Designing for High Performance: Slab-On-Grade, Part I – Controlling Moisture Flow

We’re just a few weeks away from a computer generated, self-guided interactive virtual tour of the Serenbe Residence. We’re using REVIT software to generate the 3D model, then  VIMTREK will animate the tour.

What you will see in the tour is the home’s modern design, its 3 bedrooms and 2.5 baths, its great views of the central Georgia landscape, and its outdoor kitchen and fire pit. What you won’t necessarily see are some of the details that make it a high performance home, where we employ some of the best practices of building science and energy efficient design.

Details, Details, Details…

The biggest opportunity in designing for high performance is in controlling the flow of heat, air and moisture. Do this well, and the home will be comfortable, healthy, efficient, and will last a long time. Here is the slab-on-grade detail for the Serenbe home. We’re going to walk through the moisture control methods we employed. In Part II, we’ll focus on controlling heat. Finally, in Part III, concentrate on controlling air. Moisture is first, because it should be.

Keeping water out is perhaps the most important thing you can do in a building because prevents mold, condensation, water damage, indoor comfort issues, and more.

For the first layer of control (bulk moisture) on this above grade wall, we have Nichiha®cementitious siding. Just behind that, we’ve chosen a layer of 1/4″ rain screen, called Home Slicker, to create a gap which keeps any moisture that develops behind the cladding flowing down the wall via the drainage plane and out.

For our drainage plane, we’re using the OSB layer (with its built-in water resistive barrier) of the 1″ Zip System® R-Panel. At the base of the wall, and at all openings, is continuous aluminum flashing, which is also taped to the R-Panel withZip System® tape to maintain a continuous drainage plane. In the event that moisture does get in to the wall, which would most likely be airborne, not bulk, moisture, the materials have been selected to have a permeability that allows that moisture to get out of the cavity.

This is a critical point about designing building assemblies. They MUST have drying potential, either inward or outward or both. It’s why the industry has tried to get away from putting vapor retarders, and other impermeable materials in walls (including vinyl wall coverings).

At grade level you will see that we have called out for a minimum 5% slope away from the building with an impermeable back-fill layer to prevent saturation of ground adjacent to foundation. The free-draining back-fill layer allows a free flow of any moisture toward the even more free-draining layer of stone. The continuous drainage tile is set below the top of the footing, and well below the bottom of the slab, to collect water that makes its way down and takes it to daylight, and somewhere downhill and away from the foundation.

The foundation wall is coated in damproofing, and then another drainage plane, in this case Delta-Drain®, is used to allow any bulk water that happens to make its way to the foundation wall, down to the drainage tile and away from the wall and footing. The purpose of the filter fabric is to prevent dirt and debris from clogging the perforated drain tile.

At the base of the foundation wall, we will install Delta-Footing Barrier®, a capillary break, to prevent the wicking of moisture from the continuous concrete footing in to the foundation wall, that has the potential to wick up the wall and in to the home. The granular fill layer is not only a good bed for the concrete slab to set on to minimize movement and cracking, it also works allows any moisture to freely flow away from the slab in to the soil. The 6-mil polyethylene vapor barrier helps prevent any moisture from making contact with the concrete slab that could wick up and in to the home.

The Cellofoam PermaBG Expanded Polystyrene (EPS) foam board acts not only as a thermal and insect/termite barrier, it’s also acting as a vapor barrier for any moisture that may get in to the wall. Since we’re well protected at the outside and base of the wall, the foams primary function is for controlling heat flow (which is covered in Part II of this series). Finally, as a last line of defense, the continuous sill gasket (while also helping control air – Part II) will prevent the wicking of any unlikely moisture from the foundation wall in to the frame wall.

- Written by Chris Laumer-Giddens, Architect, HVAC Designer, Building Science Professional, Certified HERS Rater