Ventilation & Humidity Control

When God created the Universe He designed it so that moisture evaporates up out of the soil and into the clouds.  This moisture later comes down as rain.  This has continued since the beginning and our water today is the same water that the dinosaurs drank.  In a crawl space this moisture evaporates up also.  The purpose of the foundation vents is to allow this moisture to exit the crawl space.  If the home does not have enough vents or if it has a high soil moisture content then this moisture can evaporate up and into your home’s substructure.  This will cause the wood moisture content to rise.

In this area, the normal wood moisture content range for wood that is not in a climate controlled setting is usually around 13% to 15% .  When the wood moisture content gets to 18% for a sustained period of time (a couple of months), the wood is able to support fungus growth (mold mildew etc) and corrective measures should be considered.  When the wood moisture content exceeds 22% corrective measures should be performed.  At a 28% wood moisture content the wood is considered to have active wood decaying fungus.  This fungus causes damage.  It is considered a wood destroying organism like termites, and therefore – corrective measures are considered necessary.  It is inevitable that this fungus will appear.  Fungus spores are ubiquitous in the air – there are also dormant microscopic fungus spores on all non-sterile surfaces.  When the surface of an organic object sustains enough moisture to support growth – fungus spores become non-dormant and grow.

Moisture problems can occur gradually.  It is not uncommon for a house that was built in 1975 to be fine for 30 years and then start to develop some minor issues.  If a house was built 30 years ago and it had a wood moisture content of 13% at the time of construction there would be no issues.  Now during the course of the year the wood moisture content rises ¼ %  to 13 ¼%.  There is still no problem.  Now suppose this went on for several years.  In four years the wood moisture content is 14%, in 8 years it is 15%, 12 years 16% and so on.  Eventually the wood moisture content elevates to a point when corrective measures should be considered.

There are multiple corrective measures that can be performed:

  1. Vapor Barriers:  A Vapor Barrier is a thin sheet of plastic that is placed on top of the crawl space soil.  This plastic holds the moisture in the soil and thereby keeps it from evaporating up and into the substructure.
  2. Increased Ventilation:  Ventilation can be increased by either adding more vents or adding vents that allow more free air to pass through.  The standard ventilator is 8”X16” but the amount of free air varies from 28sq inches to 72 sq inches.
  3. Fans/Forced Air Systems:  Moving air holds more moisture than stagnant air.  By circulating the air you can absorb moisture and exhaust it out of the crawl space.

What is Ventilation?

Poor Ventilation

Poor Ventilation

Ventilation is a system of intake and exhaust to promote the necessary free flow of air throughout the crawlspace.

Adding ventilation is a simple way to help reduce moisture buildup. Ventilation will allow air to enter and exit the space, carrying heat and moisture out with it.

There are two types of ventilation that can be used, passive and active.

Passive Ventilation. Passive ventilation relies on the passage of air in one vent and out the other, depending on the direction of the wind.

Active ventilation. Powered foundation vents are vents with small electric fans built into them. Active ventilation is used when there is a serious moisture problem, or for difficult-to-vent areas.

Sometimes the above corrective measures are insufficient.  In some cases a crawl space just remains damp.  There are a few reasons for this but typically it is a product of humidity.  Air molecules hold moisture.  When the air is moving it can hold more moisture and when it is warm it can hold more moisture.  During the day when it is 95 degrees and the humidity is 95% the air is almost saturated with moisture.  At night when the air cools to 75 degrees it is no longer able to hold this moisture and dew or condensation forms on the grass etc.  Then the next morning when it warms up the dew

Vapor Barrier

Vapor Barrier

or condensation is absorbed.  Crawl spaces are generally cool and therefore often damp.   When the warm outside air enters a crawl space, it can warm the crawl space slightly and therefore make it able to absorb the moisture.  But often times it causes another problem.  When the warm humid air enters the crawl space it cools and therefore extra moisture/condensation/humidity is created.  This is becoming more of a problem because today’s air conditioners blow cooler air and people keep their homes cooler than they did 25 years ago.  In these cases, different control measures may need to be performed.  Some of these control methods are:

  1. Treat the substructure with a wood preservative that inhibits fungus growth.
  2. Install a dehumidification system that allows the home to vent naturally but during extreme conditions the dehumidifier will automatically turn on and aid in removing humidity.
  3. Encapsulate the crawl space.  This is sometimes necessary but as a general rule we don’t recommend it.  Crawl space homes are designed by architects and their construction was overseen by county building code enforcement agents to be a vented crawl space.  Changing them to a sealed basement may not be the best answer.  In addition sometimes effluent gasses escape out of the soil – these gasses need to exit the crawl space not go up the foundation wall and potentially enter the living area.

Fans/Forced Air Systems

Horne’s Pest Control is an installer of ATMOX Controlled Ventilation Systems.  Atmox is unique in that it actually monitors your crawl space humidity and controls fans and or dehumidifiers automatically to ensure that your crawl space remains at its optimum condition.  Please click here for more information on Atmox Controlled Ventilation Systems.    ATMOX Logo



Remember moisture can be a problem.

  • Wood Destroying Fungus

    Wood Destroying Fungus

    Moisture can attack wood and trigger decay, fungus and wood rot resulting in structural damage.

  • The foundation can crack.
  • Floors can warp.
  • High moisture levels under the home can lead to mold, mildew and cause the spread of fungus spores leading to odors and unhealthy air conditions.
  • Mold spores can aggravate allergies, asthma and hay fever.
  • Moisture in the crawlspace can dampen insulation reducing its effectiveness.
  • Dark, damp area are hospitable environments for termite and other wood-destroying pests.