How to Calculate the Air-Flow Volume Your Crawl Space

Chances are that you have a vented crawl space if you live in an old house. Unfortunately, these vented crawl spaces were based on the misconception that vents would allow moisture to evaporate from the crawl space. We know that these vents actually increase the humidity level just recently. The natural air moves upwards, and that movement carries air from the crawl space vents up into the living space above; in other words, crawl space air enters to where you live. Up to 45 percent of the air in your home was once in the crawl space… scary. The uncovered soil on the crawl space ground contains dirt, mold spores and allergens. That filth moisture is entering the floor framing foundation under the house that will start wood rot; damage. If this happens, the result is an expensive structural repair.

Things You’ll Need:

  • Keen eyesight
  • Gloves
  • Goggles
  • Plastic floor covering
  • Flashlight or portable spotlight
  • Not afraid of crawling insects and animals (i.e. spider, roach, beetle, snake, rodent).
  • Space ventilator; fan system
  • Some electrical repairing/installation experience
  • Toolbox with essential tools



Benefits of unvented versus vented crawl space: In winter, warm air escaping the attic creates a pressure at the lowest openings in the home. This pressure causes cold air to enter through the vents, and rise through cracks in the subfloor to the first floor. This is known as the “cold floor syndrome” in cold climate homes that has vented crawl spaces. This cold air also carries moisture, odor and dirt from the soil into the home, which associates with bacteria growth. An unvented crawlspace helps to eliminate all these problems.


Cleaning vented space can help eliminate mold growth and all the bad conditions listed before. It can also make the plumbing system that runs through the crawl space operate more efficiently, especially in winter for frozen pipes. Utility lines last longer since they are not suffering temperature and humidity extremes. Its temperature should be (year-round) at 55 – 68 degrees F. Ducts should be well sealed with sealant to prevent possible air leakage. Air leakage increases energy consumption.


The best way to remedy moisture is to invest in a “Foundation Ventilator” system. This ventilator minimizes wood rot to the foundation by sucking out the vapor and other hazardous gases; which in turn, prevents termite, rodent and other kinds of insect invasion.


To calculate the approximate air you need to ventilate out of your crawl space, do this: multiply the square footage of the crawl space by the height – you then get the ‘total’ cubic feet. Then divide that number by “15” to determine the CFM (Cubic Feet per Minute) needed. Example formula: (l)40ft x (w)25ft x (h)3ft = 3,000 cubic feet/area. Now, divide 3,000 (cubic feet) by 15 = 200 cubic feet per minute (CFM). This means 200cu.ft./min. of airflow is needed to ventilate the crawl space for maximum efficiency. Get one with a sensor so the unit can turn off/on itself relative to the condition.


You can buy a ventilator online or at home improvement centers. Once you’ve purchased the ventilation system, follow the manufacturer’s instructions to install the equipment.

Tips & Warnings

1. The height number “3” used in the example formula in Step 4 is a variable; it’s the height of your crawl space. Some are 1.5ft, others are 2ft, still others have 3.2ft…. so adapt the number depending to the height of your own crawl space.

2. A dehumidifier can be installed also.

3. Water in crawl space is a serious condition. Attempts need to be made to keep it dry.

4. Hire a professional if you feel that you can’t handle the job yourself.


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