WINDOW PERFORMANCE
The Learning Center
Wind imparts velocity to rain and creates a pressure difference across the window. It is the combination of driving rain striking the window
and the pressure difference across the window that tends to force the water through the seals between operable sash or ventilator and
frame and through adequately-sized drainage slots or weep holes.
Wind may be defined as air in motion. According to the laws of physics, when air moving in a horizontal direction at a certain velocity
strikes a stationary vertical plate, it will exert a static pressure on the plate which is equal to 0.00256 times the square of the velocity.
Moving air striking a vertical plate is equivalent to moving air blowing against a window. What the equation says is that a 10 MPH wind can
exert a pressure of 0.256 PSF whereas a 100 MPH wind can exert a pressure of 25.6 PSF. The significance of these pressures becomes
apparent when we relate the height at which they will raise water. We refer to this as water head.
The relationship between pressure and water head is found by multiplying the pressure by 0.192. A 25.6 PSF pressure will raise the water
a height of 4.92 inches. A 6.4 PSF pressure, which would result from a constant 50 MPH wind, will raise water to a height of 1.23 inches. A
50 MPH wind will cause a column of water to rise 1.23 inches. In any window design, the sill and its upstanding leg must be designed to
accommodate the water head resulting from the anticipated velocity.
In addition to the static pressure created across an opening by wind, there are other forces that move water through an opening. They are
gravity, kinetic energy, surface tension, capillary action and air currents.
4. What affects the water-resistant qualities of a window?
< back to list of questions about window performance
3245 Miracle Drive
Murrysville, PA 15668
724-387-2991
PA Contractor ID
PA001856
The Learning Center
Wind imparts velocity to rain and creates a pressure difference across the window. It is the combination of driving rain striking the window
and the pressure difference across the window that tends to force the water through the seals between operable sash or ventilator and
frame and through adequately-sized drainage slots or weep holes.
Wind may be defined as air in motion. According to the laws of physics, when air moving in a horizontal direction at a certain velocity
strikes a stationary vertical plate, it will exert a static pressure on the plate which is equal to 0.00256 times the square of the velocity.
Moving air striking a vertical plate is equivalent to moving air blowing against a window. What the equation says is that a 10 MPH wind can
exert a pressure of 0.256 PSF whereas a 100 MPH wind can exert a pressure of 25.6 PSF. The significance of these pressures becomes
apparent when we relate the height at which they will raise water. We refer to this as water head.
The relationship between pressure and water head is found by multiplying the pressure by 0.192. A 25.6 PSF pressure will raise the water
a height of 4.92 inches. A 6.4 PSF pressure, which would result from a constant 50 MPH wind, will raise water to a height of 1.23 inches. A
50 MPH wind will cause a column of water to rise 1.23 inches. In any window design, the sill and its upstanding leg must be designed to
accommodate the water head resulting from the anticipated velocity.
In addition to the static pressure created across an opening by wind, there are other forces that move water through an opening. They are
gravity, kinetic energy, surface tension, capillary action and air currents.
4. What affects the water-resistant qualities of a window?
< back to list of questions about window performance
3245 Miracle Drive
Murrysville, PA 15668
724-387-2991
PA Contractor ID
PA001856
