Heat Loss Calculator — Wall and Insulation R-Value Estimator

ANA›Life Services Authority›National Calculator Authority›Heat Loss Calculator — Wall and Insulation R-Value Estimator

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Heat Loss Calculator — Wall and Insulation R-Value Estimator

Calculate heat loss (BTU/hr) through a wall assembly and estimate the total effective R-value from multiple insulation layers, including air films and sheathing.

Wall Area (ft²)

Indoor Temperature (°F)

Outdoor Temperature (°F)

Wall Assembly Layers (R-values per layer)

Enter 0 to omit a layer. R-values are in °F·ft²·hr/BTU.

Interior Air Film R-value

Drywall / Interior Finish R-value

Cavity Insulation R-value

Sheathing R-value

Exterior Finish R-value (siding, brick, etc.)

Exterior Air Film R-value

Additional Continuous Insulation R-value (optional)

Calculate

function heaCalc() { var resultDiv = document.getElementById("hea-result");

// --- Read inputs --- var area = parseFloat(document.getElementById("hea-wall-area").value); var tIn = parseFloat(document.getElementById("hea-temp-inside").value); var tOut = parseFloat(document.getElementById("hea-temp-outside").value); var rIntAir = parseFloat(document.getElementById("hea-r-interior-air").value); var rDrywall = parseFloat(document.getElementById("hea-r-drywall").value); var rInsulation = parseFloat(document.getElementById("hea-r-insulation").value); var rSheathing = parseFloat(document.getElementById("hea-r-sheathing").value); var rExtFinish = parseFloat(document.getElementById("hea-r-exterior-finish").value); var rExtAir = parseFloat(document.getElementById("hea-r-exterior-air").value); var rExtra = parseFloat(document.getElementById("hea-r-extra").value) || 0;

// --- Validation --- var errors = []; if (isNaN(area) || area 0) { resultDiv.innerHTML = "Please fix the following:" + errors.map(function(e){ return ""; }).join("") + ""; return; }

// --- Core Calculations --- // Total R-value = sum of all series resistances var rTotal = rIntAir + rDrywall + rInsulation + rSheathing + rExtFinish + rExtAir + rExtra;

if (rTotal Total R-value must be greater than 0. Please enter valid layer values."; return; }

// U-value (overall heat transfer coefficient) = 1 / R_total var uValue = 1.0 / rTotal;

// Temperature difference var deltaT = tIn - tOut;

// Heat loss Q = U × A × ΔT (BTU/hr) var heatLoss = uValue * area * deltaT;

// Heat loss per square foot var heatLossPerSqFt = uValue * Math.abs(deltaT);

// Annual heat loss estimate (assuming 24 hr/day, 180 heating days typical) var annualHeatLoss = Math.abs(heatLoss) * 24 * 180;

// Determine direction label var direction = deltaT > 0 ? "outward (heat loss)" : "inward (heat gain)"; var absHeatLoss = Math.abs(heatLoss);

// Layer breakdown table var layers = [ { name: "Interior Air Film", r: rIntAir }, { name: "Drywall / Interior Finish", r: rDrywall }, { name: "Cavity Insulation", r: rInsulation }, { name: "Sheathing", r: rSheathing }, { name: "Exterior Finish", r: rExtFinish }, { name: "Exterior Air Film", r: rExtAir }, { name: "Additional Continuous Insulation", r: rExtra } ];

var tableRows = layers.map(function(l) { if (l.r === 0) return ""; var pct = ((l.r / rTotal) * 100).toFixed(1); return "" + l.name + "" + l.r.toFixed(2) + "" + pct + "%"; }).join("");

// R-value rating var rating = ""; if (rTotal " + "ParameterValue" + "Total Wall R-ValueR-" + rTotal.toFixed(2) + "" + "U-Value (BTU/hr·ft²·°F)" + uValue.toFixed(4) + "" + "Temperature Difference (ΔT)" + Math.abs(deltaT).toFixed(1) + " °F" + "Heat Flow Direction" + direction + "" + "Heat Loss / Gain Rate" + absHeatLoss.toFixed(1) + " BTU/hr" + "Heat Loss per ft²" + heatLossPerSqFt.toFixed(2) + " BTU/hr·ft²" + "Estimated Annual Heat Loss*" + (annualHeatLoss / 1000).toFixed(0) + " kBTU/season" + "" + "#### R-Value Rating " + "" + rating + "

" + "#### Layer Breakdown " + "" + "LayerR-Value% of Total" + tableRows + "TotalR-" + rTotal.toFixed(2) + "100%" + "" + "* Annual estimate assumes 24 hr/day × 180 heating days at the entered ΔT. Actual usage varies by climate zone and occupancy.

"; }

#### Formulas Used

Total R-Value (series resistances):

Rtotal = Rint-air + Rdrywall + Rinsulation + Rsheathing + Rext-finish + Rext-air + Rextra

U-Value (overall heat transfer coefficient):

U = 1 / Rtotal    [BTU / (hr · ft² · °F)]

Heat Loss Rate:

Q = U × A × ΔT    [BTU/hr]

Where A = wall area (ft²), ΔT = Tindoor − Toutdoor (°F)

Equivalent forms:

Q = (A × ΔT) / Rtotal

#### Assumptions & References

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References

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