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GSR

Correct way to count square feet for lighting and ventilation

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Habitable space with natural lighting and ventilation requires 8% of the floor area be windows and 4% be openable.  How is the floor area defined in the following cases.

1. A habitable attic.  Is it the area with 7 ft or greater headroom?  5 ft or greater headroom?  Or the full area including out to where the rafters meet the floor?

2. A second story room that has an opening to the floor below.  Is it the perimeter floor area around the opening?  Or the full area as if the opening was not present?  (For example, a 16 X 28 ft room with a 8 X 20 opening to the room below, and a 4 ft wide perimeter of actual floor.)

Which provisions of IRC govern?

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4 hours ago, GSR said:

Habitable space with natural lighting and ventilation requires 8% of the floor area be windows and 4% be openable. 

That's not exactly correct. It says that "habitable rooms" shall have an aggregate glazing area of not less than 8 percent of the "floor area" of such rooms. That floor area might cover portions of the room that don't contribute to the habitable space of the room. The requirement applies to the "floor area" not the "floor area of the habitable portion of the room." 

To my eye, it's clearly talking about the gross floor area of the room. So for both your questions, the floor area for lighting and ventilation would be the same as the floor area for, say, carpeting. 

That said, different jurisdictions might interpret this differently. If it's a critical issue, call the local building department and ask. 

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The light and ventilation portion is pretty forgiving if you consider the exceptions to §303.1. If the openings are not required for egress, operable sash may be avoided altogether if their is a whole house ventilation system. (see M1507) 

As for 8%, if there is an alternative artificial light source that can provide adequate (place esoteric number here) light then one can avoid meeting the requirement. 

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