Assembly R Value Whole Wall Calculator

Purpose and model boundary

Builds two transparent parallel heat-flow paths from a complete layer schedule, reports cavity, framing, and whole-wall thermal resistance, and optionally screens a degree-day retrofit payback.

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

• film treatment: a listed route or text value.
• payback analysis: a listed route or text value.
• target check: a listed route or text value.
• interior film r: entered in h ft^2 deg F/Btu.
• exterior film r: entered in h ft^2 deg F/Btu.
• framing fraction: entered in fraction.
• assembly area: entered in ft^2.
• baseline whole wall r: entered in h ft^2 deg F/Btu.
• heating degree days: entered in deg F day/year.
• heating performance factor: entered in delivered/input ratio.
• energy cost per kbtu: entered in $/kBtu.
• installed project cost: entered in $.
• service life: entered in years.
• target whole wall r: entered in h ft^2 deg F/Btu.

The layer grid contains 12 fixed data rows plus one header row with these columns: Include, Layer label, Path, Value basis, Thickness (in), Thermal property (selected property basis), Source note. Numeric columns retain the units shown here; text, route, and include columns are intentionally unitless.

Governing relationships

Layer resistance is entered directly as R_j or calculated from one of these relationships:

R_j = t_j r_j

R_j = t_j/k_j

The first applies to an R-per-inch basis and the second to the workbook's conductivity-basis numeric route. t_j is layer thickness (in), r_j is resistance per inch, and k_j is the route-dependent thermal-property value entered in units compatible with thickness. The resulting R_j is in h-ft^2-deg F/Btu.

The insulated-path and framing-path totals are:

R_ins = R_film + sum(R_common) + sum(R_insulated)

R_frame = R_film + sum(R_common) + sum(R_framing)

R_film is zero or the entered interior-plus-exterior film resistance. Whole-wall transmittance, whole-wall resistance, and framing penalty are:

U_whole = (1-f_frame)/R_ins + f_frame/R_frame

R_whole = 1/U_whole

Penalty_frame = 1-R_whole/R_ins

U_whole is in Btu/(h-ft^2-deg F), and f_frame is framing-area fraction.

When payback is active, annual thermal savings are:

E_save = A 24 HDD (1/R_base-U_whole)/(1000 eta_heat)

E_save is in kBtu/year. A is assembly area (ft^2), HDD is heating degree-days (deg F-day/year), R_base is baseline whole-wall resistance, and eta_heat is the entered heating performance factor. Annual cost savings are:

Savings_annual = E_save c_E

Simple payback is installed project cost divided by positive annual cost savings.

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

• insulated path r (h ft^2 deg F/Btu): Total resistance through common layers and the insulated path, including entered films when selected.
• framing path r (h ft^2 deg F/Btu): Total resistance through common layers and the framing path, including entered films when selected.
• whole wall r (h ft^2 deg F/Btu): Inverse of the area-weighted parallel-path transmittance.
• whole wall u (Btu/(h ft^2 deg F)): Area-weighted sum of the insulated-path and framing-path U-values.
• annual cost savings ($/year): Annual thermal savings multiplied by the entered delivered-energy cost.
• simple payback (years): Installed project cost divided by positive modeled annual cost savings; zero means payback is unavailable.

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 and property is synthetic and illustrative.

• Parallel paths are one-dimensional and steady-state.

• The economic route uses entered degree-days, performance, and delivered-energy cost.

• Thermal bridges at corners, plates, fasteners, openings, and interfaces are not resolved.

• The workbook is not a code-compliance determination, energy model, or professional envelope design.

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 96 saved test cases, covering both calculated results and entries that should be refused, run against this exact workbook.

• U.S. Department of Energy building-envelope overview - Primary public explanation of heat transfer through building envelopes and the role of thermal resistance. Official public U.S. government source; no copied text, table, figure, or dataset is embedded.
• U.S. Department of Energy insulation guidance - Primary public context for insulation, air sealing, and project-specific evaluation. Official public U.S. government source; no copied text, table, figure, or insulation value 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 insulation, air-film, material-property, energy-price, or code table is reproduced.