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IRC Section R303: Natural Light Calculation Requirements

Mark_Nagel | Posted in General Questions on

While researching window requirements (specifically the minimum for egress/emergency exiting) I stumbled upon IRC Section 303 (reference: https://buildingcodetrainer.com/residential-light-ventilation-calculation/) which also includes this gem:

Natural Light Calculation Requirements

The aggregate glazing area for a Habitable Room shall not be less than 8 percent of the floor area of the room.

So basically all of the glazing provided in a room added up shall not be less than 8 percent of the rooms floor area. For example if two windows are provided in a bedroom, the size of both windows added together must not be less than 8 percent of the rooms square footage.

I was wanting to minimize the amount of glazing in my north-facing rooms (habitable ones), only going with (at least) the minimum for egress.  I realize that I’m in the US and perhaps not required to do 8% (I have not looked- if anyone can point me at any US code references I’d appreciate it), but trying to be a bit more proactive on applying likely future codes this IRC one seems to be a bit of a stretch.  My only thought is to, in combination with the code required egress windows, utilize transom-like windows (fixed): locate high up on the walls (I’ll have 9′ walls).

This kind of stuff starts to strain energy efficiency requirements.

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Mark,

    The IRC is international in name only. It's a US model code that many jurisdictions adopt or modify. You may live in an area where another code or no codes apply, but you are probably under the IRC.

    This is a current code requirement, not a future one. I don't see much p0int in trying to anticipate future codes. Existing buildings are not required to meet codes as they change unless they are doing large renovations. Codes change constantly I doubt whether 2% of the housing stock in Canada meets current codes.

    I've never run into a problem with minimum bedroom glazing sizes because the percentage is so low. I can't imagine designing a room with less than 8% gazing and having it be pleasant to be in. It doesn't make sense to subordinate the architecture of a house to minimize energy use. If no one want to be in a house it doesn't matter how much it costs to heat to cool.

  2. Expert Member
    Michael Maines | | #2

    Mark, Exception R303.1.2 says that you can replace window area with artificial lighting. 8% is just a good guideline for daylighting, as Malcolm notes, and it also happens to be what feels "normal." If you use much less than 8% of the floor area in glazing, the room will feel a bit like a bunker or cave. That's not always bad, and with LED fixtures you can get a code-compliant 6 footcandles affordably. https://codes.iccsafe.org/content/IRC2015/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR303.1

    1. Expert Member
      NICK KEENAN | | #3

      Michael, thanks for that, it makes more sense. On first blush it seemed like the code was getting into the business of specifying esthetics, which seems like a slippery slope -- how long before ugly colors are banned?

      But if the requirement is that a room has to have adequate light, which can be met either through natural or artificial light, that seems reasonable.

  3. Mark_Nagel | | #4

    Ah! Many thanks for the code clarification. I did not see the Exceptions clauses.

    By no means am I looking to do a caveman design/theme. Bedrooms are for sleeping (which is why I have them on the north side of the building - zone 4C): I WILL make sure I'm at/above the 8% (which will also comply with egress min). The "I'm awake and doing things" rooms, living room, kitchen etc, will have ample natural light: they're on the south side of the building.

    There is also a requirement for ventilation as openable window or door (to the outside) that's a minimum of 4% of total floor space.

    1. Expert Member
      Michael Maines | | #5

      Mark, good catch on the 4% for ventilation. I always include a whole-house ventilation system which overrides the 4%. Are you planning on having a ventilation system? You should, and it may be required.

      Regarding egress and emergency escape, be sure to read sections R310 and R311: https://codes.iccsafe.org/content/IRC2015/chapter-3-building-planning#IRC2015_Pt03_Ch03_SecR310. For emergency escape, you will need at least one window or door that opens directly to the exterior and that has a net clear opening of at least 5.7 sq.ft., with some other important criteria as well.

  4. DavidLang | | #6

    Thanks for pointing out the lighting exception. The ventilation exception is well documented, but finding info on lighting has been much harder.

    You may say that a room will feel like a cave without enough windows, but if you are talking about a basement, getting 8% windows can be rather hard

    And if you have a very large interior room, getting enough windows can become rather difficult (double the perimeter and you have 4 times the area, requiring 4 times the windows in the walls, some of which are to other interior spaces.

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