Model leakage through windows using crack-flow physics quickly. Choose units, temperatures, and room volume easily. Download results, compare scenarios, and improve energy savings now.
This estimator treats window leakage as a crack-flow process that follows a pressure power law:
Total reference flow is derived from the rated leakage per area:
Sensible heat loss uses W ≈ ρ·cp·Q·ΔT with standard air properties (ρ≈1.2 kg/m³, cp≈1005 J/kg·K).
| Case | Leakage rating | Pref (Pa) | ΔP (Pa) | n | Windows | Area each | Room volume | Typical result (CFM) |
|---|---|---|---|---|---|---|---|---|
| Baseline | 0.30 CFM/ft² | 75 | 4 | 0.65 | 6 | 15 ft² | 9000 ft³ | ~6–8 |
| Windy | 0.30 CFM/ft² | 75 | 10 | 0.65 | 6 | 15 ft² | 9000 ft³ | ~12–16 |
| Improved seal | 0.15 CFM/ft² | 75 | 4 | 0.65 | 6 | 15 ft² | 9000 ft³ | ~3–4 |
Window leakage is unintended airflow through frame joints, sash interfaces, and hardware penetrations. Ratings are measured at a standardized reference pressure, often 75 Pa, to compare products consistently. Buildings usually operate near 1–10 Pa, so pressure scaling helps estimate realistic infiltration.
Most certifications report leakage per unit area, such as CFM per square foot or liters per second per square meter. Multiply the rating by total window area to obtain a reference flow for all windows. This calculator builds total area from area-per-window and window count, reducing unit mistakes.
Operating ΔP depends on wind, stack effect, and HVAC balance. Use 2–4 Pa for typical sheltered conditions, and 8–10 Pa for exposed facades or stronger winds. If you have measured zone pressures, enter them directly. Running multiple ΔP cases is a practical way to compare comfort risk.
Crack-flow often follows a power law where flow scales with (ΔP/Pref) raised to n. Values near 0.60–0.70 are common for window cracks, with 0.65 frequently used as a default. A higher n increases sensitivity to pressure, so testing 0.60 and 0.70 can bracket uncertainty.
Air changes per hour (ACH) converts leakage flow into how quickly a room’s air is replaced. ACH depends strongly on volume, so smaller rooms can show higher ACH for the same window flow. Use ACH to compare zones and draft risk.
Sensible heat loss is estimated from airflow and the indoor–outdoor temperature difference using standard air properties. Larger ΔT values amplify the impact, so winter conditions often dominate energy penalties. Treat the watts and BTU per hour outputs as planning-level figures for comparing sealing options.
Per-window flow helps prioritize fixes when you suspect a few windows drive most discomfort. Common interventions include weatherstripping replacement, sash alignment, improving latch compression, and sealing frame-to-wall gaps. Re-run the calculator with updated ratings or measured flows to quantify improvement.
This estimator assumes steady crack-flow and does not model gust direction, multi-zone pressure balancing, or turbulent jets at specific gaps. Use it for comparisons and scenario screening, not code compliance. For detailed assessment, pair these results with blower-door tests, zone pressure diagnostics, and field observations.
Use 1–5 Pa for typical calm conditions, and 5–10 Pa for windier exposure. If you have measured zone pressures, enter those values. Scenario runs are often more informative than one number.
Not always. Many ratings use 75 Pa, but some documents specify different test pressures. Match the reference pressure to your rating source so scaling stays consistent and meaningful.
n controls how leakage flow grows with pressure difference in the power-law model. Values near 0.65 are common for cracks. If n is unknown, run 0.60 and 0.70 to bracket outcomes.
Start with weatherstripping, sash adjustment for uniform contact, tightening hardware, sealing frame-to-wall gaps, and correcting misalignment. Confirm progress by updating inputs with new ratings or measured flow and comparing exports.
No. The heat-loss estimate is sensible only, based on airflow and temperature difference. If humidity control matters, add a separate latent-load calculation for moisture infiltration.
It works best for a room or a window group where rated leakage and area are known. Whole-building infiltration includes many leakage paths and pressure interactions, so blower-door results are more reliable.
Draft sensation depends on local jets, air temperature, and where the air hits occupants, not only total airflow. A modest total flow can still feel strong if it is concentrated in one gap or directed into the room.