What “net free area” means.
Net free ventilation area is the usable opening area declared for a ventilation product, after restrictions such as screens and louvers are accounted for in the product value. It is not the outside dimension of the product, a measured airflow rate or a guarantee that the installed route is clear.
Use the exact product's published value and installation basis. Keep the product name, source document and unit alongside the number. If no value can be found, record it as unknown instead of calculating it from a photograph.
Separate the quantity from the unit basis.
| Product data says | Your record needs | Arithmetic |
|---|---|---|
| Area per individual vent | Number of installed vents serving this roof space | Quantity × free area per vent |
| Area per linear metre | Installed effective length in metres | Length × free area per metre |
| Area per linear foot | Installed effective length in feet | Length × free area per foot |
Convert units before adding results. For area, one square metre is 10,000 square centimetres; one square inch is 6.4516 square centimetres. Do not apply a simple length conversion to an area value.
A worked example with made-up inputs.
Suppose a product sheet states 80 cm² per intake vent and a roof section has six of those vents. Suppose its exhaust product declares 120 cm² per metre and the effective installed length is three metres.
Exhaust: 3 m × 120 cm²/m = 360 cm²
Recorded total: 480 + 360 = 840 cm² = 0.084 m²
These are illustrative numbers, not product recommendations or a compliant design. The difference between intake and exhaust is useful to discuss, but the total alone does not establish acceptable distribution or actual airflow.
Keep the design requirement separate.
A rule expressed as one part vent area to a given roof-space or ceiling area requires the correct area definition, ratio and units. The responsible professional should identify the applicable code provisions and assembly-specific requirements. Do not select a familiar ratio and assume it applies to every roof.
When a worksheet uses an area divided by a ratio, its result is conditional on those inputs. It does not check the installation, open paths, required distribution, product suitability or the roof's air and moisture control design.
Bring the full record to the quote review.
- The roof section and area being assessed.
- The source for the chosen design criterion.
- Each product, its free-area value, unit basis and installed quantity.
- Separate intake and exhaust totals in matching units.
- Unknowns, inaccessible areas and any proposed changes.
Use the vent-area worksheet to keep the inputs editable. Then review the route between the openings with the person responsible for the design.
Reference and method
Building Science Corporation: Understanding Attic Ventilation discusses free-area ratios and assembly context. Product-specific values must come from the selected manufacturer's documentation. The worked example above is HAUSE's illustrative arithmetic.