How To – Preventing Mold And Insulation of Heating Ducts in Crawl Spaces

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    One of our visitors from the Seattle Washington area asks about a problem with condensation on his duct work leaving some mold growth due to moisture in his crawl space.

    The problem they are having is that their home has a partial basement and a dirt crawl space under part of the home. The area that is concrete is not having a problem but the area that has open dirt is.

    Seattle is a pretty interesting climate. They have all four seasons with a warmer then normal winter. However it does get cold enough to snow and in the summer having an air conditioning system is important due to the high humidity levels and rainfall.

    Unfortunately this means that condensation can occur in both the summer and winter months.

    A solution that their local contractor gave them was to give up the insulation that surrounds their duct work in the open dirt area crawl space. This was inappropriate information because although the condensation will not remain in the insulation resulting in mold it will still form. It can form both outside of the duct and inside depending on the time of year and if the air is being heated or cooled. If condensation forms inside the duct then mold can grow inside the duct resulting in an unhealthy environment.

    The real solution for this home is to reduce the amount of humidity in the unfinished crawl space. This will require inspection of the area to make sure that runoff water from their rain gutters is not feeding the problem and also protection from the moisture in the dirt below the crawl space.

    Depending on the height of the crawl space this can be a pretty difficult task to fix.

    If the home has at least 3 to 4 feet of area then it will be easy to dig back enough room in the crawl space to install a vapor barrier and then cover it with some gravel. Some contractors will recommend that you use a vapor membrane that is specifically designed for this application however you can get the job done with 10 mill poly plastic tarp if you overlap it well and fold the edges to create a seal and depending on the problem you may want to double the layer of poly plastic.

    Make sure that you allow for a foot or more to run up the side of the home and hold the edges of the plastic in place with gravel.

    If you did find that the rain gutters or just runoff was causing a problem then you will need to install downspout extenders on your gutter.

    You will also want to build up a sloping grade around the perimeter of your house so water from the rest of your yard or neighbors will not end up under your home.

    Ventilation Under the Home

    Good ventilation in your unfinished crawlspace will reduce the amount of moisture that is retained in the dirt and porous elements like wood and insulation.

    There are specific codes on the amount of ventilation that is minimally required but depending on your area you will probably need to increase the amount of venting to obtain good results.

    You may even need to install a fan system such as is found in Attics for cooling. Fan size can probably be pretty minimal meaning you won’t be wasting a lot of electricity.

    Final Note

    Unfinished crawl spaces are not fun to deal with. There are a number of reasons that your home may have one.

    To reduce the cost of the home when a basement is not installed or to provide for access and raise the home in flood zones. I think the only reasonable reason to have an unfinished crawl space is when the home is in a Flood Zone and must be elevated however even new homes are built with perimeters that are enclosed with concrete but crawl space floors that are mud.

    That is just not acceptable to save a couple thousand dollars by leaving out a concrete slab when you have already installed a perimeter footing and concrete knee-wall. This is not only a danger for mold but will result in a lifetime of headaches for the life of the home.

    Increased levels of pests in the home, higher moisture levels not to mention the headache of servicing the mechanicals and the possibility of moisture entering the electrical system is just dangerous.

    The contractor that recommended that insulation was not necessary in their location was giving bad advice to their client. Whenever there is heating or cooling duct that is outside of the insulated portion of your home it must be insulated.

    In some areas it is even recommended to insulate duct, pipe and services that are inside the home like hot water lines in a finished basement that run from your hot water heater to your fixtures or exposed baseboard heater feeds.













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