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Old insulation in crawl space

myuschak | Posted in Green Building Techniques on

I have an existing crawl space that in the past was a wet and damp space. Now that it has been cleaned , the old insulation seems to be holding the damp musty smell. The crawl space is semi heated with the furnace and duct work in the space. Should we remove the old insulation and put nothing in its place, or should we instal new insulation and if so, what do you recommend?

Thank you
Mark Yuschak

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  1. GBA Editor
    Martin Holladay | | #1

    The walls of crawl spaces should be insulated with rigid foam insulation or spray polyurethane insulation. In some cases, it also makes sense to insulate the floor of a crawl space. In any case, a crawl space should be tightly sealed — not ventilated — and equipped with a ground cover of heavy polyethylene, EPDM, or similar material to prevent soil moisture from evaporating.
    Read more here:

  2. Allison A. Bailes III | | #2

    If advisable, encapsulating your crawl space is the way to go. This means covering the ground with a vapor barrier sealed to the walls and at all the seams, sealing up the vents in the foundation walls, doing something with the air (conditioning it directly, exhausting a small amount, or dehumidifying it), and insulating the foundation walls. The old insulation doesn't need to be removed in many houses, but it sounds like I'd recommend it for yours.

    In some cases, it's not a good idea to completely seal up the crawl space, however. What kind of furnace do you have down there? If it's an atmospheric combustion model, which uses air from around the unit for combustion, you'll need to make sure it has adequate air from outside. Otherwise you could possibly depressurize the crawl space enough to cause backdrafting, especially if there's also a gas water heater down there.

    The source for information on this topic is Advanced Energy's website, They did research on this topic early in this decade and have guidelines you can download for free from the site.

  3. Danny Kelly | | #3

    We struggle with this every day - we are down south (NC) and virtually every crawl space has high levels of moisture, some mold and a musty smell. We typically use a closed crawl for new construction but are reluctant to use it on an existing crawl. It seems almost impossible, without going through very expensive measures (mold remediation, mold retarder, air scrubbers, etc) to get rid of the mold and the musty smell. By sealing/closing the crawl - it then communicates with the conditioned house - we certainly do not want that musty air mixing with the living area of the home.

    Most experts agree that a vented crawl is not the way to go - but we found a product that seems like a good compromise. The Atmox system is a smart system of foundation vents that have fans. On a typical home - we install 2 intake fans, 3 exhaust fans (to keep a slight neg. pressure on the crawl) and seal up all other vents. There are sensors that measure the interior and exterior temperature and humidity - it calculates the dew point and only runs the fans when the conditions are better outside than they are inside the crawl. Once the conditions inside the crawl are improved, the fans trun off virtually sealing the crawl and not allowing hot/humid air to enter. We have been using this for 10 years to resolve houses with major mold problems - yearly inspections are performed with very good results. Much cheaper than closingthe crawl and do not run the risk of mixing contaminated air. By movingthe air - it should over time remove the musty smell - if that does not work than removing the insulation may be the only answer.

    Of course we still install a vapor barrier, proper air sealing and duct sealing. Cold air from the duct leakage combined with the hot/humid outside air is usually the main culpret with the mold growth.

  4. Allison A. Bailes III | | #4

    Crawl spaces communicate pretty darn well with the houses above them before they're encapsulated, and sealing them up doesn't necessarily make that any worse. If you follow Advanced Energy's recommendation to air seal the floor to isolate the crawl space from the house, it communicates less after encapsulation.

    I've encapsulated a lot of crawl spaces in the past 5 years and my experience, confirmed by the homeowners' feedback, is that the musty smell goes away when it's done correctly. I'm not a mold expert, but I know that mold needs moisture to grow. Remove that, and it's not a problem anymore.

    If venting the crawl space with lots of outside air works for you, that's great. I distrust that method, however, because it requires conditions that don't often exist in much of the South during the summer. Which part of North Carolina are you in?

    It also depends on a properly functioning control system to find the dew point and turn the fans on or off. Since that system is in the crawl space, how will the homeowners know if it malfunctions and brings in air with a dew point of 75 degrees every day for a couple of months?

    In the end, I believe a properly encapsulated crawl space will outperform a mechanically-vented crawl space across most of the southeastern US because the dew point just doesn't go low enough for ventilation to help.

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