Heat Flow & Thermal Bridging

How this fits in the series

Builds on: P6 (four control layers), P8 (air/vapor)
Leads to: P10 (assemblies), P11 (HVAC/IAQ)

Core concepts and execution implications

• Thermal bridges undermine comfort and durability.
  • Can identify and mitigate bridges in common details.
• Condensation risk depends on temperature gradients.
  • Can evaluate assemblies for cold-surface risk, not R-value alone.
• Continuous thermal control simplifies the whole system.
  • Can prioritize continuity across transitions and penetrations.

What good looks like

• Comfortable interior surfaces (floors, walls, window perimeters) in winter.
• Reduced thermal bridging at common bridge locations (rim joists, slab edges, balconies, headers, fasteners).
• Condensation control by keeping vulnerable layers warm enough and/or able to dry.
• Performance that matches expectations (avoid "I paid for R-XX but it doesn't feel like it").

Three common thermal bridges

1. Rim joist / band area

• Symptom: cold band at floor line, musty odor, condensation or mold behind finishes
• Why: thin or discontinuous insulation leaves framing exposed to exterior temps; minor air leaks supply moisture
• Mitigation: continuous exterior insulation across the band, or spray-foam the cavity and air-seal the connection

2. Slab edge

• Symptom: cold floors at perimeter, occupant complaints of "drafts" (actually radiant asymmetry)
• Why: concrete slab conducts heat directly to exterior; no thermal break
• Mitigation: insulate slab edge (exterior preferred); detail to prevent thermal bypass at foundation-to-wall transition

3. Window opening

• Symptom: condensation on frame or adjacent drywall; cold spots on thermal scan
• Why: framing around opening creates concentrated bridge; installation gap may lack insulation
• Mitigation: insulate jamb/head/sill gaps; use thermally broken frames where budget allows; return insulation to frame

Explore in PF: Thermal (C2) → Thermal/Moisture Mechanisms (E2) → Envelope Failures (F3)

Condensation rule of thumb

Cold surface + moisture source + time = damage. Condensation happens when a surface is colder than the dew point of the air touching it. The three variables you can control: keep surfaces warm (insulation continuity), reduce moisture load (ventilation, source control), and limit exposure time (drying potential, seasonal drying direction).

If you can keep interior-facing surfaces above ~55–60 °F during heating season and maintain reasonable indoor RH (30–50%), most condensation problems disappear. The failures happen where thermal bridges drop surface temps below that threshold.

Explore in PF: Thermal/Moisture Mechanisms (E2), Condensation risk in Controls (G5)

Where things go wrong

Thermal/condensation failure patterns (often misdiagnosed as "HVAC problems"): cold corners/bands from thermal bridges, condensation at window perimeters, roof/ceiling moisture from thermal bypass + air leakage, and ice dams from heat loss + melt/refreeze. Examples:

1. Rim joist cold band and musty odor
   Thin or discontinuous insulation leaves cold surfaces; minor air leakage supplies moisture; mold risk localizes. Driver: thermal bridge + minor leakage
   Field check: thermal scan rim/band area in heating season; verify continuous insulation and air seal at floor line
2. "We insulated more and now the sheathing is wet"
   Changing insulation moves the condensing surface; if the assembly cannot dry, moisture accumulates. Driver: condensation plane moved
   Field check: verify ratio of exterior to interior insulation keeps sheathing above dew point; confirm drying path

References & resources

• Performance Framework Explorer
• Performance series index