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Crawlspace insulation advice

Petrea | Posted in General Questions on

Hi – Let me describe our crawl. It is about 3 to 4 feet tall and about 20×20 feet. We have a vapor barrier, sump, and dehumidier. The space is dry because of the dehumidifier. The ceiling and walls are both insulated. We’ve always just had a crawlspace smell in the area above the crawl and can’t seem to get rid of it. I’ve tried blocking vebts and opening them but I can’t get rid of that slight smell.

One thing I haven’t tried was removing the crawl wall insulation and just leaving the ceiling. Would that allow more air movement in and out through the cement wall? Are their reasons you shouldn’t insulated both ways? Any advice will be appreciated.


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  1. Petrea | | #1

    I should add I live in an area that ranges from 0F to 100F from winter to summer so heat loss is an issue.

  2. GBA Editor
    Martin Holladay | | #2

    You didn't explain whether your crawl space is vented or sealed. If there are large vent openings in your crawl space walls, you need to seal them up and to perform the other measures described in this article: Building an Unvented Crawl Space.

    Once you have done this work, you can reassess the situation. In general, it's almost always better to have insulation on your crawlspace walls rather than on your crawl space ceiling.

    You wrote, "One thing I haven't tried was removing the crawl wall insulation and just leaving the ceiling." Most experts advise the opposite approach -- leaving the wall insulation and removing the ceiling insulation. In my opinion, however, you can leave the ceiling insulation in place -- as long as it is well secured and not moldy.

    If you've performed the measures needed to create a sealed crawl space, and you still have an odor problem, the usual remedy is to install a low-flow exhaust fan in the rim joist to slightly depressurize the crawl space. Makeup air is provided by a grille in your floor (so that conditioned air from upstairs can enter your crawl space). This should solve the odor problem.

    Don't install a fan that's too powerful. The fan needs to be rated at 1 cfm for each 50 square feet of crawl space floor area.

  3. Petrea | | #3

    Thanks for the response Martin. I do have two vents on the same side which bring air in but I doubt exhaust much. There is a strong breeze coming in from both. I tried encapsulating myself but the stagnant smell got worse. I brought in four professional who ha four completely different theories. But it sounds like the current setup of insulation wouldn't be causing my problem Anny more than when I used rigid foam and had it encapsulated.

  4. GBA Editor
    Martin Holladay | | #4

    Now that you have filled us in on your crawl space details, the mechanism by which crawl space odors are entering you house is clear. The mechanism is the stack effect.

    Here's how the stack effect works in your house: cold outdoor air enters your crawl space through the convenient vents that you have provided. Warm, conditioned indoor air leaves your house through cracks in the ceiling of the top floor of your house. As the warm air leaves your house, it pulls more cold air into your house through those crawl space vents. The air travels from the crawl space to your first floor rooms through cracks in the floor that separates your crawl space from the first floor rooms. This air brings odors with it.

    The vents in your crawl space are a large part of the problem. So an important part of the solution -- that is, the solution to your odor problem -- is to seal these crawl space vents.

    The most important steps you can take to reduce your odor problem are:

    1. Seal the crawl space vents in an airtight manner.

    2. Perform whatever other air sealing work needs to be done in your crawl space (for example, by sealing cracks in the rim joist area).

    3. Install a low-power exhaust fan as described in my first response, along with a floor register to allow makeup air to enter the crawl space from the floor above.

    4. Perform air sealing work in your attic. Here is a link to an article that describes the necessary work: Air Sealing an Attic.

    There is a good chance that the work I describe will have an important side benefit in addition to solving your odor problem: lower energy bills. There are two reasons that this work will lower your energy bills: (a) sealing up those big holes in your crawl space and attic will reduce the amount of conditioned air leaking out of your home's thermal envelope, and (b) you will probably be able to unplug your dehumidifier once the small exhaust fan and floor grille are installed.

  5. Petrea | | #5

    Thanks for all the advice, Martin.

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