Home / Guides / For builders — detailing / Net free area after mesh or a listed core
Net free area after mesh or a listed core
Oregon R327, where adopted, still wants attic and underfloor net ventilation area under R408 and R806. Covering those openings with 1/16 to 1/8 inch mesh, or with a manufactured ember- and flame-resistant vent, cuts net free area. Size the openings. This page does not invent an NFA number for a SKU (amendment PDF).
You are swapping 1/4 inch animal screen for 1/8 inch mesh, or for a listed core. The hole looks the same from the yard. The attic still has to breathe, or you chose unvented.
No g3-passed NFA chart exists. Read R327.3.2’s pointer and the listing’s published NFA. Do not use a house-level hub graphic as a calculator.
Why it matters
Ember mesh and listed cores both obstruct the opening. If you keep the same hole count and shrink the openings, moisture and heat can sit in the attic. R327 did not repeal R806. It added an ember rule on top.
ASTM E2886 does not score airflow. A pass is ember and flame through that specimen.
How it works
Net free area is the unobstructed opening that remains after the mesh or the core. Finer mesh, baffles, and intumescent matrices all reduce it. The number, if published, belongs to that model.
R327.3.2 says the minimum net area, where ventilation is provided, shall be in accordance with R408 and R806. That is a pointer, not a table on this page. This session did not open 2023 ORSC R806 as a full PDF. Read the chapter the official is using.
The paths
| Choice | What to size | What not to do |
|---|---|---|
| Prescriptive mesh | Enough opening that, after 1/16–1/8 inch mesh, R408/R806 still hold | Keep the old hole count and assume airflow is unchanged |
| Listed 8165 / E2886 vent | Use the NFA the listing or the manufacturer publishes for that model | Invent a number because a brochure said “high airflow” |
| Unvented attic | No attic ventilation openings. Other code for that assembly | Tape the old vents and call it NFA math |
See mesh versus listed vents and OSFM 8165 applications.
What people mix up
- Treating ember mesh as a free airflow upgrade.
- Using an E2886 pass as an NFA rating.
- Guessing NFA because the handbook row is silent.
- Forgetting crawlspace R408 when you only did the attic.
How to check
- Count the openings you will keep.
- If mesh, plan more area or more openings than the 1/4 inch screen needed.
- If listed, copy the published NFA for that model onto the drawing. If none is published, ask the maker in writing or pick another model.
- If the sheet says unvented, those attic openings should not exist.
Limits
This page does not print R806 tables. It does not invent SKU NFA. FireApproved is not the AHJ. Moisture design is still a building-science job.
Related records
Questions
Does 1/8 inch mesh keep the same airflow as the old 1/4 inch screen?
No. Finer mesh reduces net free area. R327.3.2 still points remaining ventilation to R408 and R806. Size more opening, or a listed model whose published NFA still meets those sections.
Where do I get the NFA for a listed vent?
From the listing, the evaluation report, or the manufacturer’s published instructions for that model. This site does not invent a number. If the row is silent, do not guess.
Can I skip airflow math with an unvented attic?
Unvented attic removes those attic ventilation openings. It does not delete R408/R806 for openings that remain, and it has its own moisture and insulation rules. See unvented attic.