Crawl space Covering dirt floor of crawl space

Published on May 31st, 2013 | by Henri


Crawl-space insulation falling down, moisture building

Q.    We live in a home with a crawl space that is only about two feet tall. The insulation has all fallen down and sometimes we have a small area with a little water. We live just 3 blocks from the ocean and want to know what kind of insulation would be best for this situation.

We also have a lot of condensation in the attic. The plywood is wet and the nails drip water. Do you recommend a ridge vent? I would appreciate any advice you can give me as we are new to this area and don’t know what to do. Thank you.

A.    I assume that the crawl-space soil is bare and not covered with a 6-mil plastic vapor retarder. This is likely to be responsible in great part for the attic’s problem. It is also likely to be responsible for the insulation falling out between the floor joists; the paper vapor retarder holding it up may have rotted away.

I suggest you remove all the insulation batts. If they are in good shape and not soaking wet, and the paper vapor retarder is still sound, use them to insulate the walls of the crawl space. If they are wet and the paper is rotten, put them in heavy contractors trash bags and throw them away.

The first thing to do is to cover the crawl space floor with 6-mil plastic being careful to bring the plastic up the walls to the outside grade level. Since you have had some water seepage, install the plastic on the walls first by stapling it to the mudsill onto which the floor joists rest. Be sure that this plastic is wide enough to cover as much of the soil as possible.

Next, put another sheet of plastic on the ground from wall to wall, overlapping the plastic that is covering the walls as much as possible. This will prevent water from seeping onto the plastic where it cannot be absorbed by the soil. If the fiberglass batts are usable, staple one of their ends to the mudsill and let them hang down to the floor, bending them to cover the floor by a foot. Place bricks, stones or whatever you can find (except untreated wood) on the flat part of the batts against the walls to hold the insulation tightly. Use a stapler to staple the flanges of adjacent batts together.

But if the fiberglass batts are not useable, adhere 1-inch thick extruded polystyrene rigid insulation (blue, grey, pink or green, but no white expanded polystyrene beadboard) to the walls after cleaning them with a stiff brush. Apply daubs of StyroBond or polyurethane caulking to the concrete and press the insulation boards into the adhesive. Then staple plastic to the mudsill, covering the insulation following the instructions above.

Now check the grade outside and make sure it slopes away from the foundation.
Next, check for any convective paths from the living quarters into the attic and seal them up. If you can do the crawl space work and take care of any convection over the winter, wait to see what happens to the attic next winter after it has had a chance to dry over the summer. If you find new condensation, then more investigation will be needed to determine the best methods to ventilate the attic.

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