Roof Ventilation Calculator
ANA›Life Services Authority›National Calculator Authority›Roof Ventilation Calculator
.calc-container { max-width: 640px; margin: 2rem 0; padding: 1.5rem; background: #fff; border: 1px solid #ddd; border-radius: 8px; box-shadow: 0 1px 3px rgba(0,0,0,0.06); font-family: system-ui, -apple-system, sans-serif; } .calc-container h3 { font-family: Georgia, serif; font-size: 1.15rem; color: #1a1a1a; margin-bottom: 1rem; padding-bottom: 0.5rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-row { display: flex; align-items: center; gap: 0.75rem; margin-bottom: 0.75rem; flex-wrap: wrap; } .calc-row label { min-width: 160px; font-size: 0.9rem; color: #333; font-weight: 500; } .calc-row input[type="number"], .calc-row select { flex: 1; min-width: 120px; max-width: 200px; padding: 0.5rem 0.6rem; border: 1px solid #ccc; border-radius: 4px; font-size: 0.9rem; font-family: system-ui, sans-serif; color: #1a1a1a; background: #fafaf8; } .calc-row input:focus, .calc-row select:focus { outline: none; border-color: var(--ac, #3d5a80); box-shadow: 0 0 0 2px rgba(26,74,138,0.12); } .calc-row .unit { font-size: 0.82rem; color: #888; min-width: 30px; } .calc-btn { display: inline-block; margin-top: 0.5rem; padding: 0.55rem 1.5rem; background: var(--ac, #3d5a80); color: #fff; border: none; border-radius: 4px; font-size: 0.9rem; font-weight: 600; cursor: pointer; font-family: system-ui, sans-serif; } .calc-btn:hover { opacity: 0.9; } .calc-result { margin-top: 1.25rem; padding: 1rem 1.25rem; background: #f0f6fc; border-left: 3px solid var(--ac, #3d5a80); border-radius: 0 6px 6px 0; display: none; } .calc-result.visible { display: block; } .calc-result-label { font-size: 0.78rem; text-transform: uppercase; letter-spacing: 0.06em; color: #666; margin-bottom: 0.25rem; } .calc-result-value { font-size: 1.6rem; font-weight: 700; color: var(--ac, #3d5a80); } .calc-result-detail { font-size: 0.85rem; color: #555; margin-top: 0.5rem; line-height: 1.5; } .calc-note { margin-top: 1rem; font-size: 0.8rem; color: #888; font-style: italic; } .calc-grid { display: grid; grid-template-columns: 1fr 1fr; gap: 0.75rem; margin-top: 0.75rem; } .calc-grid-item { padding: 0.6rem 0.8rem; background: #f8f9fa; border-radius: 4px; border: 1px solid #eee; } .calc-grid-item .label { font-size: 0.75rem; color: #888; text-transform: uppercase; letter-spacing: 0.04em; } .calc-grid-item .value { font-size: 1.1rem; font-weight: 600; color: #1a1a1a; } @media (max-width: 720px) { .calc-row { flex-direction: column; align-items: flex-start; gap: 0.3rem; } .calc-row label { min-width: auto; } .calc-row input[type="number"], .calc-row select { max-width: 100%; width: 100%; } .calc-grid { grid-template-columns: 1fr; } } .calc-chart { margin: 1rem 0; text-align: center; } .calc-chart svg { max-width: 100%; height: auto; } .calc-chart-legend { display: flex; flex-wrap: wrap; justify-content: center; gap: 0.6rem 1.2rem; margin-top: 0.6rem; font-size: 0.8rem; color: #555; } .calc-chart-legend span { display: inline-flex; align-items: center; gap: 0.3rem; } .calc-chart-legend i { display: inline-block; width: 10px; height: 10px; border-radius: 2px; font-style: normal; } .calc-related { max-width: 640px; margin: 2rem 0 1rem; padding: 1.25rem 1.5rem; background: #f8f9fa; border: 1px solid #e8e8e8; border-radius: 8px; } .calc-related h3 { font-family: Georgia, serif; font-size: 1rem; color: #1a1a1a; margin: 0 0 0.75rem; padding-bottom: 0.4rem; border-bottom: 2px solid var(--ac, #3d5a80); } .calc-related-list { list-style: none; padding: 0; margin: 0 0 0.75rem; display: grid; grid-template-columns: 1fr 1fr; gap: 0.4rem 1.5rem; } .calc-related-list li a { font-size: 0.88rem; color: var(--ac, #3d5a80); text-decoration: none; } .calc-related-list li a:hover { text-decoration: underline; } .calc-browse-all { margin: 0.5rem 0 0; font-size: 0.9rem; font-weight: 600; } .calc-browse-all a { color: var(--ac, #3d5a80); text-decoration: none; } .calc-browse-all a:hover { text-decoration: underline; } @media (max-width: 720px) { .calc-related-list { grid-template-columns: 1fr; } }
Roof Ventilation Calculator
Calculate the net free ventilation area (NFVA) required for your attic based on the 1/150 or 1/300 rule per IRC Section R806.
Attic Floor Area (sq ft)
Ventilation Ratio
1/150 — No vapor barrier or <40% humidity control 1/300 — Vapor barrier installed & humidity controlled
Intake Vent Percentage (%)
Recommended: 40–60% of total as intake (soffit vents)
Vent Net Free Area Factor (%)
Typical louver/screen factor: 50% (check manufacturer specs)
Calculate
function rooCalc() { const resultDiv = document.getElementById('roo-result');
const atticArea = parseFloat(document.getElementById('roo-attic-area').value); const ratio = parseFloat(document.getElementById('roo-ratio').value); const intakePct = parseFloat(document.getElementById('roo-intake-pct').value); const nfvaFactor = parseFloat(document.getElementById('roo-nfva-factor').value);
// --- Validation --- if (isNaN(atticArea) || atticArea 0 sq ft).
'; return; } if (atticArea > 50000) { resultDiv.innerHTML = 'Attic area seems too large (max 50,000 sq ft). Please verify.
'; return; } if (isNaN(intakePct) || intakePct 60) { resultDiv.innerHTML = 'Intake percentage must be between 40% and 60%.
'; return; } if (isNaN(nfvaFactor) || nfvaFactor 100) { resultDiv.innerHTML = 'Net free area factor must be between 1% and 100%.
'; return; }
// --- Core Formula (IRC R806) --- // Total required NFVA (sq ft) = Attic Area / Ratio const totalNFVA_sqft = atticArea / ratio; const totalNFVA_sqin = totalNFVA_sqft * 144; // convert to sq inches
// Split into intake and exhaust const exhaustPct = 100 - intakePct; const intakeNFVA_sqin = totalNFVA_sqin * (intakePct / 100); const exhaustNFVA_sqin = totalNFVA_sqin * (exhaustPct / 100);
// Gross vent area needed (accounting for screen/louver blockage) const grossFactor = 100 / nfvaFactor; const intakeGross_sqin = intakeNFVA_sqin * grossFactor; const exhaustGross_sqin = exhaustNFVA_sqin * grossFactor; const totalGross_sqin = totalNFVA_sqin * grossFactor;
// Common vent sizes for reference (sq in NFVA each) const ventSizes = [ { label: '8"×16" soffit vent', nfva: 55 }, { label: '16"×8" ridge vent/ft',nfva: 18 }, { label: '12"×12" gable vent', nfva: 80 }, { label: '14"×24" gable vent', nfva: 192 }, ];
let ventTable = ''; ventTable += 'Vent TypeNFVA Each (sq in)Qty for IntakeQty for Exhaust'; ventSizes.forEach(v => { const qtyIn = Math.ceil(intakeNFVA_sqin / v.nfva); const qtyEx = Math.ceil(exhaustNFVA_sqin / v.nfva); ventTable += '' + v.label + '' + '' + v.nfva + '' + '' + qtyIn + '' + '' + qtyEx + ''; }); ventTable += '';
resultDiv.innerHTML = '### Results ' + '' + 'Attic Floor Area' + atticArea.toLocaleString() + ' sq ft' + 'Ventilation Ratio1/' + ratio + '' + 'Total Required NFVA' + totalNFVA_sqft.toFixed(2) + ' sq ft (' + Math.round(totalNFVA_sqin) + ' sq in)' + '↳ Intake Vents (' + intakePct + '%)' + (intakeNFVA_sqin/144).toFixed(2) + ' sq ft (' + Math.round(intakeNFVA_sqin) + ' sq in)' + '↳ Exhaust Vents (' + exhaustPct + '%)' + (exhaustNFVA_sqin/144).toFixed(2) + ' sq ft (' + Math.round(exhaustNFVA_sqin) + ' sq in)' + 'Total Gross Vent Area Needed' + (totalGross_sqin/144).toFixed(2) + ' sq ft (' + Math.round(totalGross_sqin) + ' sq in)' + '↳ Gross Intake Area' + (intakeGross_sqin/144).toFixed(2) + ' sq ft (' + Math.round(intakeGross_sqin) + ' sq in)' + '↳ Gross Exhaust Area' + (exhaustGross_sqin/144).toFixed(2) + ' sq ft (' + Math.round(exhaustGross_sqin) + ' sq in)' + '' + '#### Vent Quantity Reference ' + ventTable; }
#### Formula
Total Required NFVA (sq ft) = Attic Floor Area (sq ft) ÷ Ventilation Ratio
- 1/150 rule: NFVA = Attic Area ÷ 150
- 1/300 rule: NFVA = Attic Area ÷ 300
- Intake NFVA = Total NFVA × (Intake % ÷ 100)
- Exhaust NFVA = Total NFVA × (Exhaust % ÷ 100)
- Gross Vent Area = NFVA ÷ (Net Free Area Factor ÷ 100) — accounts for louvers/screens blocking airflow
Example: 1,500 sq ft attic with 1/300 ratio → 1,500 ÷ 300 = 5 sq ft (720 sq in) total NFVA required.
#### Assumptions & References
- IRC R806 (International Residential Code): Minimum ventilation ratio is 1/150 of attic floor area; reduced to 1/300 when at least 40% and not more than 50% of required ventilation is provided by ventilators located in the upper portion of the attic, or a Class I or II vapor retarder is installed.
- Balanced ventilation: 40–60% intake (low, soffit) / 40–60% exhaust (high, ridge or gable) split is recommended for optimal airflow.
- Net Free Area (NFA/NFVA): The actual open area through which air can pass, after accounting for louvers, screens, and baffles. Typical screen reduces gross area by ~50%.
- Reference: IRC 2021 Section R806; ASHRAE 160; HUD Minimum Property Standards.
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