Vented Crawl Spaces

How Effective is a Vented Crawl Space?

When we sit down with homeowners that are looking to perform crawl space repair, many of them are frustrated by the different ideas and proposals being thrown at them via the internet and crawl space waterproofing companies. Many homeowners that have moved to East Tennessee and purchased a home with a vented crawl space may not be aware of how our humid climate can affect their crawl space. Homes up for sale will uncover crawl space problems during the home inspection process but even some home inspectors are unsure how to advise correcting crawl space issues. Even mortgage lenders and refinance companies are requiring crawl space inspections to ensure there are no hidden problems that can affect the value of the home. The most problematic crawl space design homeowners in Knoxville, Chattanooga, Lenoir City, Oak Ridge and other surrounding counties and cities face is the vented or open crawl space.

Vented Crawl Spaces in East TennesseeVented crawl space door

The most common crawl space we run across is a vented crawl space. A vented crawl space has been around the longest and is designed to allow air to pass from the inside to the outside of the crawl space through crawl space foundation vents. This design has been around for years and up until recently has been very effective. Its design is to allow the crawl space to breathe. So what causes a vented crawl space to become ineffective? I am glad you asked but before I get into that, let's look at other attributes of a vented crawl space design:

  1. Vented crawl spaces typically use fiberglass batts for insulation.
  2. Vented crawl spaces typically contain a vapor barrier installed on the floor
  3. Vented crawl spaces many times will have crawl space foundation fans.
  4. Vented crawl spaces are more likely to have insects, rodents, mold and fungus.
  5. Vented crawl spaces humidity is most likely high in the summer and low in the winter.
  6. Vented crawl spaces lose more heat and air conditioning if ductwork is present in crawl space.
  7. Subfloors and joists hold more moisture in a vented crawl space, especially when fiberglass insulation is installed.
  8. Vented crawl spaces many times allow pipes to freeze more often.
  9. Vented crawl spaces may cause the living space to smell musty if mold is present.
  10. Since heating and cooling loss occurs in a vented crawl space this can cause utility bills to be higher.

"The principal perceived advantage of a vented crawl space over an unvented one is that venting may limit radon and moisture-related decay hazards by diluting the crawl space air. Additionally, providing a vented crawl space may make sense in flood-prone areas such as coastal zones subject to hurricanes. Venting can complement other moisture and radon control measures such as ground cover and proper drainage. However, although increased air flow in the crawl space may offer some dilution potential for ground source moisture and radon, it will not necessarily solve a serious problem. Vented crawl spaces are often provided with operable vents that can be closed to reduce winter heat losses, but also potentially increase radon infiltration. Although not their original purpose, the vents can also be closed in summer to keep out moist exterior air that can have a dew point above the crawl space temperature. This approach, however, requires a high level of informed occupant participation to be successful." ~ Oak Ridge National Labs


What causes a vented crawl space to become ineffective?

Time and time again when speaking with homeowners we hear how the crawl space had never had a mold issue and then all of a sudden it begins to smell musty, insulation starts falling and visible mold appears. They many times don't know why for years the crawl space was fine and then it became an unhealthy environment. During our interview process one home improvement project that stands out more than any other that may be linked to the sudden change in the crawl space is the installation of a more powerful HVAC system.

New HVAC SystemMold covered wood in crawl space

Upgrading from a very inefficient heat and air unit to a very efficient one may be the cause of increased humidity in a vented crawl space. If you live in a home built 20 years ago and you go from a 10 to 20 year old HVAC unit to a 14 SEER system that blows cold air way more effectively than before, that cold air can affect the crawl space. We have seen homes where they upgraded the HVAC unit but not the ducts due to expense or maybe they were in good shape. Ducts installed 20 years ago have a different R value of insulation than today but even the R value installed today may not be effective at creating a thermal barrier between the warm summer crawl space air and the cold HVAC air. When a cold duct meets warm moist crawl space air, it's like a cold glass of tea on a hot summer day. Sweating occurs on the ducts and that sweat evaporates and is absorbed by the surround insulation and wood.

Batt Insulation like Fiberglass or Mineral WoolWater soaked fiberglass batt insulation

Due to the air transfer of a vented crawl space, it is common practice to insulate the subfloor with batting insulation. When cool air enter the crawl space in the winter, batting installed in the floor creates a thermal barrier to help keep flooring warmer. Batt style insulations are also home to pests like rodents, opossums, snakes, ants, cave crickets and more. Batting will also absorb humidity and hold it against the surrounding wood studs and subfloor. Any plumbing leaks can also be absorbed and even hidden by batt insulation. Hidden plumbing leaks and water soaked batt insulation can cause mold to grow and even wood rot fungus to set it. Wood rot fungus will eventually destroy any wood it feeds on.

Add More Vents or Foundation Fans

Some companies in our area are still preaching more ventilation is better. The average humidity in East Tennessee is 70%. It is recommended to keep a crawl space at 45% to 55%. Adding more vents or foundation vent fans increase humidity in the summer and can also be the catalyst that created a more mold filled crawl space. If you install more vents or fans be sure to install a humidity reader to make sure they are working as promised. Chances are you will be asking for a different solution when you see the results.

Control Crawl Space Humiditycrawl space encapsulation

Controlling crawl space humidity is the key to a healthy crawl space and a healthy home. It is the same for basements too. A wet moldy basement or crawl space can affect resale value and maybe even your health. Dust mites and mold flourish in damp environments so make sure you choose PROVEN methods to creating a clean, dry, healthy and efficient crawl space.

About Michael Church

Michael Church has been involved with indoor air quality since 2005 and feels the unhealthy crawl space is one of the major problems causing poor indoor air quality.


  1. Judy Wilson on April 21, 2016 at 8:01 pm

    Thanks for explaining how I can control crawl space humidity by installing better ventilation. I think you’re right about how it’s important to make sure that it’s properly ventilated to prevent mold growth. You’re right about how mold flourishes in damp environments, so I should do more to keep my crawl space clean and dry. You said something interesting about how a good vented crawl space is designed with fiberglass batts for insulation and a vapor barrier installed on the floor. Maybe I should install those things to improve the condition of my crawl space. Thanks for posting this!

    • Michael Church on April 25, 2016 at 2:10 pm

      Thanks for your comment. Just remember a vented crawl space only works if the make-up air humidity is low. If you are having trouble controlling humidity, you may want to look into a dehumidifier.

  2. Sue on July 26, 2019 at 2:02 pm

    I would say that it would be beneficial to have a vented crawl space in the winter, if the homeowner has a gas furnace. This would allow oxygen to get to the furnace. There is a concern that if the crawl space is sealed off, this could become a problem in the winter months. In the summer, they can be sealed to ward off excess humidity.

    • Michael Church on July 26, 2019 at 2:26 pm

      Great point – this is why we do a hybrid system per EPA standards to both ventilate and control humidity. Thanks for the comment.

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