Expert: Daniel Humphrey - 12/15/2008

Question
We have a slab built home with 1 x 6 constructed walls and fiber-cement siding. Is it possible to earth-berm lower exterior walls without compromising the structure with moisture damage, etc.? Difficult finding contractors in our area that are experienced with green construction techniques. Would like directions for this type of berming if it is advisable. Thanks so much. M

Answer
Dear Maxene,

You've identified the main problem with earth berming--moisture damage. The pressure from the ground can also damage the walls.

To keep ground water away from the siding, you will need to prevent the earth from actually touching it. Starting with the cement siding, a waterproof membrane is sometimes applied and flashed so the siding overlaps above the new ground level. Next, a layer of stacked rock is placed against the wall to provide channels for water to drain away. Then in the earth itself, some artificial structure, such as road or drain fabric, is placed laterally for several feet in layers to alleviate hydrostatic pressure of the earth against the wall. And finally, the ground is contoured so that water flows away from the building. Gutters and downspouts are installed so that the roof water is taken away from the perimeter.

Inside, the walls to be backfilled are first sheathed with 1/2" plywood and a vapor retarder (plastic) applied beneath the sheetrock to prevent any interior air from moving into the wall cavity. Outlet boxes are sealed. The sheetrock is sealed all the way around with caulk and the finished normally, with a vapor retarding paint.

The main moisture problem with earth berming done properly comes from the interior moisture that gets into the wall and cannot get out. By sealing off the air leaks in the outside wall, the small amount of outside air circulation that keeps the insulation dry is eliminated. For this reason, the air/vapor barrier must be absolute.

Hope this helps,

Daniel