Estimate a night-cooling airflow range from living area and ceiling height. See the attic vent and open-window areas that go with it.
PNNL’s Building America guide gives a planning range of 0.2 to 0.375 CFM per cubic foot of living space. This tool uses that volume method, so a taller ceiling changes the result directly. The appropriate point within the range depends on nighttime conditions, heat gains and the cooling strategy.
| Result | Calculation | What it does not establish |
|---|---|---|
| Airflow range | Floor area × average height × 0.2 to 0.375 | An exact product, local-code approval or a cooling guarantee. |
| Attic exhaust guideline | CFM ÷ 750 = net-free vent area in sq ft | Actual airflow or the outside dimensions of a screened vent. |
| Open-window guideline | CFM ÷ 1,000 × 4 = screened open area in sq ft | The total glazed area of the window. |
| Air-change rate | CFM × 60 ÷ house volume | Continuous whole-building ventilation compliance. |
The living-space volume is 16,000 cubic feet. Multiplying by 0.2 and 0.375 gives the range. At its upper end, the guidelines call for 8 sq ft of attic net-free area and 24 sq ft of open screened-window area.
13,500 cubic feet gives 2,700 to 5,062.5 CFM. The displayed upper endpoint rounds up to 5,063 CFM. A selected 4,500 CFM fan gives 20 air changes per hour, with 6 sq ft net-free attic area and 18 sq ft of open-window area under these guidelines.
Screens and louvers obstruct part of a vent. Use the net-free area stated for the actual vent, add the relevant exhaust areas, and convert square inches to square feet by dividing by 144. Do not automatically halve every vent’s outside area: products differ.
The ordinary attic-ventilation ratio used for moisture control does not by itself establish enough exhaust area for a whole-house fan. Follow the selected fan’s instructions and local requirements.
A whole-house fan pulls air through the living space during suitable outdoor conditions. An attic-only fan does not do that job. Continuous ventilation design and fans ducted directly outdoors also have different requirements, so this tool’s attic-area calculation does not apply to them.
A fan may be a poor cooling choice when outside air stays warm, smoky or very humid. An installer also needs to assess combustion appliances, air leaks from garages or crawlspaces, insulated dampers and the ceiling opening. None of those checks can be inferred from floor area.
Area-based rules assume a ceiling height. This tool uses PNNL’s volume multipliers and returns a range; climate and cooling strategy still determine the final selection.
No. The optional check only compares documented net-free area with the stated airflow guideline. It does not check duct resistance, vent placement, local code, combustion safety or actual installed airflow.
No. PNNL says to open enough windows before operation. Inadequate intake can depressurize the home and draw unwanted air from other spaces. Follow the product’s operating instructions and have the installation assessed.
Primary guidance checked October 12, 2026. The numeric results are planning references for fans exhausting into a vented attic.
Volume multipliers, the nine-foot-ceiling example, HVI-916 airflow, attic/window area guidelines and operation and combustion limitations. The source discusses approximately 12 to 23 ACH; the exact multipliers used here give 12 to 22.5 ACH.