Bedroom 8% natural light requirement? Can someone explain?

Discussion in 'Remodel Forum & Blog' started by snokel, Mar 20, 2014.

  1. snokel

    snokel Member

    Jan 29, 2014
    I am framing my basement and my building inspector approved my permit but said my existing egress windows the builder installed must provide 8% of the square footage of the bedroom for the
    natural light requirement. Since the windows are already in place and I think they are 48x36 sliders, and he said you can not use the actual size you have to measure the glass only to come up with
    the sq footage of the window, which I am guessing would be around 10 sq feet for a 48x36, so the max room I could have would be 125 sq feet, so a bit bigger than 10x12.

    I just don't understand how this natural light requirement assists in egress in any way, specially at night..

  2. Dana

    Dana In the trades

    Jan 14, 2009
    The 8% number is not about egress at all, only about natural light.

    The 8% glazing to floor area ratio is a requirement of the IRC 2009 section 303.1 which reads:

    R303.1 Habitable rooms. All habitable rooms shall have an aggregate glazing area of not less than 8 percent of the floor area of such rooms. Natural ventilation shall be through windows, doors, louvers or other approved openings to the outdoor air. Such openings shall be provided with ready access or shall otherwise be readily controllable by the building occupants.

    The minimum openable area to the outdoors shall be 4 percent of the floor area being ventilated.


    1. The glazed areas need not be openable where the opening is not required by Section R310 and an approved mechanical ventilation system capable of producing 0.35 air change per hour in the room is installed or a whole-house mechanical ventilation system is installed capable of supplying outdoor ventilation air of 15 cubic feet per minute (cfm) (78 L/s) per occupant computed on the basis of two occupants for the first bedroom and one occupant for each additional bedroom.

    2. The glazed areas need not be installed in rooms where Exception 1 above is satisfied and artificial light is provided capable of producing an average illumination of 6 footcandles (65 lux) over the area of the room at a height of 30 inches (762 mm) above the floor level.

    3. Use of sunroom additions and patio covers, as defined in Section R202, shall be permitted for natural ventilation if in excess of 40 percent of the exterior sunroom walls are open, or are enclosed only by insect screening.

    So for ventilation purposes if it's an operable window and you get more than 4% of floor area as free air when fully open you're good on the natural ventilation part, but you still need the 8% of the see-thru stuff when it's closed to meet the letter of code.

    If you put in permanently hard-wired LEDs sufficient to meet the 65lux everywhere minimum in exception #2 you can probably skate on the natural illumination.

    This is an old part of the code- prior to IRC 2006 it had been a 10% glazing requirement, but since window area has such poor thermal performance relative to wall area (losing 3x the amount of heat per square foot as typical walls) there was pressure to reduce it for energy-use reasons, which is why it's now "only" 8%.
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  4. snokel

    snokel Member

    Jan 29, 2014
    Thanks Dana.
    Here is the wisc code, it does not have the exceptions the IRC has:

    SPS 321.05  Natural light and natural ventilation.
    (1)  Natural light. All habitable rooms shall be provided with natural light by means of glazed openings. The area of the glazed openings shall be at least 8% of the net floor area, except under the following circumstances:
    (a) Exception. Habitable rooms, other than bedrooms, located in basements or ground floors do not require natural light.
    (b) Exception. Natural light may be obtained from adjoining areas through glazed openings, louvers or other approved methods. Door openings into adjoining areas may not be used to satisfy this requirement.
    SPS 321.05(2)

    It looks like I could install a louvered vent,transom window in the wall to the adjoining rec room which has two windows or maybe even a door with a full length window and add a ventilation van to supplement the window.

    I actually went and met with the building inspector and he didn't mention this exception. I agree with almost all the code requirements except this one, 8 % is a silly requirement for a basement bedroom, who hangs out
    in a spare bedroom duing the day, most people use these basement bedrooms as a guest room etc and people are only in there at night when there is no light anyways.
    Last edited: Mar 21, 2014
  5. snokel

    snokel Member

    Jan 29, 2014
    I emailed the building inspector and he siad I would have to prove the light from the adjoining area would reach the area, my reply was the code does not specify a distance or state that the adjoining area must meet the 8% rule. We shall see, if he says no, then I am just going to call it a fitness room :)
  6. dj2

    dj2 In the Trades

    Aug 13, 2013
    Apply for a variance or appeal to the inspector's supervisor.
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