IRC Section R303: Natural Light Calculation Requirements
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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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.
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
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.
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.
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.
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.