Crawl Space Ventilation Calculator
Vent area or fan CFM for a crawl space.
Enter your values
ⓘ Live results
Results
- Required net free area
- 0.67 sq ft
- In square inches
- 96
- Typical 8x16 vents needed
- 2
- Fan option (5 ACH)
- 250 CFM
- Crawl volume
- 3,000 cu ft
Quick answer
Traditional code asks for 1 square foot of net free vent area per 150 square feet of crawl space, or 1 per 1,500 with a vapour barrier.
NFA = area / 150 (or /1500 with ground cover)
How to use the Crawl Space Ventilation Calculator
- 1Measure the crawl space footprint and average height.
- 2State whether a class I vapour barrier covers the ground.
- 3Either fit vents totalling the net free area, or seal and fit a fan/dehumidifier.
Worked example
Using the values the calculator loads with:
Inputs
- Crawl space area1000 sq ft
- Vapour barrierYes
- Average height3 ft
Results
- Required net free area0.67 sq ft
- In square inches96
- Typical 8x16 vents needed2
- Fan option (5 ACH)250 CFM
- Crawl volume3,000 cu ft
Frequently asked questions
Vent or seal?
In humid climates sealing with a liner and dehumidifier usually outperforms venting.
What is net free area?
The open airflow area after louvres and insect screen, roughly 50 sq in for a typical 8x16 vent.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.