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R-49 is the common target for the roof or ceiling in cold and mixed climates. That is about 20 inches of blown fibreglass, 15 inches of blown cellulose, 16 inches of fibreglass batt or about 8 inches of closed-cell spray foam. Air seal the ceiling plane before adding any of it.
Size a whole house fan by the air it moves: square feet × ceiling height × air changes per hour ÷ 60. Most homes target a full air change every 2 to 4 minutes, and California's Energy Code sets a floor of 1.5 CFM per square foot. The attic must let that air out: at least 1 square foot of net free vent area for every 750 CFM. Enter your home to get the fan size and venting.
With these inputs:
| Result | Value |
|---|---|
| Fan size | 5,333 CFM |
| Attic vent net free area | 7.1 sq ft |
| By air changes | 5,333 CFM |
| Title 24 minimum (1.5 CFM/sq ft) | 3,000 CFM |
| One full air change every | 3 minutes |
How a whole house fan works. A whole house fan sits in the ceiling of a central hallway and pulls cool evening air in through open windows, pushing the warm house air up into the attic and out through the attic vents. It replaces air conditioning on nights when the outside air is cooler than inside, at a fraction of the power. It only works with windows open, and the attic vents have to be big enough to let the air out — undersized vents push hot attic air back into the house and can backdraft gas appliances. Attic venting. The 750 CFM per square foot rule is for net free area: the open area of a vent after louvers and screens. A typical 14 × 24 inch gable vent has around 1 sq ft of NFA; a 4-ft ridge vent section and soffit vents each list theirs. Existing roof ventilation counts. If the attic is short, add gable or ridge vents before installing the fan. Insulation and the ceiling opening. The fan's ceiling opening is a big hole in the thermal envelope in winter. Choose a model with insulated, self-closing dampers, or cover the opening with an insulated box for the heating season, and keep attic insulation away from the fan housing as the manufacturer directs.
Volume = floor area × ceiling height. · Airflow by air changes = volume × ACH ÷ 60. · Fan size = the larger of that and 1.5 CFM × floor area (California Title 24, Part 6, §150.1(c)12). · Attic net free vent area = fan CFM ÷ 750 sq ft.
| Home | By air changes | Title 24 minimum | Fan size | Attic vent area |
|---|---|---|---|---|
| 1,000 sq ft | 2,667 CFM | 1,500 CFM | 2,667 CFM | 3.6 sq ft |
| 1,500 sq ft | 4,000 CFM | 2,250 CFM | 4,000 CFM | 5.3 sq ft |
| 2,000 sq ft | 5,333 CFM | 3,000 CFM | 5,333 CFM | 7.1 sq ft |
| 2,500 sq ft | 6,667 CFM | 3,750 CFM | 6,667 CFM | 8.9 sq ft |
| 3,000 sq ft | 8,000 CFM | 4,500 CFM | 8,000 CFM | 10.7 sq ft |
Attic vent area is net free area (NFA), not the vent's outside dimensions.
Multiply your floor area by ceiling height, then by 20 air changes an hour, and divide by 60. A 2,000 sq ft home with 8 ft ceilings is 16,000 cu ft, so 5,333 CFM; California's minimum for the same home is 3,000 CFM (1.5 per sq ft).
At least 1 square foot of net free vent area per 750 CFM of fan airflow — about 7 sq ft for a 5,000 CFM fan. That is California Title 24's requirement and the common manufacturer rule.
Up to a point. More CFM cools the house faster, but it needs more attic venting and more open window area, and it's louder. Many people buy a two-speed fan sized for hot nights and run it on low.
Enough that the fan pulls air in quietly instead of straining: open windows in the rooms you want cooled, wider in the bedrooms at night, and follow the fan maker's stated minimum open window area for its size.
Not at the same time. Run the fan in the evening and overnight when it's cooler outside, then close the windows and switch to AC as the day heats up.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.