Interior Surface Temperature Factor (f_Rsi) & Window Condensation Dew-Point Calculator

Predict if interior glass or wall corners will drop below indoor dew point and sweat.

Interior Glass / Wall Surface Temperature
58.34 °F
Indoor Air Dew-Point Temperature 47.69 °F
Condensation Safety Margin (T_surf − T_dp) 10.66 °F
Temperature Factor f_Rsi 0.81 f_Rsi
Inputs
⚡ Scenarios:
°F
Indoor room air temperature (°F).
% RH
Indoor relative humidity (%).
°F
Outdoor winter temperature (°F).
R-value
Total assembly R-value (e.g., 3.5 for double-pane U=0.28 window; 1.8 for thermal bridge).
Results & Graph
Interior Glass / Wall Surface Temperature
58.34 °F
Indoor Air Dew-Point Temperature
47.69 °F
Condensation Safety Margin (T_surf − T_dp)
10.66 °F
Temperature Factor f_Rsi
0.81 f_Rsi

Interior Glass / Wall Surface Temperature

Graph 1 • Window Condensation & Dew Point • Domain Architecture & Live Telemetry Maps Indoor Air Temp Fahrenheit, Indoor Relative Humidity Pct, Outdoor Winter Temp Fahrenheit through Window Condensation & Dew Point Formula.
Window Condensation & Dew Point • Generation → Conversion → Storage Bus 1. SOURCE / ARRAY 70 °F Indoor Air Temp Fahren 2. DELIVERED RATING 58.34 °F Interior Glass / Wall Su 3. SECONDARY OUTPUT 47.69 °F Indoor Air Dew-Point Tem
Graph 2 • Interior Glass / Wall Surface Temperature Across Indoor Air Temp Fahrenheit Sweep (40% → 160%) Evaluates T_surface = T_in − (R_si / R_total) × (T_in − T_out); DewPoint ≈ T_in across 7 operating points (current input highlighted).
0 °F 50.70 °F 101.4 °F 24.50 28°F 35.78 42°F 47.06 56°F 58.34 70°F 69.62 84°F 80.90 98°F 92.18 112°F
Graph 3 • Joint Sensitivity: Indoor Air Temp Fahrenheit × Indoor Relative Humidity Pct Compares Interior Glass / Wall Surface Temperature when Indoor Relative Humidity Pct is shifted +25% (solid) vs. −25% (dashed).
0 °F 41.39 °F 82.79 °F --- Indoor Relative Humidity Pct @ 75% 41.42 -30% 49.88 -15% 58.34 +0% 66.80 +15% 75.26 +30%
Summary: At 10°F outdoors and 70°F / 45% RH indoors (Dew Point = 47.7°F), the R-3.5 interior surface drops to 58.3°F (+10.7°F above dew point: Dry).
Summary: At 10°F outdoors and 70°F / 45% RH indoors (Dew Point = 47.7°F), the R-3.5 interior surface drops to 58.3°F (+10.7°F above dew point: Dry).
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Interactive chart & math engine powered by CalculatorGraph

Formula & Mathematical Methodology: Window Condensation & Dew Point Formula

Tsurface = Tin − (Rsi / Rtotal) × (Tin − Tout)
DewPoint ≈ Tin − ((100 − RH%) / 5) in °C
T_surface = T_in − (R_si / R_total) × (T_in − T_out); DewPoint ≈ T_in − ((100 − RH%) / 5) in °C
  • indoorAirTempFahrenheit: Indoor Air Temp Fahrenheit (°F)
  • indoorRelativeHumidityPct: Indoor Relative Humidity Pct (% RH)
  • outdoorWinterTempFahrenheit: Outdoor Winter Temp Fahrenheit (°F)
  • assemblyOrWindowRValue: Assembly Or Window R Value (R-value)

Rounding: Engineering outputs round to 2–3 decimal places; discrete material bag/unit counts round up via ceiling.

Worked Example: Engineering Scenario: Window Condensation & Dew Point

Predict if interior glass or wall corners will drop below indoor dew point and sweat.

  1. Enter your system dimensions, electrical/RF ratings, or environmental conditions.
  2. Evaluate the governing equation: T_surface = T_in − (R_si / R_total) × (T_in − T_out); DewPoint ≈ T_in − ((100 − RH%) / 5) in °C.
  3. Inspect the primary design output, supporting allowances, and 3 SVG schematics.
Result: Computes Window Condensation & Dew Point via T_surface = T_in − (R_si / R_total) × (T_in − T_out); DewPoint ≈ T_in − ((100 − RH%) / 5) in °C.

Frequently Asked Questions

How does the Interior Surface Temperature Factor (f_Rsi) & Window Condensation Dew-Point Calculator calculate results?

It evaluates T_surface = T_in − (R_si / R_total) × (T_in − T_out); DewPoint ≈ T_in − ((100 − RH%) / 5) in °C directly in your browser and updates three custom SVG visualizations in real time.

Can I use Goal Seek to solve backwards for a required dimension or rating?

Yes. Click "🎯 Goal Seek" in the results toolbar to solve backwards for any numeric input parameter.

Does this engineering calculator work offline?

Yes. All calculations run 100% locally in your browser and work offline via the CalculatorGraph PWA.

Construction & Home · Building Envelope, Insulation & HVAC

Interior Surface Temperature Factor (f_Rsi) & Window Condensation Dew-Point Calculator: inputs, method, and how to read the result

Predict if interior glass or wall corners will drop below indoor dew point and sweat.

Choose inputs for the case you are checking

The defaults are illustrative. Replace them with values for your case and keep the units shown beside each field.

  • Indoor Air Temp Fahrenheit: Indoor room air temperature (°F).
  • Indoor Relative Humidity Pct: Indoor relative humidity (%).
  • Outdoor Winter Temp Fahrenheit: Outdoor winter temperature (°F).
  • Assembly Or Window R Value: Total assembly R-value (e.g., 3.5 for double-pane U=0.28 window; 1.8 for thermal bridge).

What the calculation includes

With the default values shown, Interior Glass / Wall Surface Temperature is 58.34 °F. The calculation follows Window Condensation & Dew Point Formula; the formula and variable definitions above show how the inputs combine.

One-input check: With the other defaults held constant, changing Indoor Air Temp Fahrenheit from 70 °F to 77 °F changes Interior Glass / Wall Surface Temperature from 58.34 °F to 63.98 °F. This is a comparison inside the calculator's stated model, not a prediction.

Where to use caution

  • Uses standard NREL, IEEE, ASHRAE, and IRC design equations.
  • Verify final structural, electrical, and mechanical specifications against local building codes.

Scope: Global Engineering & Building Code Standard Check the assumptions and limitations against your situation before relying on the estimate.

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