2024 IRC Diffusion port area for unvented attic using air permeable insulation, 1:600 or 1:150?
Raft
| Posted in Building Code Questions on
The 2021 IRC states diffusion port area requirement as 1:600 (i.e. 1 square foot of diffusion port for every 600 square feet of ceiling) in subsection 5.2.2:
Did the 2024 IRC keep this unchanged at 1:600 or did they increase port area to 1:150 (i.e. 1 square foot of diffusion port for every 150 square feet of ceiling)?
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Replies
Yes it looks like they changed to 1/150
This article from earlier this month is still pointing to the 2021 IRC and 1:600 vapor port requirement.
https://buildingadvisor.com/best-approach-to-hot-climate-cathedral-ceiling/
Furthermore, I can't find 2024 IRC online....only 2021 IRC and the 2024 IBC.
Are you (or anyone else) sure it is the 2024 IRC that states 1:150? I know the 2024 IBC changed to 1:150 from 1:600....but IRC is different than IBC.
You are right, I was looking at the IBC, but the 2021 IBC also has the 1/600 ratio. It would be very strange if they only updated the IBC requirements and not the IRC, but I'm sure stranger things have happened. Also, If you read much of Lstiburek articles on vapor diffusions ports, he states that the required area is actually the same as what you need for a standard vent. I think the IRC 2021 underestimated, especially with their low perm requirements.
A standard ridge vent only needs to be 1:600 because with an equal area of 1:600 dedicated to soffit vents the total vent area comes out to be 1:300.
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
Right, but it is not split top and bottom and it is the only vent