Issues and regulations regarding Egress Windows

Egress windows are a requirement for all new construction projects.  I think the code dates back farther than 1983, but has only recently (say the last 10 years) been enforced.  The purpose of the code is to allow a fireman access to a burning home through the opening of a window.  In theory the clear opening has to be large enough to allow the fireman entry with an oxygen tank strapped on his or her back, which actually makes sense. The code has the following requirements:

  1. No clear opening height dimension may be less than 24″.
  2. No clear opening width dimension may be less than 20″.
  3. For the first floor the square footage of clear opening must be 5 square feet (SF) or higher.
  4. For second floor the SF of clear opening must be 5.7 SF or higher.
Figure 1.

A typical vinyl single hung will require a 3’0 x 5’0 window to meet the 5.7 SF minimum.  This is greatly affected by the height of the sill tower which will limit the vertical clear opening.  There are two competing design objectives when considering a window, the first of which is the Egress issue and the second of which is a concept more pertinent to users of windows in high wind zones such as coastal areas–that is high design pressures (DP).  In order to achieve say a DP of 50, the window has to withstand a blowing rain of 54 miles per hour (windload chart). (Figure 1. shows MGM’s in-house test wall used to simulate a 54mph wind load.) In order to achieve this the sill tower has to be 1.440 inches tall. (also see the windload chart, column 7 to find 1.440).  To paint an example to try to explain this, assume one were to put a bucket of water on the ground with a plastic tube mounted vertically in the pale of water.  Cap off the tube but vent it to atmosphere.   Now blow air down on the surface area of the water at a speed of 54 mph.   the water will rise in the tube exactly 1.440.  This is exactly what happens to a window that is exposed to a blowing rain of 54mph.  So, in order to have a window pass a DP50 water resistance test, the sill tower has to be at least 1.440 tall.  It’s simple physics, but this simple physics limits the ability to pass egress regulations.  Many company’s have to have a 3’0 x 5’2 size single hung to pass egress for these reasons.  Another way to pass egress is to install casement windows.  They will provide a large clear opening because the majority of the width and height of the window can be included in the egress calculations.  In basements it is a different story. You may need add a window for light and also meet egress requirements.

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