated ψ-values from Accredited Construction Details or BR 497; custom values from thermal modeling accepted.

Appendix A: Envelope Testing and Commissioning

Air Permeability Testing

Blower door test (BS EN ISO 9972 / ASTM E779):

• Pressurize building to 50 Pa; measure air flow rate required to maintain pressure
• Result expressed as m³/(h·m²) at 50 Pa (envelope area basis) or air changes per hour at 50 Pa (ach50, volume basis)
• UK Building Regs Part L: max 8.0 m³/(h·m²); recommended ≤5.0; best practice ≤3.0
• Passive House: ≤0.6 ach50 (extremely airtight; ~1.0 m³/(h·m²) typical)
• Testing: required on completion for new buildings in UK (since 2006); sampled or all units
• Common leakage paths: service penetrations, wall-to-floor junctions, window frames, loft hatches, electrical sockets on external walls

Water Penetration Testing (Curtain Wall)

• AAMA 501.1: Field test with calibrated spray rack at 34 liters/m²/hour with 137 Pa pressure differential
• AAMA 503: Volumetric test — measure total water penetration volume
• EN 12155: Lab test under static and dynamic pressure (up to 600 Pa static, 250 Pa gusting)
• CWCT Standard (UK): Lab test to 600 Pa static; no water penetration beyond air barrier
• Hose test (AAMA 501.2): Field diagnostic — hand-held nozzle at 22 liters/min, 300 mm distance, systematic sweep. Identifies leak locations.

Thermal Imaging (Infrared Thermography)

• BS EN 13187: Qualitative detection of thermal irregularities in building envelopes
• Conduct during heating season: min 10°C interior-exterior temperature differential
• Pre-dawn preferred (no solar loading on facade)
• Identifies: thermal bridges, missing insulation, air leakage paths, moisture ingress
• Cannot quantify U-values; qualitative assessment only (color palette indicates relative surface temperature)

Appendix B: Key Standards and Code References

Envelope Performance Rules of Thumb