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Best practice? Insulating a vented crawl space

My old house is in Atlanta and built in 1925. The house is a Mediterranean style and built with terra cotta blocks, covered in stucco on the outside.

The crawl space slopes about 30 degrees from front to back. We do not have water problems. The dirt floor has some flimsy plastic that needs to be enhanced. There are three 1-foot square vents to the outside. The only entrance is through an insulated, exterior door. Total square footage of the basement is about 1800 square feet.

All of the plumbing and HVAC ducts run through this area. The HVAC system is installed in an area of the crawlspace that has a concrete floor. The HVAC ducts are insulated.

There is no insulation on the walls or the subfloor. The hardwood floor above is "leaky," to say the least.


  1. Would you insulate the walls or the subfloor or both?
  2. If insulating the subfloor, can I use spray foam?
  3. Should I close the vents?
  4. Should I add an HVAC supply into the crawlspace area?
  5. How should I handle the risk of Radon? (We have not tested. There is no easy way to vent to the attic.)
    Re: Best practice? Insulating a vented crawl space

    Instead of insulating the crawl space, which, without proper measures to control typical crawl space moisture can have disastrous consequences, depending on the kind of insulation you use, I would suggest you consider a process called crawl space encapsulation or crawl space conditioning instead.

    This process consists in sealing the vents to keep the moisture from the outside air getting into the crawl space, and then completely isolating the crawl space from the ground and the air by lining the whole space with a sturdy, thick (preferably a multi layered 20mil) poly sheet. It is like a pool liner.

    You then seal the crawl space air tight with caulk and mastic tape. This process will include the crawl space within the internal envelope of the building.The space is then conditioned by running a dehumidifier or a crawl space conditioning system.

    Crawl space encapsulation eliminates moisture problems, cold or hot drafts, improves indoor air quality and numerous studies conducted by independent organizations such as Advanced Energy, Building Science Corp. and Habitat for Humanity, makes your whole house 18% more energy efficient.

    A professionally installed encapsulations system also serves as a passive radon mitigation system.Depending on the level of radon emissions in your area, that might be all you need. If not, radon mitigation systems can be installed with the encapsulation in place.

    Crawl space encapsulation is also recommended as Best Practice by the U.S. Department of Energy/EERE Building America Initiative.

    I am adding some information on the matter I suggest you look into.
    This is the Advanced Energy Website, with all the studies and many videos on the process.

    This is the link to the PDF document from the U.S. Department of Energy - EERE

    This is a website all about crawl spaces with some more information:

    Re: Best practice? Insulating a vented crawl space

    Wow. Thank you for a comprehensive response! I appreciate the time you took to pull this together. I have some reading to do!

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