Condensation Dew Point Risk Calculator

Purpose and model boundary

Calculates interior and exterior dew points with the Magnus relation, maps a steady-state temperature gradient through a complete assembly, and identifies the evaluated interface with the smallest dew-point margin.

Do not treat this result as an energy-code determination, Manual J, HVAC or ventilation design, moisture-control guarantee, structural or concrete design, permit approval, equipment selection, cost quote, or endorsement by a cited source. Site climate, assemblies, room use, ventilation, windows, material properties, geometry, manufacturer instructions, locally adopted codes, and approval thresholds remain the user's responsibility.

Inputs and units

• temperature unit: a listed route or text value.
• moisture source: a listed route or text value.
• interface scope: a listed route or text value.
• film treatment: a listed route or text value.
• interior temperature: entered in deg F/deg C.
• exterior temperature: entered in deg F/deg C.
• interior relative humidity: entered in fraction.
• exterior relative humidity: entered in fraction.
• interior film r: entered in h ft^2 deg F/Btu.
• exterior film r: entered in h ft^2 deg F/Btu.
• selected interface number: entered in row number.
• alert band: entered in deg F/deg C.

The layer grid contains 10 fixed data rows plus one header row with these columns: Include, Layer label, Layer R (h ft^2 deg F/Btu), Evaluate interface, Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

Temperatures are converted to Celsius before the moisture calculation. For air temperature T (deg C) and relative humidity RH (fraction), the Magnus relationship and dew point are:

gamma = ln(RH) + 17.625 T/(243.04+T)

T_dp = 243.04 gamma/(17.625-gamma)

T_dp is in deg C. Interior or exterior T_dp is selected by the moisture-source route.

Total thermal resistance is:

R_total = R_film,in + sum(R_j) + R_film,out

R_total is in h-ft^2-deg F/Btu, with film terms included only under the active route. At interface j, cumulative resistance from the interior is R_cum,j; interface temperature is:

T_j = T_in - (T_in-T_out) R_cum,j/R_total

The dew-point margin is:

M_j = T_j-T_dp

M_j is in deg C and is converted to a deg F difference by multiplying by 9/5 when Fahrenheit display is selected. The workbook reports the minimum eligible margin.

Calculation sequence

1. Validate every active selector, scalar bound, type, and every fixed-grid entry.
2. Apply the selected route and ignore values from inactive fields.
3. Normalize unit-dependent inputs into the workbook's calculation basis.
4. Evaluate every included row or candidate and aggregate the active schedule.
5. Calculate primary and supporting outputs, populate the authored chart series, and apply derived numeric checks.
6. Return the workbook status with the computed results; invalid input is never converted into a plausible engineering answer.

Outputs and interpretation

• interior dew point (deg F/deg C): Interior-air dew point in the selected display unit.
• source dew point (deg F/deg C): Dew point selected from interior or exterior air for interface comparison.
• worst interface: Interface label with the smallest temperature-minus-dew-point margin.
• worst interface temperature (deg F/deg C): Modeled temperature at the worst evaluated interface in the selected unit.
• minimum dew point margin (deg F/deg C): Worst interface temperature minus selected source-air dew point; negative means the plane is at or below dew point.

The chart is an explanatory view of the workbook's named chart ranges. It does not add a separate calculation or replace the numeric outputs.

Validation and status logic

The workbook evaluates Model_Status in the order shown. The first matching condition wins.

Assumptions and limitations

• Every shipped layer, film, temperature, and humidity value is synthetic and illustrative.

• The assembly is one-dimensional and steady-state.

• Layer R-values and films are user-verified inputs.

• The dew-point comparison is a thermal-plane screen only.

• Vapor diffusion, air leakage, rain wetting, capillary transport, sorption, thermal bridges, and drying are not modeled.

• A CHECK is not a mold, durability, or condensation diagnosis.

Restrictions and non-computing states

Each numeric control offers a working range chosen for usable entry, which is not always the limit this calculator enforces; a value outside that limit is rejected with a message rather than quietly accepted. The calculator additionally enforces allowed choices, active-field types, complete fixed-grid rows, and conditional or cross-field rules before computing.

Unsupported route values, incomplete rows, out-of-domain inputs, relational failures, nonfinite derived arithmetic, and workbook error tokens are non-computing states. Hidden inputs remain in the values sent to the calculator but must not affect results while those fields are inactive.

Errors and warnings

A rejected entry means the submitted state failed the published input rules. Workbook NOT VALID identifies an invalid active domain or unsupported derived arithmetic. Workbook CHECK retains a finite result while flagging a planning condition that needs review. OK means only that the delivered workbook logic closed within the entered assumptions; it is not code compliance, professional approval, or a safety guarantee.

This version is checked against 80 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• National Weather Service dew-point explanation - Primary public weather-service context for dew point and relative humidity. Official public U.S. government source; the Magnus relation is independently implemented without copied text, tables, or figures.
• U.S. Department of Energy moisture-control guidance - Primary public context for moisture transport limits and the need for project-specific evaluation. Official public U.S. government source; no copied standard text, table, figure, or dataset is embedded.
• NIST temperature unit guidance - Primary public source for temperature-scale conversion context. Official public U.S. government metrology source; no copied text, table, or figure is embedded.

Licensing conclusion: No licensed, proprietary, manufacturer, utility, product-price, code-table, climate table, material table, forecast, or external dataset is embedded. The calculator independently implements general heat-transfer, moisture, airflow, energy, geometry, and quantity arithmetic from cited public and official sources; all defaults, labels, property rows, equipment assumptions, diagrams, and test values are synthetic illustrations. No moisture-design standard, material table, or climate dataset is reproduced.