Heat Flow Rate Through Glass Calculator
Enter values and click calculate.
Understanding Heat Flow Through Glass Windows
The rate of heat flow through a glass window is a fundamental concept in thermodynamics and building efficiency. It measures how quickly thermal energy passes from a warm environment to a colder one through the medium of glass. This process is primarily driven by conduction.
The Heat Conduction Formula
The formula used to calculate the rate of heat flow (Q) is based on Fourier's Law of Heat Conduction:
Q = (k × A × ΔT) / d
- Q: The rate of heat flow, measured in Watts (W).
- k: The thermal conductivity of the glass (W/m·K). Higher values mean the material conducts heat more easily.
- A: The total surface area of the glass window (m²).
- ΔT: The temperature difference between the inner surface and outer surface of the glass (°C or K).
- d: The thickness of the glass (m). Note that our calculator takes input in mm and converts it to meters automatically.
Example Calculation:
Suppose you have a single-pane window with an area of 1.5 m² and a thickness of 4 mm (0.004 m). The thermal conductivity of glass is 0.8 W/m·K. If the temperature outside is 5°C and inside is 20°C, the temperature difference is 15°C.
Q = (0.8 × 1.5 × 15) / 0.004 = 4,500 Watts.
This means 4,500 Joules of energy are escaping through the glass every second.
Suppose you have a single-pane window with an area of 1.5 m² and a thickness of 4 mm (0.004 m). The thermal conductivity of glass is 0.8 W/m·K. If the temperature outside is 5°C and inside is 20°C, the temperature difference is 15°C.
Q = (0.8 × 1.5 × 15) / 0.004 = 4,500 Watts.
This means 4,500 Joules of energy are escaping through the glass every second.
Factors Influencing Thermal Loss
To improve energy efficiency in a building, we look at several variables:
- Glass Thickness: Increasing the thickness of a single pane reduces heat flow, though this is often limited by weight and structural constraints.
- Surface Area: Larger windows naturally allow more heat to pass through. This is why highly efficient homes often limit window sizes on north-facing walls (in the northern hemisphere).
- Insulation: This calculator focuses on conduction through a solid. In reality, modern windows use double or triple glazing where air or argon gas is trapped between panes, significantly reducing the "k" value and overall heat loss.
Why Thermal Conductivity (k) Matters
Standard soda-lime glass has a thermal conductivity of roughly 0.8 to 1.05 W/m·K. In contrast, materials like wood have a much lower conductivity (around 0.12), and metals like aluminum have much higher conductivity (around 200). This explains why glass feels cold to the touch in winter; it is actively conducting heat away from your skin much faster than a wooden frame would.