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 Proud Green Home at Serenbe, a design-build collaboration with The Imery Group. 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, we will concentrate on controlling air. Moisture is first because it should be.

Designing for High Performance, Part I, Controlling Moisture Flow

Keeping water out is perhaps the most important thing you can do in a building because it 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.

Designing for High Performance, Part I, Controlling Moisture Flow

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 with Zip System® tape to maintain a continuous drainage plane. In the event that moisture does get into 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 a minimum 5% slope away from the building with an impermeable back-fill layer to prevent saturation of the ground adjacent to the 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.

Designing for High Performance, Part I, Controlling Moisture Flow

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 into the home. The granular fill layer is not only a good bed for the concrete slab to sit on to minimize movement and cracking it also 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.

Designing for High Performance, Part I, Controlling Moisture Flow

The Cellofoam PermaBG Expanded Polystyrene (EPS) foam board acts not only as a thermal and insect/termite barrier. It also acts as a vapor barrier for any moisture that may get into the wall. Since we’re well protected at the outside and base of the wall the foam’s 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 into the frame wall.

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

PROJECT UPDATE: Permit drawings for this home are being submitted this week, and we should be under construction the first week of September. We’ll be following the progress closely with pictures and videos. Average energy bills for the home should be “zero” (a.k.a. Net-Zero). The home will be featured on ProudGreenHome.com. 

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8 Responses so far.

  1. Milan Jurich says:

    Chris,
    Excellent level of detailing! Thanks for sharing. You are also the first I’ve seen to incorporate the Zip R Sheathing into a wall assembly as it’s a relatively new product. I’m considering wrapping it on the exterior with Benjamin Obdyke’s HydroGap product as an extra layer of protection around window flashings instead of relying solely on the installers of the Zip tape.

  2. Thanks, Milan!

    That’s an interesting approach that I have considered in lieu of the Home Slicker. The advantage of the HydroGap is that it’s thinner, which makes the overall thickness of the wall less (not by much). We were going to use Home Slicker on a house here in Atlanta until the thickness of it changed all the details at the exterior wall (trim, windows). It has more thickness than the HydroGap, and would have pushed the siding out too far…almost flush with the window trim. The product was introduced too late, otherwise we could have integrated it during design. The HydroGap would have worked well. Instead, the builder wanted to cut 1″ strips of building felt and doubled them up at 16″ intervals all the way up the exterior wall, giving us a small gap for drainage. Seemed to do the job pretty well, and cost less than any rain screen product on the market.

  3. Looks good. Do you have details for shallow frost protected foundation designs?

  4. [...] Part I of this series on designing for high performance, we discussed the control of moisture flow at [...]

  5. Sydney says:

    Chris, I’m surprised to see the insulation board on the foundation wall. Can you comment on the impact that it has on either moisture or heat transfer (that would not be otherwise taken care of by the poly, for instance)?

  6. Hi, Sydney,
    Thanks for your comment and question.
    Check out Part II (controlling heat flow), if you haven’t already. I have found that heat flow in a slab is predominately through the edge. We also have to deal with exposing the foundation for termite inspections. The insulation on the inside of the foundation wall allows us to deal with both. http://lgsquaredinccom.ipage.com/wordpress/www.lgsquaredinc.com/part-ii-2-designing-high-performance-foundation-slab-on-grade-controlling-heat-flow/
    As far as controlling moisture flow, we could stop the poly just about anywhere we want around the slab edge. In this detail, we’re showing it stopping at the insulation/inside of foundation wall. The moisture flow from the foundation wall to the slab (if any) will be dealt with by the insulation between the slab and wall.
    This detail is the one we chose among a number of solutions because of how well it addresses these issues, as well as being simple and cost effective.

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