Condensation forms where the temperature drops below the dew point. On a pipe, that is visible. Inside a wall, it is hidden.
The temperature gradient
In winter, each layer of a wall is at a different temperature:
Indoor air: 70°F
Drywall: 69°F
Insulation: 70°F → 25°F (across R-15)
Sheathing: 25°F
Air gap: 22°F
Cladding: 20°F
Outdoor air: 20°F
The temperature at each interface:
T_interface = T_indoor − (ΔT_total × R_cumulative / R_total)
Where:
ΔT_total = T_indoor − T_outdoor = 50°F
R_total = 20
At the sheathing inner face:
R_cumulative = R_drywall(0.5) + R_cavity(15) + R_studs(2.5) = 18
T = 70 − (50 × 18/20) = 70 − 45 = 25°F
The dew point check
Indoor air at 70°F and 40% RH:
Dew point ≈ 45°F
Compare to each interface:
Interface Temp vs Dew Point Risk
──────────────────────────────────────────────────
Drywall surface 69°F > 45°F safe
Insulation cavity varies crosses 45°F ⚠️
Sheathing inner face 25°F < 45°F CONDENSATION
Sheathing outer face 22°F < 45°F CONDENSATION
The condensation plane is at the sheathing — 20°F below the dew point.
Why this destroys buildings
Visible pipe condensation:
→ noticed in days
→ fixed with insulation
→ cost: $50
Hidden wall condensation:
→ noticed in months or years
→ mold, rot, structural damage
→ cost: $5,000–$50,000+
The vapor barrier rule
Cold climate: vapor barrier on INTERIOR (warm side)
Hot-humid climate: vapor barrier on EXTERIOR (warm side)
Mixed climate: smart vapor retarder (adjusts with humidity)
Wrong side = moisture trapped against cold surface = guaranteed failure
The quick check
1. Calculate indoor dew point from T and RH
2. Calculate temperature at each wall interface (R-value ratio)
3. If any interface < dew point → condensation risk
4. Fix: add exterior insulation, lower indoor RH, or add vapor retarder
