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Evgenii Konkin
Evgenii Konkin

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Quick Tip: How to Find the Condensation Plane Inside a Wall

#hvac #engineering #quicktip #buildingscience

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
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The temperature at each interface:

T_interface = T_indoor − (ΔT_total × R_cumulative / R_total)
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Where:

ΔT_total = T_indoor − T_outdoor = 50°F
R_total  = 20
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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
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The dew point check

Indoor air at 70°F and 40% RH:

Dew point ≈ 45°F
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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
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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+
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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
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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
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