Window Draft Energy Savings Calculator

Estimate how much heat escapes through drafty windows and how much you can save by sealing them. Calculate payback period for weatherstripping, caulk, and window insulation film.

windows
sq ft
\u00B0F

Quick Facts

Stack Effect
Warm Air Rises
Air leaks are worst on upper floors where warm air presses against window frames
DIY Cost
$3\u20138 per window
Weatherstripping and caulk cost $3-8 per window and last 3-5 years
Film Savings
10\u201320% Heat Loss
Window insulation film kits reduce window heat loss by 10-20%
Detection
Incense or Candle
Hold a lit incense stick near window edges; flickering smoke reveals drafts

Your Results

Calculated
Annual Heat Loss
-
Energy escaping through window gaps per year
Annual Energy Cost
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Money spent heating air that leaks out
Annual Savings from Sealing
-
Assume 90% air leak reduction with proper sealing
DIY Payback Period
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Estimated sealing cost: -

Efficiency Assessment

Enter your window details to estimate heat loss and sealing savings.

Key Takeaways

  • Air leakage through window gaps is a major source of heat loss in homes. The DOE estimates that air leaks account for 25 to 40 percent of the energy used for heating and cooling in a typical home.
  • Weatherstripping and caulk are among the cheapest energy upgrades you can make. The materials cost $3 to $8 per window, and the work takes about 15\u201330 minutes per window with no specialized tools.
  • The payback period for DIY window sealing is often measured in weeks or months, not years. A $30 weatherstripping project can pay for itself within a single heating season.
  • Window insulation film adds a second layer of trapped air that boosts R-value by roughly 0.5 to 1.0, reducing window heat loss by 10\u201320 percent for about $3\u20135 per window.

How Window Drafts Waste Energy

A draft is uncontrolled air moving through a gap between the window sash and its frame. Warm indoor air escapes through the top and sides while cold outdoor air is pulled in through the bottom and sides. This air exchange effectively turns your heating system into an outdoor heater.

The physics is straightforward. Air has a specific heat of about 0.018 BTU per cubic foot per degree Fahrenheit. A window with a 1/8-inch gap around its entire perimeter on a standard 3-by-4-foot window creates an opening of roughly 2.6 square inches. At a typical 40-degree temperature difference, the heat loss through that single window can reach 500,000 BTU per month in heating season, roughly equivalent to 5 therms of natural gas or 146 kWh of electricity.

How This Calculator Works

Heat loss (BTU/hr) = gap area \u00D7 air flow rate \u00D7 specific heat of air \u00D7 temperature difference \u00D7 60 min/hr
Gap area: window count \u00D7 windows perimeter (2\u00D7width+2\u00D7height) \u00D7 gap width. For a 3x4 ft window with 1/8-inch gap: (2\u00D73 + 2\u00D74) \u00D7 12 inches/ft \u00D7 0.125 in = 21 sq inches of gap area.
Fuel cost per million BTU: natural gas $12/MMBTU (at $1.20/therm), electric resistance $44/MMBTU (at $0.15/kWh), heating oil $25/MMBTU, propane $28/MMBTU, heat pump $15/MMBTU (COP 3.0).

Worked Example

5 drafty 12 sq ft windows, 1/8-inch gaps, natural gas heat, 40\u00B0F temperature difference:

  • Gap area per window = (2\u00D73 + 2\u00D74) \u00D7 12 \u00D7 0.125 = 21 sq inches
  • Total gap area = 5 \u00D7 21 = 105 sq inches
  • Air flow \u2248 105 \u00F7 144 = 0.73 sq ft \u00D7 1.5 ft/sec \u2248 1.1 cfm air leakage (simplified)
  • Heat loss \u2248 1.1 \u00D7 60 \u00D7 0.018 \u00D7 40 \u00D7 24 hrs \u00D7 180 heating days \u00D7 0.75 duty cycle \u2248 2.9 million BTU/year
  • At $12 per MMBTU for natural gas: roughly $35 per year in wasted heating cost
  • DIY sealing cost: about $30 for 5 windows. Payback: under one heating season.

Real-World Numbers

The DOE estimates that the average home has enough air leakage to equal leaving a window wide open all winter. A comprehensive air sealing project \u2014 windows, doors, outlets, attic hatches \u2014 typically costs $200\u2013500 in materials and reduces heating and cooling bills by 10\u201320 percent, paying back in 1\u20133 years. Window sealing alone is the highest-return piece of that project.

Important Note

This calculator estimates convective heat loss from air leakage through window gaps. It does not account for conductive heat loss through the glass itself, which is addressed by window insulation film. For double-pane windows, conductive loss through the glass is often larger than air leakage loss. If your windows are draft-free but still cold to the touch, window film or replacement is the solution, not weatherstripping.

Frequently Asked Questions

How do I know if my windows are drafty?
The most reliable DIY method is the incense stick test: hold a lit incense stick near the window edges, frame, and corners on a cold or windy day. If the smoke wavers or blows sideways, you have an air leak. You can also use a candle, though smoke is easier to see. Another method is to feel along the edges with a damp hand — you will feel the temperature change where cold air is infiltrating. Thermal cameras and blower door tests (done by energy auditors) give more precise results.
Should I weatherstrip or caulk — what is the difference?
Use weatherstripping for moving parts and caulk for stationary gaps. Weatherstripping compresses and springs back, so it works on window sashes and door edges that open and close. Caulk is a permanent sealant meant for fixed joints between the window frame and surrounding wall trim. Most draft problems require both: caulk around the frame perimeter and weatherstripping along the sash where it contacts the sill and stop molding.
When is weatherstripping not enough and I should replace the window?
Replace rather than seal when the window frame itself is warped or rotted, when the glass is cracked or the seal is broken on a double-pane unit (you will see fogging between the panes), or when the window is single-pane and the U-value is the problem, not air leakage. A properly sealed single-pane window still loses about 10 times more heat through the glass itself than a double-pane window would. Sealing extends the life of a window that still has structural integrity; replacement is warranted when the frame is compromised.
How accurate is this heat loss estimate?
The calculation is an estimate based on simplified air infiltration physics and average heating season assumptions. It is accurate enough for deciding whether sealing is worth doing and estimating payback periods, but not precise enough for engineering calculations. Actual heat loss depends on your specific climate, wind exposure, how often the window is opened, and the exact gap geometry. Use the results as directional guidance, not precise engineering figures.